Geophysical-Geotechnical Characterization of Mud Volcanoes in Cartagena Colombia

Abstract

In this research, the mud diapirism phenomenon in the Membrillal sector in Cartagena is characterized to analyze its spatiotemporal evolution. The goal is to geomorphologically, geotechnically, and geologically characterize the area to zone regions with the greatest susceptibility to geological hazards and provide an updated diagnosis of the phenomenon. This study is conducted due to the risks that mud diapirism poses to infrastructure and the safety of local communities. Understanding the behavior of these structures is essential for designing effective mitigation measures and optimizing urban planning in areas affected by this phenomenon. The methodology used includes collecting secondary data and implementing geophysical, geotechnical, and laboratory tests. Among the techniques employed are the Standard Penetration Test (SPT), the excavation of test pits, and electrical resistivity tomography, which revealed mud deposits at different depths. Laboratory studies also evaluated the physical and mechanical properties of the soil, such as Atterberg limits, grain size distribution, moisture content, and expansion tests, in addition to physic-chemical analyses. Among the most relevant findings is the presence of four active mud vents and four mud ears, representing an increase compared to the previous study that only recorded three mud vents. The tests revealed mud deposits at 1.30 m and 10 m depths, consistent with the geotechnical results. Laboratory tests revealed highly plastic soils, with Liquid Limits (LL) ranging from 44% to 93% and Plastic Limits (PL) ranging from 14% to 46%. Soil classification showed various low- and high-plasticity clays (CL and CH) and silty clays (MH), presenting challenges for structural stability and foundation design. Additionally, natural moisture content varied between 15.8% and 89%, and specific gravity ranged from 1.72 to 2.75, reflecting significant differences in water retention and soil density. It is concluded that diapirism has increased in the region, with constant monitoring recommended, and the Territorial Planning Plan (POT) has been updated to include regulations that mitigate the risks associated with urban development in affected areas.

1. Introduction

Mud diapirism and mud volcanism are remarkable yet often overlooked geological phenomena on Earth, documented across diverse terrestrial and marine environments globally [1,2,3]. These geological features are similar in formation mechanism, as both result from buoyant forces driving upward movement, yet they are distinguished by their progression and surface visibility. Mud diapirs remain confined to the subsurface and are unable to breach the overlying strata, whereas mud volcanoes eventually breach these layers and emerge visibly on the surface or seafloor, forming distinct geomorphological structures [4].

Diapirism is a geological process that primarily manifests in three forms: serpentinization, salt, and mud. According to Delgado [5], serpentinization can occur due to regional deformation, either at a constant temperature or with a thermal increase. This phenomenon is prominent in subduction and continental collision zones, where previously altered upper mantle rocks rise owing to their lower density and tectonic pressure. In addition to serpentinization, Benavides [6] noted that salt domes are intrusive salt structures that penetrate thick layers of sedimentary rocks. They are distinguished from other geological deformations associated with salt by their generally circular or elliptical shape in horizontal sections and horizontal dimensions of the same order of magnitude or smaller than their vertical dimensions [6].

The phenomenon of mud diapirism has been the subject of research and study in various regions worldwide and is crucial for understanding geological dynamics and the processes that shape the Earth’s crust. Wan [7] used physical models to simulate the formation of mud diapirs, and observed how pressure and sediment composition affect their development. Physical modeling is a fundamental tool in geology for replicating and studying natural processes in controlled environments [8]. Wan [7] states, “Physical simulation experiments provide valuable data that help decipher the underlying mechanisms in the formation of mud volcanoes and mud diapirs”. These models allow for the observation of fluid and sediment dynamics under specific conditions, providing crucial insights into the factors that influence the formation of these structures [8].

Globally, research has revealed the influence of mud diapirs on the formation of geological structures, exploration of natural resources, and evolution of sedimentary basins. For instance, countries like Mexico, the United States, the United Kingdom, and Iran exhibit this phenomenon. An illustrative example is the Gulf of Mexico, where pioneers such as Nettleton and Barton [9] conducted physical experiments to unravel the dynamic processes underlying the genesis of salt diapirism. They aimed to interpret the shapes and development patterns associated with this phenomenon in that region [9]. These research efforts have significantly contributed to our understanding of how mud diapirs interact with local geology and have had a lasting impact on how the exploration and exploitation of natural resources are approached worldwide.

In this context, for example, in Taiwan, the origin of mud diapirs and volcanoes is attributed to a combination of factors, including excessive accumulation of pressure in sedimentary layers, tectonic compressive forces, and the presence of gaseous fluids. Notably, gas-rich fluids play a crucial role in intensifying post-diapiric intrusion, clearly manifesting through significant gas emissions [10].

In Colombia, mud diapirism is a geological phenomenon characteristic of the Sinú Belt, a region of rock formations located along the Colombian coast between the Gulf of Urabá and the city of Barranquilla. It is characterized by the uplift and deformation of soft sediment layers due to fluid pressure. These deformations create dome-shaped structures and are influenced by factors such as sediment compaction, seismic activity, and erosion. It is essential to highlight that mud diapirism significantly impacts the topography and morphology of the coastal area [11]. The diapirs along the Colombian coast are primarily composed of pelagic and hemipelagic muds, which constitute the base of the sedimentary sequence as documented by Duque-Caro [12] in its biostratigraphic profiles. Mud Diapirism, driven by the weight of turbiditic sediments on lower-density pelagic and hemipelagic layers, has been identified as a crucial factor in the regional accretion process in the Sinú-San Jacinto terrain. This phenomenon induces discordant intrusions of deeper materials, leading to the deformation of the upper layers and the formation of diapiric structures. The interaction between diapirism and the weight of turbiditic sediments has resulted in the mobilization of underlying pelagic layers, contributing to the appearance of contorted anticlinal structures in the region. This process has been essential in the geological evolution of the area, demonstrating the complex relationship between diapirism, the accretion process, and the structural configuration of the Sinú-San Jacinto terrain in Colombia [12]

In this context, due to the differences in density, viscosity, and fluidity of the materials that emerge at varying intervals, simple orifices or vents of different sizes (ranging from 0.5 to 60 m) or cones (with heights between 0.6 and 20 m and base diameters from 1 to 30 m) can form. The mud sources or vents of this type of volcano are classified based on their shape and the slope of the cone flanks. These classifications include Type A, which represents cones with slopes greater than 20°; Type B, with slopes ranging from 5° to 20°; Type C, with slopes less than 5°; Type D, which refers to caldera-crater shapes with diameters of several meters; and Type O, which denotes depressions or craters only a few centimeters wide [13], Figure 1 illustrates these different types of mud sources.

A specific case that illustrates the importance of these studies is the mud volcanic eruption in the Membrillal area, located southwest of the city of Cartagena, in the department of Bolívar, Colombia. This region is characterized by the emission gases, predominantly methane, the predominant gas in this type of volcano. Trace amounts of ethane, propane, butane, and CO₂ can also be found, which, at high concentrations, can cause symptoms such as nausea, diarrhea, loss of appetite, disorientation, headache, excitement, rapid breathing, amnesia, and other effects on the central nervous system [15]. The Membrillal district experienced its first mud eruption in 2013, destroying a house. The Colombian Institute of Geology and Mining (INGEOMINAS) immediately intervened. Ten years after that initial eruption, Membrillal is experiencing a reactivation of this phenomenon, resulting in the collapse of several houses in the area. This recurrence highlights the need for constant monitoring and detailed analysis of diapiric activity in the region and control of anthropogenic effects that may accelerate the volcanism process.

Research on mud volcanoes in Cartagena’s Membrillal area is vital due to the geotechnical and environmental risks these phenomena pose to local communities and infrastructure. This study is fundamental because it contributes to scientific knowledge in the field of geology and geomorphology of Membrillal, providing new insights into how geological conditions may shift over time as a result of both natural processes and anthropogenic interventions, along with recommendations for the proper use of soils and conducting relevant studies for their evaluation in evolving conditions. In this context, the aim of the study is to analyze the spatiotemporal evolution of mud volcanoes present in the Membrillal sector using secondary information, geophysical, geotechnical, geomorphological, and physicochemical tests, in order to zone and identify areas with higher susceptibility to geological hazards, thereby generating an updated diagnosis of the phenomenon in the study area.

2. Experimental Area

Membrillal is a rural community located in Cartagena de Indias, with coordinates 10°20′4.82″ N and 75°28′33.41″ W. CARDIQUE [16] also conducted a socioeconomic assessment of Membrillal that highlights key indicators affecting the population impacted by mud diapirism. Seventy-three percent of households are owner-occupied, of which 79% are legally documented, facilitating relocation in the face of geological risks. Between 81% and 93% of residents have lawful access to basic utilities (electricity, potable water, natural gas, sewage, and waste collection), whereas 2% do so illegally, increasing sanitary vulnerabilities (Figure 2).

Only 26% of the working-age population earns enough to meet basic needs, reflecting economic precariousness. Although 99% report no disabilities, 1% have motor or physical impairments, underscoring the need for inclusive relocation plans. Approximately 32% of residents express willingness to move to a safer area, illustrating their risk perception concerning the phenomenon. These findings stress the urgency of implementing sustainable strategies and mitigation programs to address socioeconomic vulnerabilities and the geological threat, reinforcing the need for institutional intervention to ensure stability and an improved quality of life in the region.

2.1. Geological and Geomorphological Setting

The Colombian Caribbean region is characterized by the convergence of the Nazca, Caribbean, and South American tectonic plates, featuring non-magmatic processes and low seismicity [17]. The Caribbean plate migrates northeastward relative to both American plates, accommodated by right-lateral (dextral) and left-lateral (sinistral) strike-slip faults along its northern and southern margins, respectively. Oceanic lithosphere from the North and South American plates is being subducted beneath the Caribbean and Lesser Antilles zones. Meanwhile, the Cocos and Nazca plates subduct beneath the western and southwestern edges of the South American plate. Sedimentary units in the Colombian Caribbean Basin were deposited in marginal basins that lie between two distinct geological provinces: an oceanic basement to the west and a continental basement to the east [18]. Thus, this NW directed accretionary wedge result from eastwards, low-angle subduction of oceanic rocks and sediments belonging to the Caribbean plate under arc terranes along the northwest edge of the South American continent that belong to the South American plate.

The Sinú—San Jacinto Basin is located in the northwest corner of South America in the Caribbean coastal zone of Colombia. This geological province is controlled by trasnpressional regime caused by the NE migration of the Caribbean plate, which has been interacting with the NW margin of South America since the Creatceous, to the accretion of allochthonous terrains from the southwest. This process has defined, over geological time, the current geomorphological and lithostructural characteristics of the Colombian Caribbean plate [18,19,20,21]. This region has 85% flat to undulating relief and some low–elevation hills, associated with lowland coasts, and includes coastal plains, alluvial plains, salt flats and floodplains, mangrove plains, coastal lagoons, beaches and sandbanks, dune fields, platforms and reef bars, among other geomorphological features. The other 15% of the littoral corresponds to cliffs, where the Sierra Nevada de Santa Marta (SNSM) stands out, with the Cristóbal Colón and Simón Bolívar Peaks over 5700 masl, the serranías de Jarara, Macuira, Carpintero, and Cocinas in the La Guajira Peninsula; and the serranías de Abibe, Ayapel, Darién, and San Jerónimo in the southwestern sector. Mud diapirism and the alluvial delta of the Magdalena River are important geological features of this region [22].

The Sinú rocky belt, where mud volcanism is common, exhibits a lower degree of folding and uplift than the San Jacinto belt, and its lithification degree is relatively low. In the southwestern region of the Colombian Caribbean (Córdoba and Antioquia), the influence of diapiric processes on the current morphostructural configuration of the northwestern Colombian Caribbean is more evident. In this area, large symmetrical synclinal basins oriented in a northeast direction are commonly found, bordered by pressure ridges, narrow anticlines, and homoclinal ridges. These structures are associated with right-lateral strike-slip faults with reverse components, where manifestations of mud volcanism are often observed [23].

On the other hand, in the northern part of the Sinú belt (Cartagena-Barranquilla), the tectonic framework is characterized by the presence of tightly folded anticlines, homocli-nal ridges, and curved pressure ridges oriented in a NEE direction. Additionally, locally deformed synclinal folds are found, affected by faults in E-W and NW-SE directions, from the Dique Canal to the Galerazamba region. In this zone, convergence plate tectonics pre-dominates over diapiric processes [24].

The rocks of the San Jacinto belt, present in the study area southwest of Barranquilla and east of Cartagena, are characterized by highly folded and faulted terrains with elevations exceeding 650 m. These terrains, including the easternmost part of the Sinú belt, have been considered by Mantilla [20] as the fossil part of an accretionary prism, which behaves as the backstop of the active frontal accretionary sedimentary prism corresponding to the westernmost sector of the Sinú belt (Figure 3).

The block configuration oriented in a north-south direction controlled, during the Late Miocene to Pliocene, the wedging of sedimentary sequences in the eastern part of the Sinú belt. This configuration led to the formation of pop-up structures in the western part of the San Jacinto belt, particularly in the area between Santa Catalina and Campo de la Cruz, where a series of homoclinal ridges are bounded by right-lateral transpressive faulting [25].

This configuration generates differential regional uplifts and the formation of local basins bounded by active transpressive faults, especially in the central part of the Sinú belt. The tectonics, favored by mud diapirism, influences the scalloped configuration of the coast and the surrounding geomorphology, characterized by elevated synclinal ridges, tilting of reef platforms, formation of local marine terraces, and abrasion platforms at various heights. In addition, it gives rise to large bays accompanied by coastal lagoons [24] (Figure 4).

The Membrillal area, located within the Sinú Fold Belt, presents a complex geological configuration spanning from the Miocene to the Quaternary. The main stratigraphic units include the Arjona, Bayunca, and La Popa Formations, as well as unconsolidated Quaternary deposits. The Arjona Formation, of Miocene-Pliocene age, comprises siliceous and calcareous mudstones at its base, intercalated with chert, followed by dirty sandstones and mudstones, representing deep marine deposits that extend towards the eastern and southeastern parts of the study area. The Bayunca Formation, also of Miocene-Pliocene age, is characterized by alternating layers of fine sandstones and clayey siltstones containing mollusk fossils and erosion-resistant nodules, and correlates with the Tubará and Zambrano Formations. The La Popa Formation consists of a succession of sandstones, mudstones, conglomerates, and limestones deposited in a shallow epicontinental environment, with abundant fossils suggesting depths of less than 200 m. Finally, the Quaternary deposits are composed of unconsolidated alluvial and lacustrine sediments, such as gravels and clays, associated with fluvial activity from rivers and streams. These geological features, along with the presence of tectonic structures such as faults and folds, reflect the tectonic and sedimentary dynamics that have shaped the region over time [25,26,27].

The structural geology of the Membrillal community highlights multiple faults and tectonic lineaments that govern the area’s geodynamics. Particularly, faults are oriented N78°E, traversing sectors such as Zaragocilla, Buenos Aires, Las Sierras, Las Gaviotas. These faults exhibit normal displacement ranging from 30 to 50 cm, with subsidence occurring in the northern block.

Additionally, the northern sector of La Popa hill reveals a fault system oriented N60°W, extending to the Tesca swamp and closely linked to structural tilting and changes in bedding dip around Villa Rosita. In Membrillal and its surroundings, tectonic structures have facilitated minor fractures that act as escape pathways for muddy materials. Recent seismic records indicate ground uplift ranging from 30 to 70 cm, characterized by reverse-type movement in a northwest orientation, confirming active tectonic dynamics in the area [16] (Figure 5).

The geomorphology of Membrillal is equally complex, featuring structural and depositional units that reflect the interaction of tectonic and exogenous processes. Among the structural origin units, active mud volcanoes stand out, formed by the emission of gases and liquids mixed with fine sediments, which can generate explosive eruptions reaching up to 15 m in height. The estimated diameter of the mud diapirism in the area is approximately 0.8 km². Slopes are defined by the alignment of the terrain with the stratification of the bedrock, while structural backslopes have slopes perpendicular to the rock stratification, marked by differential erosion processes and the presence of scarps. Structural hills, on the other hand, are rounded, low-elevation landforms shaped by weathering and differential erosion. Regarding depositional origin units, the coastal plain is a low-altitude area formed by marine deposits and, in some cases, human and industrial activities. Low terraces and floodplains correspond to sedimentary areas of alluvial origin, composed of sand and clays, and are highly prone to flooding. These geomorphological characteristics reflect a dynamic landscape where the interaction of tectonic, sedimentary, and climatic processes gives rise to a variety of landforms that define the natural environment of the region [25,26,27] (Figure 6).

2.2. Historical Background of Mud Diapirism in Membrillal

The expressions of mud diapirism in the Membrillal area of Cartagena, Colombia, has been studied primarily through scientific investigations, geological hazard assessments, and the testimonies of local residents. The lack of a complete historical record of diapiric events in the area has made these sources essential in reconstructing the timeline and understanding the impact of the phenomenon on both the local environment and the population.

Key to this understanding has been the lack of extensive geological and geotechnical studies. One of the studies conducted by the Corporación Autónoma Regional del Canal del Dique, CARDIQUE [16], assessed the geological hazard posed by mud diapirism and identified areas where mud deposits are present at varying depths, as well as the distribution of mud vents and gas emissions. Using geophysical methods such as Electrical Resistivity Tomography, researchers were able to map the subsurface conditions and gain insights into the extent of diapiric activity. Combined with geotechnical data extracted with SPT, coupled with laboratory analysis of soil samples, this approach has proven critical in identifying the high-risk zones and understanding the geotechnical challenges posed by the expansive and highly plastic clays found in the region.

In addition to the previous study, official geological hazard assessments from institutions like INGEOMINAS [28] have provided the broader geological context of Membrillal mud diapirism. These assessments have established that the phenomenon is linked to tectonic activity along the Pasacaballos Reverse Fault, contributing to the formation of mud volcanoes. Similarly, Di Luccio et al. [29] found that mud volcanoes along Colombia’s Caribbean coast are closely tied to tectonic faults and significant hydrocarbon deposits. The study classified these volcanoes using a vulnerability index, showing that the nearby areas face substantial risk, exacerbated by both tectonic activity and human settlement patterns in high-risk zones.

In terms of infrastructure, the continuous subsidence caused by the upward movement of mud and gas has led to the deterioration of foundations and structures. Local government reports, such as those from the Office for Disaster Risk Management (OAGRD) [30,31,32,33,34,35,36], have documented the direct impact of these eruptions on housing. In 2021, OAGRD documented that over 30 homes were damaged, and around 100 people were displaced due to ongoing mud eruptions and the risk for further geological activity. These findings underline the vulnerability of the community, especially in regions like Membrillal, where the land is in a constant flux due to the interaction of tectonic forces and the extruding mud.

However, the implications of this geological activity extend beyond mere observation; they have significant consequences for the local infrastructure. The first notable diapiric event occurred on 19 August 2012, when an explosion from the diapir caused substantial damage to a house, necessitating immediate evacuation due to the associated risks. This event marked the beginning of a series of incidents affecting the housing stability in the area. On 1 August 2022, a technical assessment conducted as part of risk management efforts reported the collapse of a residential structure. Subsequently, on 12 October of that year, documentation indicated that 28 houses had sustained structural damage; this figure was later revised to 33 houses on 25 October 2022, highlighting the significant impact on structural integrity, including the deterioration of walls, beams, columns, and floor slabs. Due to the imminent risk of collapse, several of these residences were evacuated. By 9 November 2022, the situation had further deteriorated, with the number of affected houses rising to 64. As of now, the total number of impacted houses stands at 70, of which 15 have been evacuated, emphasizing the persistent challenges faced by residents in the Membrillal community.

Moreover, the study from Restrepo [37] about the Coastal Subsidence in Cartagena de Indias, highlighted that the proximity of the Membrillal mud volcano to local settlements further amplifies the potential risks. Studies have indicated that coastal regions like Membrillal, where sedimentary and mud diapirism is active, are increasingly vulnerable to not just seismic events but also to the rising sea levels associated with both tectonic subsidence and global climate change. Subsidence rates, as identified by GPS and InSAR data, show a concerning trend of vertical land motion, further compounding the risks to local populations. These subsidence rates, combined with projections of rising sea levels, suggest that the area could face compounded environmental hazards in the coming decades.

Given these insights, it is crucial for the authorities to continue monitoring these geological features and prioritize strategies that mitigate the risks to human life, environment, and infrastructure. This includes the implementation of more effective land-use planning, the relocation of at-risk communities, and the development of robust disaster response systems that can swiftly address the threats posed by mud diapirism and its associated hazards (Figure 7).

3. Methods

This study adopts a multidisciplinary approach to characterize the mud diapirism phenomenon in Membrillal, integrating geotechnical, geophysical, and physicochemical techniques. Methods such as Electrical Resistivity Tomography (ERT), the Standard Penetration Test (SPT), and analyses of soil’s physical and chemical properties were implemented to assess material distribution, ground strength, and mineralogical composition. This methodological combination provides a comprehensive view of soil behavior and its implications for regional stability. The resulting data enhance the understanding of active geological processes and offer essential information for risk mitigation related to these geological features.

The decision analysis for spatiotemporal and susceptibility analysis adopts elements from the previous Membrillal mud volcano study developed by CARDIQUE [16] from 2013 and observing its limitations (Figure 8).

A systematic review was performed to examine Membrillal mud diapirism status, searching on databases such as Scopus and ScienceDirect. Additionally, information requests were submitted to governmental agencies, including OAGRD (Oficina Asesora para la Gestión del Riesgo de Desastres), SGC (Servicio Geológico Colombiano), the Mayor’s Office of Cartagena, and academic databases from Caribbean regional universities to assess the current state of research in this field. Since there is no clear regulatory framework in Colombia for evaluating the hazard, risk, and susceptibility of mud volcanism, a susceptibility analysis was carried out using the collected data. Direct engagement with the community provided empirical data and practical insights of mud diapirism behavior through time.

3.1. Geological Analysis

The geological analysis in this study comprised two key components: a geophysical survey using geotomography and a detailed inventory of mud diapirism in Membrillal. The geophysical survey was conducted with a Terrameter LS2 system, providing detailed subsurface imaging of the study area and facilitating the identification of contrasts in geophysical properties. Meanwhile, the characterization and documentation of mud diapirism followed the methodology proposed by Higgins, enabling the systematic classification and recording of diapiric structures in the region. The integration of these approaches allowed for a comprehensive assessment of the phenomenon, yielding critical insights into its distribution, morphology, and geological dynamics.

3.1.1. Mud Diapirism Inventory in Membrillal

To undertake this component, exploration began in collaboration with the COMBAS group, composed of local community members. The main objective was to conduct a detailed characterization of the key points associated with diapirism in the study area. Fieldwork started at the principal dome on Calle de las Flores and extended to other known vents, allowing for the identification and classification of these features by type and current status. Each vent was assessed to determine whether it was active or inactive, as well as the type of fluids or gases emitted. This systematic approach enabled the creation of a detailed photographic record and precise GPS location of each structure. The resulting data were digitized using ArcGIS PRO software (Version 3.4) for geospatial analysis and visualization. This characterization was made in summer time (Figure 9).

3.1.2. Geophysical Data

A Terrameter LS2 unit was employed to acquire electrical resistivity data, chosen for its accuracy and user-friendly design. The collected data were initially processed using Terrameter ToolBox (Version 2.0.2.0) and then further refined with the specialized software Res2dInv (Version 5.02), created by Geotomo Software, based in Penang, Malaysia, where anomalous readings were removed and irregularities corrected to ensure the final models had errors below 5%. During interpretation, models exhibiting low resistivity were prioritized, as they suggest areas potentially containing mud and diapiric structures. The measurements were precisely georeferenced to guarantee accurate spatial positioning within the study area. This comprehensive methodology combined advanced technology with geophysical and geological approaches to address mud diapirism and support risk management in the community. Tomography surveys were conducted under moderately moist soil conditions due to recent rainfall, as shown in Figure 10.

3.2. Geotechnical Analysis

Geotechnical exploration in Membrillal involved both field tests and laboratory analyses to characterize the physical and mechanical properties of the soils in the study area. Initially, the Standard Penetration Test (SPT) was conducted according to ASTM D1586 (Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils), providing data on the resistance and compaction of subsurface strata. Subsequently, test pits were excavated throughout the study area to examine stratigraphy and ground conditions in situ. Laboratory testing further complemented these investigations by assessing parameters such as grain size distribution, Atterberg limits, moisture content, and density—among other relevant physical properties. This comprehensive approach enabled a detailed subsurface characterization and offered critical insights into the geotechnical behavior of the zone.

3.2.1. Standard Penetration Test (SPT)

The study was conducted based on SPT results [38]. A sampling strategy was designed to cover the study area effectively. Strategic locations for the boreholes were selected based on preliminary terrain analysis and the study’s specific objectives. A total of 18 boreholes were drilled at depths of 6 m and 8 m, distributed throughout the study area, and 4 pits were located as shown with dimensions of 1.5 × 1.5 × 1.5 m corresponding to height, length, and depth, as shown in Figure 11. A total of 49 samples from SPT and 4 samples from tests pits were collected.

Soil samples were extracted at 0.50 m intervals for the SPT and at a 1.5 m depth for the excavation pits to ensure adequate representation of the soil profile. The samples were placed in sealed containers that minimized any alterations capable of influencing test results. A detailed record was kept for each borehole, including the depth reached, a description of the encountered materials, and any relevant observations, thus enabling the correlation of test outcomes with local ground conditions.

The samples were subsequently transported to the Geotechnical Laboratory of the University of Cartagena under conditions designed to preserve their original properties, using packaging that prevented physical or chemical modifications. Once in the laboratory, the samples were cleaned, separated, and conditioned according to the specific requirements of each test, thereby maintaining their integrity for further analysis (Figure 11).

3.2.2. Laboratory Plan for Soil Samples

The laboratory tests were divided into groups to properly correlate soil data parameters. For SPT samples, the laboratory tests included were grain size analysis, Atterberg limits, specific gravity, granulometric analysis, and moisture content. For samples from the pits, tests included moisture content, grain size analysis, Atterberg limits, specific gravity, unconfined compressive strength, and pinhole test. Additionally, chemical tests were performed to assess the composition and potential contaminants present in the samples.

A semi-quantitative phase composition analysis by X-ray diffraction (XRD) and chemical composition determination by X-ray fluorescence (XRF) were conducted. Additionally, samples were analyzed with a Scanning Electron Microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) tests.

• Atterberg Limits (ASTM D4318)

This test followed the [39] standard for the 18 samples extracted from the standard penetration test and four boreholes. According to [39], the Atterberg limits test was used to determine the plastic and liquid properties of the soil. Each sample was prepared by passing it through a No. 40 sieve to remove oversized particles. For the liquid limit, the material was mixed with water to form a paste and placed in the Casagrande cup; a groove was then made, and the number of blows required to close the groove was recorded. This procedure was repeated at different moisture contents, and the results were plotted against the number of blows, enabling the determination of the moisture content at which 25 blows were needed to close the groove.

• Specific Gravity (ASTM D854)

The specific gravity of the soil material was determined in accordance with ASTM D854 [40] standard, which is designed to establish the relative density of solid soil particles. A dry, pulverized soil sample was placed in a pycnometer, which was partially filled with distilled water to remove air bubbles and then filled to top capacity, sealed, and weighed. The weights recorded included the pycnometer containing both water and soil (W3), the pycnometer filled only with water (W2), and the empty pycnometer (W1).

• Particle Size Distribution (ASTM D3740)

Granulometric analysis was performed in accordance with ASTM D3740 [41]. A soil sample was dried in an oven at 110 ± 5 °C, and a quantity of 500 to 1000 g was then weighed. The sample was vigorously washed with distilled water in a suitable container to remove fine particles, passing the suspension through a 0.075 mm (No. 200) sieve until the water ran clear. The retained material was dried again, weighed, and subsequently sieved through a series of sieves with openings ranging from 75 mm to 0.075 mm. The sample was placed in the sieve stack and shaken for 10 to 15 min. The mass retained on each sieve was recorded, and the percentages of retained and cumulative mass passing were then calculated.

• Moisture Content (ASTM 2216)

This test was conducted according to the ASTM 2216 [42] standard. A representative soil sample from each borehole was taken and weighed on a precision balance to obtain its initial mass. The sample was then dried in an oven at 105 ± 5 °C for 24 h.

• Unconfined Compression (ASTM 2166)

The unconfined compression test was conducted following the guidelines of ASTM D2166 [43] on soil samples extracted from the test pits. Cylindrical specimens were prepared, with initial height, diameter, and weight recorded. Each sample was then placed in a uniaxial compression machine and subjected to an increasing axial load until failure. Throughout the test, the applied load and corresponding deformation were recorded, allowing for the calculation of maximum compressive strength and axial strain.

• Pinhole Test (ASTM D4647)

The pinhole test was conducted on samples extracted from the SPT in accordance with ASTM D4647 [44]. Each cylindrical specimen was drilled with a 1 mm hole at its center and then connected to a water pressure source. Gradual loads of 5, 18, and 38 cm were applied for five minutes each, while the water flow rate and any signs of soil dispersion were closely monitored. The results were compared with ASTM guidelines to classify soil dispersivity, maintaining reproducibility and reliability by using distilled or deionized water throughout the test.

• Soil Expansiveness (Lambe Test)

The expansion test was conducted on samples extracted from the pit samples, following the guidelines of Lambe [45]. Each sample was placed in a cylindrical expansion mold, filled in three layers, and compacted. The initial volume of the mold was measured before being placed in an expansion chamber at a controlled temperature of 25 °C. The sample remained in the chamber for a specified period, typically 24 h, with expansion measurements taken at regular intervals. The total expansion was determined by comparing the initial and final volumes, providing a quantitative assessment of the soil’s expansion potential under controlled moisture conditions.

• Semi-quantitative phase composition analysis by X-ray diffraction (XRD);
  Chemical composition determination by X-ray fluorescence (XRF)

In this study, physicochemical analyses were performed on selected samples from the SPT.

Table 1. Main Parameters for the X-ray Diffraction (XRD) Analysis.

Parameter	Detail
Instrument	Malvern Panalytical X-ray Diffractometer 1
Detector	Pixel 3D
X-ray Source (λ = 1.541)	CU, at 45kv and 40 mA
Goniometer	Omega/2θ with the reflection Transmission Spinner (4-s rotation)
Step Size	0.02°
Time per Step	52 s
Semi-quantification Method	Rietveld method with High Score Plus software (Version 5.1)
Crystallographic Database	ICSD FIZ Karlsruhe 2012-1

¹ The enterprise product specifications.

summarizes the main parameters for the X-ray Diffraction (XRD) analysis, and

Table 2. Main Parameters for the X-ray Fluorescence (XRF) Analysis.

Parameter	Detail
Instrument	X-ray Fluorescence (XRF) spectrometer 1
Moisture Determination	Drying at 110 °C for 6 h
Loss on Ignition	800 °C for 2 h
Sample Preparation	Fusion method, analyzed using the WROXI (Version 1.0) quantitative application

¹ The general product specifications.

outlines the X-ray Fluorescence (XRF) methodology. These approaches provided detailed insights into the mineralogical and chemical composition of the soil, which are crucial for understanding the behavior of mud diapirism in the study area.

• Scanning Electron Microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) test

The microscopy analysis was performed at the MicroCore Microscopy Center of the University of the Andes (Bogotá, Colombia) using a JEOL JSM 6490-LV Scanning Electron Microscope (manufactured by JEOL Ltd. in Akishima, Tokyo, Japan.). This equipment enables the observation of samples at scales ranging from centimeters to nanometers in both high and low vacuum modes and allows for chemical analysis through energy-dispersive spectroscopy (EDS). Selected samples from the SPT, exhibiting relevant characteristics for the study, were sent to the laboratory for detailed analysis.

4. Results and Discussion

Following the completion of the laboratory tests under the relevant standards, detailed data on the mechanical and physical properties of the soils were collected. The tests included particle size distribution, Atterberg limits, permeability, expansion, compression, specific gravity, moisture content, unit weight, pinhole test, and soil classification, as shown in

Table 3. Laboratory results of soil samples recovered from the SPT.

Spt	Depth	Description	SPT (Blows/Foot) 1	Wn 2 (%)	Consistency Analysis			Passing N °4	Passing N °200	Clasification	Gs 10	γt 11	γd 12
	(m)				LL 3 (%)	PL 4 (%)	PI 5 (%)	(%)	(%)	SUCS 6		(kN/m3)	(kN/m3)
spt 1	4	Grayish brown clay	4	56.8	89	42	47	100	98.2	CL 7	2.75	15.1	9.6
	5	Gray clay with ash	4	59.5	87	42	45	100	98.2	CL	2.72	12.2	7.6
	8	Brownish gray clay	4	51.6	88	42	46	100	98.9	CL	2.73	12.7	8.3
spt 2	2	Grayish-brown clay	8	53	92	41	51	100	98.8	CL	2.74	12.1	7.9
	5	Grayish brown clay	7	50	90	42	48	100	99.3	CL	2.67	12.9	8.6
spt 3	3	Grayish brown clay	4	57.2	90	42	48	100	99.3	CL	2.75	11	7
	6	grayish brown clay	6	52.9	91	42	49	100	98.9	CL	2.74	12	7.8
spt 4	2	Yellowish brown clay	7	19.2	75	39	36	100	98.9	MH 8	2.73	-	-
	5	Light brown clay with gray streaks	18	28.9	89	43	46	100	98.7	CL	2.73	14.7	11.4
spt 5	6	Light brown clay	8	41.5	77	37	40	100	95.1	CH 9	2.75	13.6	9.6
spt 6	1.5	Yellowish brown clay with traces of gravel	9	26.7	49	29	20	97.4	78.5	ML	2.74	14.3	11.3
	4	Yellowish clay with gray streaks and oxide	10	48	93	46	47	100	97.3	CL	2.75	-	-
	6	Yellowish brown clay with gray streaks and red oxide	20	43.9	82	41	41	100	97.4	MH	2.74	-	-
spt 10	2	Light brown silty clay with gray streaks and oxide	4	38.8	67	39	28	100	96.2	MH	2.73	-	-
spt 14	5	light brown clay with gray streaks and oxide	3	40.5	83	36	47	100	96.5	CH	2.74	-	-
spt 17	3.5	Light brown clay with gray streaks	3	42.9	78	34	44	95.3	85.1	CH	2.72	-	-
spt 18	2	Grayish brown clay with streaks of oxide and gypsum	7	36.5	83	33	50	100	97.5	CH	2.74	-	-

¹ Number of blows required for the spoon to penetrate one foot into the ground, reflecting its resistance. ² Moisture Content. ³ Liquid Limit. ⁴ Plastic Limit. ⁵ Plasticity index. ⁶ Unified Soil Classification System. ⁷ Low Plasticity Clay. ⁸ High Plasticity Silt. ⁹ High Plasticity Clay. ¹⁰ Specific gravity. ¹¹ Total density. ¹² Dry density.

and

Table 4. Laboratory results of the soil samples recovered from the test pits.

Test Pits	Depth (m)	Description	Soil Sample	Wn 1 (%)	Consistency Analysis			Passing N °4	Passing N °200	Clasification	Gs 6
					LL 2 (%)	PL 3 (%)	PI 4 (%)	(%)	(%)	SUCS 5
Test pit 1	1.5	Yellowish-brown clay with gray streaks	1	89	59	26	33	100	92.3	CL 7	2.75
Test pit 2	1.5	Dark brown clay	1	50.3	57	25	32	100	91.6	CH 8	2.75
Test pit 3	0.8	Dark brown silty clay with traces of gravel	1	15.8	49	24	25	98	79.6	CL	2.72
Test pit 4	1.5	Yellowish-brown clay with limestone fragments	1	23	54	25	29	100	87.8	CH	2.74

¹ Moisture Content. ² Liquid Limit. ³ Plastic Limit. ⁴ Plasticity index. ⁵ Unified Soil Classification System. ⁶ Specific gravity. ⁷ Low Plasticity Clay. ⁸ High Plasticity Clay.

4.1. SPT and Test Pits

SPT and test pits results are summarized as shown in Table 3 and Table 4 respectively.

According to the results obtained in Table 3, soil composition variability is evident, with a predominance of clayey soils interspersed with layers of ash, oxides, and organic matter. This heterogeneity can be attributed to complex geological processes, including mud diapirs and possible historical volcanic activity, which have influenced the original soil properties. The presence of these characteristics suggests a geological formation modified by volcanic activity, necessitating a detailed layer-by-layer analysis to assess its behavior and suitability for construction.

The liquid limits (LL) and plastic limits (PL) exhibit significant variability, with values indicating high plasticity. LL ranges from 44% to 93%, while PL varies between 14% and 46%, reflecting high plasticity in most soils. This suggests the presence of expansion and contraction issues, common in areas affected by diapirism and volcanic activity. The plasticity index (PI), ranging from 20% to 52%, also indicates a wide spectrum of plasticity, from medium to high. High plasticity may impact structural stability, particularly under varying moisture conditions.

The classification according to the Unified Soil Classification System (USCS) classifies most samples into subgroups exhibiting behavior from moderate to poor performance. This classification aligns with the evidence of highly expansive soils with variable strength. The low to medium plasticity clays (CL) and high plasticity clays (CH), along with some silty clays (MH), present additional challenges in terms of design and structural stability in the study area. High plasticity can lead to expansion and contraction issues, which must be accounted for in foundation engineering.

Soil strength, evaluated through the Standard Penetration Test (SPT), exhibits a broad range of values, from soft to low-strength soils (values below 8 blows per foot) to denser soils (values above 18 blows per foot). This variability reinforces the need for meticulous foundation design, as areas with soft soils may pose stability concerns under structural loads. The specific gravity (Gs) ranges from 2.6 to 2.8, corresponding to clayey soils and aligning with observed values. The obtained data are crucial for understanding soil behavior under varying conditions and loads, as well as for the effective planning of construction projects and geological studies related to mud diapirism.

Table 5. Description of soil samples from the CARDIQUE [16] study.

Spt	Soil Sample	Depth (m)	Description
Spt 1	1	1.00–3.50	Dark gray silty clay with oxide streaks (Mud)
	3	5.00–6.50	Mud (Light gray silty clay with clumps)
Spt 2	3	5.00–6.50	Dark brown clay with gray streaks and oxide
	5	8.50–10.00	Dark Havana clay with oxide streaks
Spt 3	1	1.00–3.50	Light gray clay with gray streaks and oxide

presents the description of soil samples from the CARDIQUE [16] study.

The results presented in

Table 6. Laboratory results obtained from the CARDIQUE [16] study.

Spt	Soil Sample	Wn 1 (%)	Consistency Analysis			(Blows/Foot) 5 (Golpes/pie) 1 (pie)	Γt 6 (ton/m3)	Γd 7 (ton/m3)	Gs 8	OM 9 (%)	Passing		AASHTO 10	SUCS 11
			LL 2 (%)	PL 3 (%)	PI 4 (%)						N° 4 (%)	N° 200 (%)
Spt 1	1	25.8	79	30	49	1	1.62	1.29	2.59	10.5	100	98.3	A-7	CH 12
	3	69.5	82	31	51	4	1.84	1.09	-	-	100	98.5	A-7	CH
Spt 2	3	42.9	59	25	34	4	1.84	1.29	2.75	3	100	96.9	A-7	CH
	5	66.1	68	26	42	5	1.88	1.13	2.75	-	100	97.9	A-7	CH
Spt 3	1	66.9	78	29	49	1	1.62	0.97	2.59	8.5	100	97.9	A-7	CH

¹ Moisture Content. ² Liquid Limit. ³ Plastic Limit. ⁴ Plasticity index. ⁵ Number of blows required for the spoon to penetrate one foot into the ground, reflecting its resistance. ⁶ Total density. ⁷ Dry density. ⁸ Specific gravity. ⁹ Organic matter content. ¹⁰ American Association of State Highway and Transportation Officials. ¹¹ Unified Soil Classification System. ¹² High Plasticity Clay.

confirm the findings of the Standard Penetration Test (SPT). Test pit 1 exhibits a natural moisture content of 89%, whereas test pit 2 registers 50.3%, indicating potential saturation or significant water content influenced by soil composition. Test pit 3 has a natural moisture content of 15.8%, suggesting low water retention, while test pit 4 presents 23%, implying drier and potentially more stable soil conditions. These moisture variations correlate with the plasticity observed in the SPT results, reinforcing differences in water retention capacity and their impact on soil stability.

The consistency limits, including the liquid limit (LL), plastic limit (PL), and plasticity index (PI), provide key insights into soil plasticity. Test pit 1 has an LL of 59%, a PL of 26%, and a PI of 33%, indicating moderately plastic clay. Test pit 2 exhibits an LL of 57%, a PL of 25%, and a PI of 32%, also reflecting considerable plasticity. Test pit 4, with an LL of 49%, a PL of 24%, and a PI of 25%, suggests a clay of lower plasticity compared to the previous ones. Meanwhile, test pit 5 shows an LL of 54%, a PL of 25%, and a PI of 29%, indicating moderate plasticity. These findings are consistent with the SPT values, confirming a broad plasticity range in the soils of the study area.

Particle size distribution, measured by the percentage of material passing through sieves No. 4 and No. 200, provides essential information about soil texture and composition. Test pits 1, 2, and 4 exhibit 100% passing through sieve No. 4, indicating the absence of coarse particles. In contrast, test pit 3 shows 98% passing through the same sieve, suggesting a minor presence of coarser particles such as gravel. Regarding sieve No. 200, test pit 1 contains 92.3% fine material, test pit 2 has 91.6%, test pit 4 has 79.6%, and test pit 5 has 87.8%. These values indicate that all tested soils are predominantly fine-grained, with a high content of clays and silts. These findings align with the Standard Penetration Test (SPT) results, which reveal a wide range of strength and plasticity in these fine-grained soils.

The soil classification based on the Unified Soil Classification System (USCS) provides a standardized categorization according to soil properties. Test pits 1 and 3 are classified as CL (low-plasticity clay), indicating moderate expansion and contraction potential due to moisture variations. In contrast, test pits 2 and 4 are classified as CH (high-plasticity clay), suggesting a higher susceptibility to significant volumetric changes with moisture fluctuations, which may impact soil stability. This classification is consistent with the SPT results, which indicate high plasticity and variable strength.

Additionally, the expansion tests conducted on test pit 1 indicate an expansion pressure of 1.95 kg/cm², suggesting a significant potential for soil expansion when subjected to moisture variations. This result is consistent with the high moisture content, indicating a high-water retention capacity, a characteristic typical of expansive soils. The Atterberg limits also suggest that the soil is a clay with moderate to high plasticity, which is indicative of expansive behavior, despite being classified as CL (low-plasticity clay). While this classification suggests low to moderate plasticity, the consistency limits and high moisture content indicate that the soil may exhibit expansive behavior under specific moisture conditions.

In the study conducted by CARDIQUE [16] in 2013, boreholes were drilled to a depth of 10 m, with representative samples analyzed from three boreholes, as detailed in Table 5 and Table 6 of the referenced study. In contrast, the present study has conducted a total of 18 boreholes—17 drilled to a depth of 6 m and one additional borehole reaching 8 m. This methodology enables a more extensive and detailed assessment of the geotechnical and mechanical properties of the soil within the study area, providing updated and expanded data compared to those obtained in the previous study.

From the above, it is evident that the results of this study, summarized in Table 5 and Table 6 show a predominant composition of dark gray silty clays with oxide veins. The liquid limit (LL) values in the CARDIQUE [16] study ranged from 68% to 82%, while the plastic limit (PL) varied from 25% to 31%. The moisture content in that study fluctuated between 25.8% and 69.5%. When comparing these results with those obtained in the present research, it is observed that the LL and PL values in the more recent study tend to be higher, indicating greater plasticity in the current soils. Similarly, the plasticity index (PI) also shows consistent trends. In the CARDIQUE [16] study, the PI ranged from 34% to 51%, reflecting a wide range of medium to high plasticity. This study, in turn, reports PI values that indicate highly plastic soils.

This increase in LL and PL values in current soils, compared to those reported in 2013, suggests a greater capacity for deformation and expansion under moisture conditions. Such an increase in plasticity could indicate higher diapiric activity in the region. The movements and pressures generated by the upward migration of muds can alter the soil composition and structure, increasing its plasticity. This may reflect changes in the composition of the diapiric material; if the emerging muds contain a higher proportion of expansive clays or fine-grained materials, this could explain the higher LL and PL values observed.

Regarding soil classification, both studies show consistency. In 2013, CARDIQUE classified the soils primarily as high-plasticity clays (CH). The present study maintains this classification, indicating that the soils are predominantly low to medium-plasticity clays (CL), some high-plasticity clays (CH), and silty soils (MH).

Beyond these findings, it is relevant to consider potential external factors that may have influenced the differences observed between the 2013 study and the present one. Climatic variations, land-use changes, and human activities, such as construction, may have altered ground conditions, contributing to the increased plasticity and variability of the current soils. Additionally, investigating tectonic and volcanic activity in the region is crucial, as these processes can significantly influence soil dynamics and composition over time.

4.2. Results of Mud and Gas Emission Vents Detected

This classification was made according to Higgins and Saunders [46], as shown in

Table 7. Mud and Gas Emission Vents.

N	West Coordinate	East Coordinate	Seep/Vent	Vent Type	Sediment Type	Inactive/Active	Fluids/Gases	Altitude	Slope
1	75° 28′ 35.69″ W	10° 19′ 58.75″ N	Dome	O	Clay	Inactive	None	56	12.34°
2	75° 28′ 35.87″ W	10° 19′ 58.84″ N	Vent	B	Clay	Active	Mud	55	11.86°
3	75° 28′ 35.87″ W	10° 19′ 58.84″ N	Vent	B	Clay	Active	Mud	55	11.86°
4	75° 28′ 35.99″ W	10° 19′ 57.26″ N	Seep	-	Clay	Active	Gas, fluid, and mud	54	4.49°
5	75° 28′ 35.99″ W	10° 19′ 57.26″ N	Seep	-	Clay	Active	Fluid	54	4.49°
6	75° 28′ 37.64″ W	10° 19′ 58.09″ N	Seep	-	Clay	Active	None	51	6.68°
7	75° 28′ 34.52″ W	10° 19′ 57.24″ N	Boca	O	Clay	Inactive	None	57	7.74°
8	75° 28′ 33.31″ W	10° 19′ 59.68″ N	Seep	-	Clay	Inactive	None	55	2.35°
9	75° 28′ 33.31″ W	10° 19′ 57.24″ N	Seep	-	Clay	Active	Fluid	55	3.0°
10	75° 28′ 29.86″ W	10° 20′ 2.24″ N	Vent	O	Clay	Active	Gas, fluid, and mud	54	2.32°

Note: It is important to highlight that the field visits for the inventory of mud and gas emission vents were conducted during the rainy season.

.

In the study conducted by CARDIQUE in 2013, a classification of mud and gas seeps in the study area was carried out, as shown in

Table 8. Mud and Gas emission zone identified by CARDIQUE 2013.

Identified Zone	Vent Number	Reference Site	Flat Coordinates
			North	East	Altitude (m)
Emission Zone No. 1	1.1	Household block I	1,634,746.26	846,786.77	65.044
	1.2	Football Field	1,634,802.28	846,830.46	63.998
Emission Zone No. 2	2.1	Household block O	1,634,890.32	846,921.65	60.431

.

When comparing the three mud emission vents identified by CARDIQUE in 2013 with those found in the present study, a significant increase in the diapirism phenomenon is observed. In the current study, nine mud emanations have been identified, including the three previously described by CARDIQUE, along with six additional ones, in addition to the main dome. It has been noted that the height of the emission vents identified by CARDIQUE differs from the current height of the same vents. This discrepancy is likely due to landslides that have occurred in the lateral area of the emission field. According to CARDIQUE’s study, the vents in question had a diameter of 30 cm. Currently, these vents have evolved into fissures or small cracks in the ground, which emit mud at certain times of the year.

Additionally, vent 2.1, identified as number 10 in the present study, has experienced an increase in diameter from 20 cm to 80 cm. This increase may be attributed to several interrelated factors, such as an increase in diapiric activity that raises pressure in the subsurface, changes in the composition of the mud that promote greater erosion, and the erosion or collapse of the surrounding terrain. It may also be related to an increase in the frequency of emanations, local geological changes, and interactions with vegetation or rainwater.

4.3. Results from Electrical Resistivity Tomography

After an exhaustive processing of the data, in which corrections were made to a few anomalous points typical of this type of study, different resistivity inversion models were obtained at the sites of the TRE lines illustrated in Figure 10.

Figure 10, with their starting and ending coordinates described in

Table 9. Star/end coordinates of ERT Lines.

Line	Starting Latitude	Starting Longitude	Ending Latitude	Ending Longitude
ERT 1	10°19′58.34″ N	75°28′34.83″ O	10°19′58.76″ N	75°28′36.66″ O
ERT 2	10°20′3.24″ N	75°28′37.27″ O	10°19′58.14″ N	75°28′37.71″ O
ERT 3	10°19′58.34″ N	75°28′34.84″ O	10°19′58.79″ N	75°28′36.67″ O
ERT 4	10°19′56.86″ N	75°28′33.22″ O	10°19′59.89″ N	75°28′32.76″ O
ERT 5	10°19′57.84″ N	75°28′33.55″ O	10°19′57.35″ N	75°28′31.74″ O

. As a quality criterion, only models with an error of less than 5% were considered for the analysis.

In this study, for the mud contour values, the colors used in these specific models correspond to the areas represented by blue, with darker shades indicating higher values. The average resistivity values correspond to soils with a high presence of clays, which are abundant in the area and can be observed from the surface. Finally, the areas with the highest resistivity, which are mostly found in the superficial zones, correspond to anthropogenic fills.

In line 1 (see Figure 12), areas indicative of potential mud presence is identified between 16 m and 60 m (relative to the start of the line) at depths of approximately 5 m. A significant zone is also noted between 98 m and 130 m (relative to the start of the line), reaching depths of up to 8 m.

In line 2 (see Figure 13), areas indicating potential mud presence are observed throughout the entire resistivity model. In this case, the mud zones reach depths of up to 10 m in specific locations.

In line 3 (see Figure 14), a pronounced concentration of mud is identified in the dome region, along with additional smaller areas suggesting potential mud presence. This may have contributed to the structural failure of the dwelling previously located in this area. In this instance, the mud is situated at shallower depths ranging from 1.3 m to 4 m.

In line 4 (see Figure 15), the survey was conducted over the community soccer field. According to testimonies from residents, this site was previously characterized by significant mud presence before its conversion into a soccer field. The resistivity survey results corroborate these observations, indicating extensive zones with a high probability of mud presence at depths of less than 2 m.

In line 5 (see Figure 16), the survey was conducted over the soccer field, specifically in the area reported by the community to have a higher incidence of mud and associated material landslides. In alignment with these community observations, a significant zone with a high probability of mud presence was identified, extending to depths of up to 6 m. Additionally, in the most distal section of the resistivity survey, a zone of elevated resistivity was detected, indicative of anthropogenic fill, which the community has indicated was implemented as part of repairs to the soccer field.

4.4. Scanning Electron Microscopy (SEM) Analysis

It provided high-resolution insight into the morphological characteristics of the particles, along with their distribution and surface composition. These analyses delivered critical data on the texture and structure of the clays present, enhancing the geotechnical study with microscopic-level details.

In Figure 17a, crystals observed primarily correspond to gypsum, a hydrated calcium sulfate. The high concentration of oxygen and sulfur (Figure 18a) is consistent with the chemical formula of gypsum, where oxygen is associated both with sulfate and water, while sulfur is found in the sulfate group. The presence of calcium is also a key component of gypsum, acting as the main cation in the mineral structure. Mud diapirs are primarily composed of clays, salts, and other minerals. The notable presence of gypsum indicates that diapiric processes could be interacting with or altering deposits such as gypsum, suggesting that these formations could be a relevant part of the material transported by the diapirs.

This slightly contrasts with the Sakhalin study [47], where sulfides such as pyrite and marcasite were the predominant minerals rather than sulfates like gypsum. Nevertheless, both environments appear to be heavily influenced by the mobilization of minerals through diapiric processes. Similarly, although large amounts of gypsum were not identified in the Kerch-Taman study [48], other sulfates like barite and celestine were found. This suggests that gypsum in the samples may be related to a different precipitation environment, where diapiric processes favored its formation.

In Figure 18b and Figure 17b, the EDS analysis and SEM image of the sample indicate a composition rich in phyllosilicates, such as kaolinite, which are typical minerals in clays. These minerals are essential in mud diapirism processes, as their plasticity and ability to flow facilitate the ascent of materials from deep levels to the surface. Additionally, the composition shows significant amounts of carbonates, represented by calcium and carbon, which are common in marine or sedimentary environments. Finally, minor elements such as sodium, magnesium, and sulfur indicate the presence of salts and sulfates, characteristic of sedimentary environments rich in fluids. This type of composition is common in regions where mud diapirism mobilizes salt-laden materials from significant depths

Additionally, the Sakhalin study [47] observed silicates along with carbonates such as siderite. Both investigations emphasize the relevance of clay minerals and carbonates in the mudflow processes, highlighting that these formations facilitate the vertical movement of material through geological strata. This mineralogical profile is quite similar to the findings of the Kerch-Taman study [48], where phyllosilicates, such as kaolinite and illite, along with carbonates and salts, were also identified, contributing to the mobilization of materials from depth.

In Figure 17c and Figure 18c, show the results indicate a composition mainly of oxides (52.84% O) and silicon (23.19% Si), suggesting the presence of clay minerals and silicates, typical in clayey soil environments. Aluminum (6.74%) and iron (6.35%) reinforce the hypothesis of minerals such as kaolinite or illite, which are common in soils with high clay content. The presence of carbon (7.07%) may be related to organic matter or contaminants, while minor elements such as sodium (0.97%), magnesium (1.29%), and potassium (1.56%) could originate from secondary minerals. In the context of the research, this mineralogical composition suggests that the analyzed soil has high plasticity and water retention capacity, which supports the results obtained from laboratory tests. These are typical characteristics of soils prone to deformation and vertical movements, key factors in diapiric processes.

On the other hand, studies in Sakhalin [47] also reported similar clay minerals, confirming that both environments share properties that facilitate the movement of mud. However, samples show a higher content of silicon oxides. This profile matches the mineral characteristics observed in the Kerch-Taman study [48], where high levels of silicates were also found in clay-rich soils, which exhibited high plasticity and significant water retention capacity.

In Figure 17d and Figure 18d, show the SEM analysis of the sample reveals an elemental composition predominantly composed of oxygen (51.44%), calcium (31.54%), and carbon (14.87%), with a lower presence of silicon (2.08%). The high oxygen concentration is consistent with the presence of clay minerals and carbonates. The elevated proportion of calcium may indicate a significant abundance of calcite or dolomite, suggesting that the soil contains a notable calcareous component, which could be related to the diapiric activity in the region. The low amount of silicon indicates that silicate minerals, typically associated with clays such as kaolinite, are less dominant than carbonates. This type of composition may influence the plasticity and mechanical behavior of the soil.

The Sakhalin study [47] also found carbonates to be prominent, with siderite standing out as a key mineral. This suggests that carbonates are a common feature in these diapiric environments, though the proportions of present minerals vary. This composition aligns with the findings of the Kerch-Taman study [48], where abundant authigenic carbonates were identified, influencing the plasticity and mechanical behavior of the soil.

In Figure 17e and Figure 18e, this chemical profile suggests that the soil has a composition rich in clay minerals, sulfides, and carbonates, which are common in areas of diapiric activity, where the mobilization of sediments and fluids influences mineralogy. The significant presence of sulfur and calcium may be related to chemical alteration processes and the precipitation of secondary minerals in a reducing environment, which is characteristic of regions with volcanic mud activity.

This type of mineral composition is consistent with findings from Sakhalin [47], where sulfides such as pyrite and sphalerite are common in these environments. In both cases, the interaction of volcanic fluids with sediments seems to play a key role in the observed mineralogy, as chemical alteration processes favor the formation of new minerals. These findings are comparable to those of the Kerch-Taman study [48], which also identified sulfides alongside clays and carbonates, indicating mineralogy influenced by the mobilization of materials and fluids from deep layers.

4.5. Semi-Quantitative Phase Composition Analysis by X-Ray Diffraction (XRD) and Chemical Composition Determination by X-Ray Fluorescence (XRF)

Table 10. Sample Information—Borehole 12.

Identification	Borehole 12	Moisture (%) w/w	31.7
Preparation	Perla	Loss on Ignition (%) w/w (L.O.I)	4.5

presents the information of borehole 12. This laboratory test was executed with Borehole 12 (Figure 19) and Borehole 17 (Figure 20) samples and the results are presented in

Table 11. Result of chemical composition determination by XRF—Borehole 12.

Name	Compound	Composition (%) w/w
Silicon (IV) Oxide	SiO2	57.92
Titanium Dioxide	TiO2	0.89
Aluminum oxide	Al2O3	17.82
Iron (III) oxide	Fe2O3	8.32
Manganese (III) oxide	Mn3O4	0.13
Magnesium oxide	MgO	2.93
Calcium oxide	CaO	1.92
Sodium oxide	Na2O	1.41
Potassium oxide	K2O	2.66
Phosphorus pentoxide	P2O5	0.13
Sulfur trioxide	SO3	0.92
Vanadium pentoxide	V2O5	0.15
Chromium (III) oxide	Cr2O3	0.07
Nickel (II) oxide	NiO	ND
Copper (II) oxide	CuO	0.06
Zinc oxide	ZnO	0.08
Strontium oxide	SrO	0.06
Barium oxide	BaO	ND
Hafnium dioxide	HfO2	ND
Lead (II) oxide	PbO	ND
Zirconium dioxide	ZrO2	0.04
Loss on Ignition	L.O.I.	4.50

,

Table 12. Results of the semi-quantitative phase composition analysis by XRD—Borehole 12.

Score	Name	Composition (%)	Chemical Formula
26	Low Quartz	38.3	SiO2
11	Biotite	2.6	H2Al1.996Fe2.554KO12Si2.45
15	Kaolinite 1A	10.1	H4Al2O9Si2
18	Illite 2M1	29.9	H3Al4KO12Si2
17	Coesite	3.6	SiO2
11	Cristobalite	0.8	SiO2
14	Anorthite	14.7	Al2Ca0.71Na0.25O8Si2

,

Table 13. Sample Information—Borehole 17.

Identification	Borehole 17	Moisture (%) w/w	35.6
Preparation	Perla	Loss on Ignition (%) w/w (L.O.I)	16.5

,

Table 14. Result of chemical composition determination by XRF—Borehole 17.

Name	Compound	Composition (%) w/w
Silicon dioxide	SiO2	40.51
Titanium dioxide	TiO2	0.46
Aluminum oxide	Al2O3	9.99
Iron (III) oxide	Fe2O3	4.79
Manganese (III) oxide	Mn3O4	0.13
Magnesium oxide	MgO	1.98
Calcium oxide	CaO	21.61
Sodium oxide	Na2O	1.17
Potassium oxide	K2O	1.56
Phosphorus pentoxide	P2O5	0.14
Sulfur trioxide	SO3	0.82
Vanadium pentoxide	V2O5	ND
Chromium (III) oxide	Cr2O3	ND
Nickel (II) oxide	NiO	ND
Copper (II) oxide	CuO	0.05
Zinc oxide	ZnO	0.07
Strontium oxide	SrO	0.11
Barium oxide	BaO	0.09
Hafnium dioxide	HfO2	ND
Lead (II) oxide	PbO	ND
Zirconium dioxide	ZrO2	0.04
Loss on Ignition	L.O.I.	16.5

and

Table 15. Results of the semi-quantitative phase composition analysis by XRD—Borehole 17.

Score	Name	Composition (%)	Chemical Formula
27	Low Quartz	28.8	SiO2
28	Calcite	45.9	CaCO3
20	Illite 2M1	16.5	H3Al4KO12Si2
14	Anorthite	8.9	Al2Ca0.71Na0.25O8Si2

.

5. Conclusions

Standard Penetration Tests (SPT) and Test Pits

• The geotechnical characterization has identified the presence of expansive clays, whose volumetric behavior varies significantly with changes in moisture, affecting ground stability and increasing the risk of landslides. Standard Penetration Tests (SPT) are essential for evaluating soil resistance and load-bearing capacity, which are key aspects of infrastructure planning and risk assessment in areas affected by mud diapirism.
• Laboratory tests such as granulometric analysis and Atterberg limits have provided detailed information on the physical and mechanical properties of the soil, essential for modeling its behavior under different loading and hydration conditions.
• A high heterogeneity in soil composition is evident, with predominant clays interbedded with ash, oxide, and organic matter. The liquid limit (LL) and plastic limit (PL) values range from 44% to 93% and 14% to 46%, respectively, reflecting high plasticity and potential problems of expansion and shrinkage.
• The SUCS classification indicates soils with variable behavior, ranging from low-plasticity clays (CL) to highly plastic clays (CH) and silty clays (MH), posing challenges for structural design.
• Natural moisture presents significant variability: test pit 1 records 89%, while test pit 3 records only 15.8%, reflecting differences in water retention and its impact on soil stability.
• Specific gravity values range between 1.72 and 2.75, indicating differences in density and mineralogical composition of the material.
• Expansion tests show high expansion pressure in one of the test pits, confirming a significant volumetric deformation potential due to moisture changes, consistent with its high content of expansive clays.
• The comparison with the 2013 CARDIQUE study reveals a less detailed assessment at that time, emphasizing the importance of data updates for more precise geotechnical characterization.

Inventory and Evolution of Mud Vents

• The geological characterization has revealed a significant increase in the activity and distribution of mud diapirism over time.
• The 2013 inventory identified three mud vents, while the present study has recorded ten emissions, including six new ones and the main dome.
• The diameter of the vents has increased, reflecting an evolution in diapiric activity. For example, vent 10 increased its diameter from 20 cm to 80 cm.

Electrical Tomography

• Electrical tomography has identified low-resistivity zones associated with the presence of mud and diapiric structures.
• This method has facilitated the mapping of mud diapirism distribution, revealing depths ranging from 1.30 m to 10 m.
• The integration of tomography with other geological analysis methods has improved the understanding of the phenomenon and its temporal evolution.

Scanning Electron Microscopy and Chemical Analysis

• Scanning Electron Microscopy has allowed the characterization of the mineralogical composition of the clays, a fundamental aspect for evaluating their influence on soil stability and diapiric dynamics.
• In the Membrillal region, chemical analysis reveals a majority composition of oxides, with 52.84% oxygen and 23.19% silicon, suggesting the presence of clay minerals such as kaolinite and illite.
• The presence of aluminum (6.74%) and iron (6.35%) reinforces the hypothesis of the formation of these minerals, while carbon (7.07%) and other minor elements such as magnesium and calcium indicate a high-water retention capacity.
• These characteristics directly affect soil stability, generating deformations and vertical movements associated with mud diapirism.

Future Research Directions

• Implementation of geotechnical and satellite sensors for real-time monitoring of diapiric activity.
• Integration of advanced geophysical techniques (refraction seismics, shear waves) to improve subsurface characterization.
• Evaluation of the influence of climate variability on the stability of highly plastic soils.
• Use of advanced mineralogical techniques (XRD, infrared spectroscopy) for a more detailed analysis of soil composition.
• Development of innovative methods for stabilizing expansive soils to enhance the feasibility of infrastructure projects.
• Environmental impact studies and the relationship between climate change and the evolution of mud diapirism.
• Creation of an interinstitutional database on mud diapirism for comparative analyses and geotechnical risk modeling.