Application of Multiple Geophysical Exploration Methods in the Exploration of Marine Sand Resources in the Northern Offshore Waters of the South China Sea

Abstract

Marine sand, in addition to oil and gas resources, is the second-largest marine mineral resource. The rational development and utilization of marine sand resources are conducive to the growth of the marine economy. In the process of marketing marine sand in China, local authorities are required to delineate auctioned sand mining areas after a general survey, commonly referred to as preliminary exploration. Marine sand can be categorized into surface marine sand and buried marine sand. Buried marine sand deposits are buried beneath the sea floor, making it challenging to locate them due to their thin thickness. Consequently, there exist numerous technical difficulties associated with marine sand exploration. We conducted the preliminary research work in the waters off Guangdong Province of the South China Sea, employing a reduced drilling and identifying a potentially extensive deposit of marine sand ore. In this study, various geophysical methods such as sub-bottom profile survey, single-channel seismic survey, and drilling engineering were employed in the northern offshore waters of the South China Sea. As a result, two distinct marine sand bodies were delineated within the study area. Additionally, five reflective interfaces (R₁, R₂, R₃, R₄, and R₅) were identified from top to bottom. These interfaces can be divided into five seismic sequences: A₁, B₁, C₁, D₁, and E₁, respectively. Three sets of strata were recognized: the Holocene Marine facies sediment layer (Q₄^m), the Pleistocene alluvial and pluvial facies sediment layer (Q₃^al+pl), as well as the Pleistocene Marine facies sedimentary layer (Q₃^m). In total, two placers containing marine sand have been discovered during this study. We estimated the volume of marine sand and achieved highly favorable results of the concept that we are proposing a geologic exploration approach that does not involve any previous outcropping analogue study.

1. Introduction

Marine sand is a significant marine solid ore resource and the second-largest marine mineral resource after oil and gas resources [1,2]. It serves as a crucial natural sand source [3]. Marine sand can be categorized into surface marine sand and buried marine sand [4]. Surface marine sand refers to the marine sand directly exposed on the seabed surface, which is considered an “active sand source”. On the other hand, buried marine sand is located below the sea surface and covered with a certain thickness of other sediment, making it a “stable sand source” [4]. The demand for sand is experiencing exponential growth due to the expansion of construction activities and transportation infrastructure development [5,6,7]. Finkl et al. (2022) have reported that Barrier island restoration in coastal Louisiana requires large volumes of sediments [8]. All nations worldwide are engaged in extensive research and development of marine sand minerals [9]. To ensure high-quality industry development, Wang Qiong et al. (2021) suggest guiding enterprises towards employing new technologies and processes for marine sand production [10]. Reasonable utilization of marine sand resources not only promotes the advancement of the maritime economy but also contributes to environmental protection during mining activities [11]. In the process of marketing marine sand in China, local marine authorities are required to delineate auctioned sand mining areas after a general survey, commonly referred to as preliminary research. Subsequently, detailed exploration can be conducted once the preliminary research has identified the mining area.

According to the current situation of exploration and development of marine sand resources, China has formed a set of exploration and evaluation technology system of marine sand resources [11]. The exploration work is divided into three stages: general survey, detailed survey, and exploration [12]. The establishment of a Quaternary sequence geological model is a crucial prerequisite for the successful implementation of large-scale marine sand exploration [13]. Previous studies primarily employed seabed sediment sampling and columnar samplers to investigate sediment sources and related information. While single-channel seismic data are commonly used in fault research, seabed strata analysis and marine sand search [14,15,16,17,18,19,20,21,22], a coarse resolution cannot define the details of sediment textures or thickness and lateral variation of sedimentary bodies [23].

The survey coverage of marine sand resources in China is currently limited, with a relatively small number of investigated sea areas and the passage of time. Given the potential variability in the occurrence of marine sand, it is imperative to expand the scope of surveys. Therefore, there is a pressing need for rapid and highly accurate investigation methods [12]. The conventional methods of ore bodies or petroleum plays exploration have always placed great emphasis on studying outcrops [24,25,26,27,28]. However, the concept that we are proposing a geologic exploration approach does not involve any previous outcropping analogue study. The more precise analysis of single-channel seismic data with few drillings is efficient for precise exploration of marine sands. Moreover, this approach is not only more cost-effective but also more efficient and precise.

2. Geological Setting

The study area is situated at the southeastern South China (Figure 1). According to previous research [29,30,31]. The marine sedimentary strata in the study area can be classified into three layers from top to bottom, as follows:

(1) Holocene Marine facies sediment layer (Q₄^m)

This layer is widely distributed and consists of gray sandy mud, mud, argillaceous sand, and sand. It primarily comprises silty clay with occasional thin layers of fine and coarse sand. The plasticity ranges from flow plastic to soft plastic. Some shell debris are present in the upper part, while a small amount of organic matter can be found locally.

(2) Pleistocene alluvial and pluvial facies sediment layers (Q₃^al+pl)

Formed during the late Pleistocene to early Holocene period (10~37 ka ago), this layer represents a time of frequent regressions and transgression. Intense sedimentation occurred mainly in ancient river environments followed by shallow marine environments.

Existing data indicate widespread distribution of this layer in the study area with well-preserved riverbed facies characterized by coarse sediment particles and lithology ranging from yellowish-brown to shallow yellow-brown gravelly sands, sands, muddy sands, and mud–sand–gravels.

(3) Pleistocene Marine facies sediment layer (Q₃^m)

Pleistocene marine facies sediment layers (Q₃^m) are found throughout the mining area. The drilling revealed one or two layers of marine sedimentary strata, with rock properties dominated by gray, light gray, grayish-white, bluish-gray, grayish-black, yellowish-gray sandy clay, mud, muddy sand, sand, gravelly sand, and grayish-black, yellowish-gray sandy clay, mud, muddy sand, sand, gravelly sand, and mud–sand–gravel mixtures.

Wang S.J. et al. (2003) contend that the coastal sand sediment belt in the South China Sea exhibits remarkable richness, which spans from the western region of the Pearl River Estuary to the northern area of the Leizhou Peninsula [32]. The report also highlights the presence of an ancient coastal sand body, with average thickness of approximately 10 m, in the nearshore region of the northern South China Sea within Guangdong Province [33].

Figure 1. (a). Regional satellite image map. (b). Geographical location and geotectonic map of the study area (revised after [34]).

Figure 1. (a). Regional satellite image map. (b). Geographical location and geotectonic map of the study area (revised after [34]).

3. Methodology

3.1. Single-Channel Seismic Profile Acquisition

The marine sand ore layer is primarily distributed in the shallow coastal area, exhibiting a relatively scattered pattern with minimal variations in physical properties (particle size, sorting, etc.), inadequate compaction, and limited impedance differences. Generally characterized by thin thickness ranging from a few meters to ten meters, the exploration accuracy and resolution requirements for marine sand bodies are exceptionally high [2]. Therefore, in order to achieve the goal of exploration, it is imperative to rely on advanced seismic survey and other geophysical and drilling engineering operations simultaneously. Single-channel seismic imaging has emerged as a prominent approach due to its remarkable advantages in terms of efficiency, resolution, and cost-effectiveness [35,36]. After undergoing meticulous treatment, the single-channel seismic data acquired during marine sand resources survey exhibit enhanced capability in distinguishing various geological formations, including sand layers, mud layers, and ancient river channels. Consequently, this refinement significantly augments the precision of marine sand exploration [37]. In recent years, the marine sand survey projects in numerous cities within Guangdong Province have also adopted the sub-bottom profile and single-channel seismic survey methods, yielding favorable outcomes [38,39].

The Geo-Spark 2000X negative electrode discharge spark seismic system, with a maximum energy output of 2000 J and a detection depth greater than 40 m, was utilized in this study. A total of 24 receivers were deployed. The spacing between the first 12 receivers is 1 m, while that among the last 12 receivers is 2 m.

The single-channel seismic detection system primarily comprises three components: the focal source, receivers, and acquisition workstation. The focal source emits low-frequency acoustic wave signals through an electric spark, while the receiver captures the echo signals reflected from the seabed and stratum to generate an acoustic image. The acquisition workstation will receive the signals collected by the receiver and record them in hard disk in “SEG-Y” format.

The selection of parameters of the single-channel seismic detection system is based on the analysis of test results and the overall design requirements of the project. For instance, the principle for selecting recording range aims at achieving optimal vertical resolution while ensuring a certain penetration depth. Similarly, the principle for selecting excitation energy aims to ensure clear reflections at both deep and shallow depths without significant multiple interference.

In this investigation, the source and receiver towers were positioned fixedly behind the stern, respectively. The source energy of 1000 J was utilized with equal time excitation at an interval of 1 s, while the data acquisition length was set to 0.15 s.

After the collection of single-channel seismic data, including data unwinding and header finishing, the following steps were undertaken: (1) Definition of a 2D observation system; (2) Application of band-pass filtering within the frequency range of 100–300–3000–3200 Hz; (3) Identification and extraction of reflected seismic phases from the seabed; (4) Removal of data located above the reflected seismic phase; (5) Implementation of automatic gain control; (6) Conversion from time to depth using seawater velocity at 1500 m/s and sediment velocity at 1600 m/s. The aforementioned processing tasks for single-channel seismic data were performed utilizing TomoPlus 7.0 software.

This study employs multiple geophysical detection techniques, including sub-bottom profile survey and single-channel seismic survey. The measuring lines are arranged based on a 200 m by 200 m grid pattern, consisting of 13 east–west survey lines (designated as B-DDL-1 to B-DDL-13) and 13 north–south survey lines (designated as B-DDJ-1 to B-DDJ-13). Please refer to Figure 2 for the layout diagram of the composite detection line. The total length of the surveyed lines amounts to 76 km.

By using SonarWiz7 software, we determined the elevation positions of both the top and bottom layers of these marine placers. After GIS treatment and analysis combined with water depth data, information regarding area coverage, average thickness, volume, and resource estimation for each marine placer was obtained.

3.2. Collection of the Sub-Bottom Profiles

The objective of the sub-bottom profile is to investigate the shallow stratum structure (3–5 m) in the study area. With a total length of 76 km, this profile utilizes a higher signal frequency compared to single-channel seismic surveys, allowing for good penetration through mud layers but experiencing significant attenuation within sand layers. By integrating data from single-channel seismic exploration, this characteristic can be leveraged to effectively identify the shallow stratum structure, which has been verified through drilling, thereby enhancing the accuracy of sand detection [4].

The sub-bottom profile survey is conducted using the EdgeTech 3400-OTS instrument. The instrument parameters include a working frequency range of 2~16 kHz, vertical resolution ranging from 6 to 10 cm, sand bottom penetration capability up to 6 m, mud bottom penetration capability up to 80 m, and a maximum working water depth of 100 m.

Before the operation, the transmitting frequency is meticulously adjusted to a range of 3~10 kHz based on considerations of water depth and substrate conditions. All necessary measures are taken to minimize noise and other sources of interference, thereby enhancing the signal-to-noise ratio and ensuring the integrity of recorded data. Additionally, the recording time delay is fine-tuned to align with the recording range of each corresponding measuring line.

During the study, based on the stratum profile data, we interpreted the acoustic reflection interface to delineate the shallow strata and fractures as well as the distribution of shallow gas. Additionally, sediment sampling and drilling provided geological data for drawing a formation geological profile. By analyzing the material composition, structure, and buried geological bodies (including structures), we assessed the shallow stratum structure of the research area.

3.3. Drilling Engineering Verification

The operating vessel is securely positioned for drilling using four anchor ropes (two at the bow and two at the stern). Each drilling operation ensures the acquisition of a 40 m sample and adheres to the following guiding principles: if a non-sand layer is encountered between 40 and 50 m, terminate the drilling and sampling process; if a sand layer persists within the 40–50 m range, conclude drilling and sampling at 50 m. The retrieval rate of all drill cores meets the specified criteria, with no less than 85% comprising mud layers and no less than 60% consisting of sand layers. Geological drilling samples are documented, photographed, disassembled, and preserved on-site to ensure their suitability for subsequent geomechanical testing.

4. Results

4.1. Characteristics of the Sub-Bottom Profile Data

The sub-bottom profile data were processed using SonarWiz7 software. Based on the analysis of grayscale variations and wave morphology reflected by changes in the reflection intensity, the reflection interfaces R₁, R₂, and R₃ were identified from the sub-bottom profile in the study area:

R₁ represents a highly reflective seabed surface with strong amplitude and consistent continuity. The undulating pattern observed in this interface reflects variations in seabed topography.

R₂ exhibits strong amplitude reflections with good continuity, significant fluctuations, and distinct stratigraphic interfaces.

R₃ displays strong amplitude reflections with good continuity, substantial fluctuations, and clear formation boundaries.

4.2. Single-Channel Seismic Data Characteristics

In the sub-bottom profile, the mud layer exhibits “easy penetration”, while in the single-channel seismic profile, it displays “good stratification”. The single-channel seismic signature of the mud layer is characterized by layered or weak amplitude reflections with consistent medium strength to medium weakness and good continuity (if the seabed surface consists of a mud layer). Conversely, the sand layer is identified by disorderly reflections with poor continuity and medium-strong amplitude [4].

According to the principle of seismostratigraphy, we integrated the seismic phase reflection structure characteristics, wavegroup characteristics, and reflection interface characteristics of each profile. We conducted continuous tracking of five main seismic reflection interfaces in the study area within a single seismic profile: R₁, R₂, R₃, R₄, and R₅ from top to bottom. The key features of these five reflection interfaces are as follows:

R₁ interface: This is the seabed reflection surface exhibiting strong amplitude and high energy with good continuity. It varies with fluctuations in the seabed and represents the most distinguishable reflection interface.

R₂ interface: Distributed throughout the entire area This interface demonstrates medium-strong amplitude and good continuity similar to that of seabed reflections. The relationship between the seabed and R₂ is conformable contact.

R₃ interface: Also distributed across the entire area. This interface exhibits overall medium-strong amplitude and good continuity approximately parallel to overlying reflection layers.

R₄ interface: Similarly distributed across the whole area. This weak-medium amplitude interface shows general continuity approximately parallel to overlying reflection layers.

R₅ interface: Found throughout the entire region. This low-medium amplitude reflects poor continuity with a large fluctuation range.

4.3. Drilling Engineering

Based on the integrated analysis of the sub-bottom profile profiles and single-channel seismic data, we have preliminarily identified the marine sand enrichment region. Consequently, four drilling sites have been selected, each with an approximate depth of 40 m (Table 1, Figure 3).

• The YXZK-B1 and YXZK-B3 boreholes are distributed in the northern region of the study area, where both holes have been identified. The average thickness of the sand layer is approximately 11.24 m.
• The YXXK-B2 and YXZK-B4 boreholes are located in the southern part of the study area, with both holes being discovered. The average thickness of the placer is around 11.47 m.

Table 1. Drilling lithology table.

Drilling Number	Depth/m	Layering/m	Lithology
YXZK-B1	30.6	0~4.30	Silt
		4.30~6.60	Sandy silt
		6.60~9.10	Silty sand
		9.10~12.6	Sandy silt
		12.60~14.20	Silty sand
		14.20~16.50	Silty sand
		16.50~22.90	Mud
		22.90~24.50	Pebbly muddy sand
		24.50~27.70	Silty sand
		27.60~32.40	Argillaceous sandy gravel
		32.40~34.70	Pebbly muddy sand
		34.70~39.20	Pebbly muddy sand
		39.20~40.00	Pebbly muddy sand
YXZK-B2	30.74	0~1.60	Silt
		1.60~6.70	Silt
		6.70~16.30	Sandy silt
		16.30~18.40	Silty sand
		18.40~24.60	Mud
		24.60~29.00	Silty sand
		29.00~40.10	Muddy sand gravel
		40.10~42.50	Gravel mud
YXZK-B3	30.71	0~2.50	Sandy mud
		2.50~5.60	Pebbly mud
		5.60~6.80	Silty sand
		6.80~18.80	Silt
		18.80~19.90	Silty sand
		19.90~22.90	Sandy silt
		22.90~25.50	Pebbly muddy sand
		25.50~31.80	Mud gravel sand
		31.80~35.30	Muddy sand gravel
		35.30~40.70	Gravel mud
YXZK-B4	30.81	0~5.90	Gravel mud
		5.90~7.50	Pebbly muddy sand
		7.50~19.00	Gravel mud
		19.00~22.10	Silty sand
		22.10~35.90	Muddy sand gravel
		35.90~37.30	Sandy silt
		37.30~38.00	Sandy silt
		38.00~39.00	Muddy sand gravel
		39.00~40.00	Gravel mud

Table 1. Drilling lithology table.

Drilling Number	Depth/m	Layering/m	Lithology
YXZK-B1	30.6	0~4.30	Silt
		4.30~6.60	Sandy silt
		6.60~9.10	Silty sand
		9.10~12.6	Sandy silt
		12.60~14.20	Silty sand
		14.20~16.50	Silty sand
		16.50~22.90	Mud
		22.90~24.50	Pebbly muddy sand
		24.50~27.70	Silty sand
		27.60~32.40	Argillaceous sandy gravel
		32.40~34.70	Pebbly muddy sand
		34.70~39.20	Pebbly muddy sand
		39.20~40.00	Pebbly muddy sand
YXZK-B2	30.74	0~1.60	Silt
		1.60~6.70	Silt
		6.70~16.30	Sandy silt
		16.30~18.40	Silty sand
		18.40~24.60	Mud
		24.60~29.00	Silty sand
		29.00~40.10	Muddy sand gravel
		40.10~42.50	Gravel mud
YXZK-B3	30.71	0~2.50	Sandy mud
		2.50~5.60	Pebbly mud
		5.60~6.80	Silty sand
		6.80~18.80	Silt
		18.80~19.90	Silty sand
		19.90~22.90	Sandy silt
		22.90~25.50	Pebbly muddy sand
		25.50~31.80	Mud gravel sand
		31.80~35.30	Muddy sand gravel
		35.30~40.70	Gravel mud
YXZK-B4	30.81	0~5.90	Gravel mud
		5.90~7.50	Pebbly muddy sand
		7.50~19.00	Gravel mud
		19.00~22.10	Silty sand
		22.10~35.90	Muddy sand gravel
		35.90~37.30	Sandy silt
		37.30~38.00	Sandy silt
		38.00~39.00	Muddy sand gravel
		39.00~40.00	Gravel mud

Figure 3. Upper Pleistocene alluvial and pluvial sediment sand layer (YXZK-B3 drilling hole, 30~35 m depth).

Figure 3. Upper Pleistocene alluvial and pluvial sediment sand layer (YXZK-B3 drilling hole, 30~35 m depth).

5. Discussion

5.1. Geophysical Interpretation and Identification of the Sedimentary Strata

5.1.1. The Sub-Bottom Profile Data

According to the sub-bottom profile data, this study classifies two sets of reflection sequences, C₁ and C₂, on the sub-bottom profile as follows (Figure 4):

Layer C₁: It represents the reflection sequence between R₁ and R₂, which corresponds to a set of surface deposition layers exhibiting medium-strong amplitude and strong reflection energy. Additionally, it displays a parallel/subparallel capping reflection structure. The thickness of layer C₁ is approximately 5 m. Based on drilling core data analysis, this layer can be identified as seabed surface mud primarily composed of silt silty clay and silty clay lithology.

Layer C₂: This refers to the reflection sequence between the reflection interface R₁ and R₂. Notably, this layer exhibits robust reflection energy enabling clear observation of ancient river channels along with boundaries from sand layers. According to drilling core data analysis, the sediment layer consists mainly of silt, fine sand, and medium-coarse sand mixed with silty clay. Due to secondary reflections impacting the sand layer, weak formation reflection energy is observed below this layer, making identification of its bottom boundary challenging.

5.1.2. The Single-Channel Seismic Profile Data

According to the seismic reflection interface division of a single-channel seismic profile and the internal structural characteristics of the sequence, combined with an analysis of existing drilling data, five interfaces (R₁, R₂, R₃, R₄, and R₅) are classified into five seismic sequences (A₁, B₁, C₁, D₁, and E₁). The characteristics of these divided seismic sequences are described as follows (Figure 4):

Seismic sequence A₁ (R₁-R₂): The top interface R₁ represents the seabed, while the bottom interface R₂ is a reflection interface. The internal reflection layer group is characterized by medium-low frequency with strong amplitude and good horizontal continuity. It generally exhibits parallel reflection characteristics. This stratum is considered to be the youngest in the study area belonging to Holocene River-Sea mixed facies. Based on drilling data analysis, this layer consists mainly of sandy mud or silty clay.

Seismic sequence B₁ (R₂-R₃): The top interface R₂ represents a reflection interface, while the bottom interface R₂ also serves as a reflection interface. The internal reflection layer group is characterized by low frequency with strong amplitude and relatively better continuity overall. It exhibits an overall parallel and flat reflection structure but may have partial chaotic-appearing reflective structures. The relationship between B₁ and A₁ is conformable contact. According to drilling data analysis, this layer primarily consists of silty clay and medium-coarse sand, showing shallow marine depositions facies with ancient river formations in certain positions.

Seismic sequence C₁ (R₃-R₄) exhibits a top reflection interface R₃ and a bottom reflection interface R₄, with an internal layer group showing low frequency, medium-weak amplitude, generally continuous reflections. The overall structure is characterized by parallel and flat reflections, while partial areas display chaotic-appearing reflective structures. Notably, the relationship between C₁ and B₁ is also conformable contact. Based on drilling data analysis, this layer primarily consists of silty clay, medium coarse sand, and silt, showing shallow marine depositions facies.

Seismic sequence D₁ (R₄-R₅) exhibits a reflection interface at the top, denoted as R₄, and another reflection interface at the bottom, referred to as R₅. This sequence is characterized by a chaotic structure with medium-high frequency and medium-strong amplitude. The reflections show poor continuity. Based on drilling data, this layer primarily consists of medium to coarse sand, silt, and silty clay, showing marine sedimentary deposits facies.

Seismic sequence E₁ (below R₅) is characterized by a top interface represented by R₅, exhibiting chaotic structures with medium-low frequency and amplitude, as well as poor continuity of reflection. Based on drilling data, this layer predominantly consists of silty clay and rounded gravel, showing marine depositional facies.

According to the integrated analysis of geophysical data and drilling results, the sedimentary strata in the study area can be shown in Figure 4:

Holocene Marine facies sediment (Q₄^m): Sequence A₁ is a marine–fluvial mixed facies. It is a set of argillaceous deposits, distributing throughout the region. Underlying sequence B₁ comprises shallow marine silty clay and medium coarse sand layers with localized occurrences throughout the study area. Sequences A₁ and B₁ correspond to the Quaternary Holocene marine facies sediment (Q₄^m) in the study area.

Alluvial and pluvial facies sediment (Q₃^al+pl): Sequence C₁ represents shallow marine deposits consisting primarily of silty clay, medium coarse sand, and silt, while sequence D₁ comprises marine sedimentary strata composed mainly of medium coarse sand, silt, and silty clay. Both sequences B₁ and C₁ correspond to the upper alluvial and pluvial deposits (Q₃^al+pl), which are significant sand formations in the study area.

Pleistocene Marine facies sediment (Q₃^m) in sequence E₁ predominantly consists of silty clay and rounded gravel, which corresponds to the geological formation found in the study area.

5.2. Resource Potential Evaluation

Through lithology analyzing of drilling, the marine sand layer in the study area basically corresponds to the results of joint interpretation of sub-bottom profile survey and single-channel seismic survey data, and ore was found in all four boreholes. The lithology of the borehole confirms the presence of a marine sand. By integrating drilling lithology, sub-bottom profile survey, and single-channel seismic data, we were able to calculate the volume and thickness of the marine sand layer. The buried sand deposits are primarily distributed within sequence D₁ and sequence E₁. Through comprehensive application of multiple geophysical methods and four drilling engineering techniques in this study, we identified two marine placers along with single-channel seismic profile data that delineate their approximate locations. The three-dimensional spatial distribution is illustrated in Figure 5. The northern placer exhibits average thickness of approximately 11.0 m over an area of about 2.5 km², with estimated resources totaling around 25 million m³; it appears thicker towards northwestern regions while thinner towards southeastern areas. On the other hand, the southern side displays average thickness of about 11.5 m covering an area roughly measuring 2.1 km², with estimated resources amounting to approximately 21 million m³; similar to its counterpart in northwestern regions, it also thins out towards southeastern areas (

Table 2. Estimation table of marine sand resources in the exploration area.

Placer	Average Thickness (m)	Area (km2)	Volume (Million m3)	Mud Content (%)	Resource Volume (Million m3)
V1	11.0	2.5	29	13	25
V2	11.5	2.1	24	13	21

).

5.3. Importance of Our Approach

The current survey coverage of marine sand resources in China remains limited due to a restricted number of investigated sea areas and temporal constraints. Given the potential variability in marine sand occurrences, it becomes imperative to expand the scope of surveys by employing rapid and highly accurate investigation methods.

The conventional methods of exploring ore bodies or petroleum plays traditionally place significant emphasis on the study of outcrops. The conventional methods may necessitate the implementation of over 20 drillings within a comparable region to our study area, thereby entailing significant time and financial investments. However, our proposed geologic exploration approach does not involve any prior analysis of outcrops. A more precise analysis of single-channel seismic data with limited drilling proves to be efficient for accurately exploring marine sands. Furthermore, this approach is not only cost-effective but also enhances efficiency and precision.

6. Conclusions

In this study, we conducted sub-bottom profile measurements, single-channel seismic surveys, and offshore engineering geological drilling in the research area of marine sand in the northern South China Sea. By combining these methods with sample test data analysis, we identified a significant marine sand deposit and comprehensively mapped the distribution of marine sand resources in the study area. Furthermore, we estimated the volume of marine sand and achieved highly favorable results of the concept that we are proposing a geologic exploration approach that does not involve any previous outcropping analogue study.

This study demonstrates the effective application of multiple geophysical exploration methods in investigating marine sand resources in the northern offshore waters of the South China Sea. The exploration of marine sand resources can yield favorable results through the combined use of various geophysical exploration techniques, reducing the need for extensive drilling engineering verification. In this study, five reflective interfaces were identified as R₁, R₂, R₃, R₄, and R₅ from top to bottom. These interfaces correspond to A₁, B₁, C₁, D₁, and E₁, respectively. Three sets of strata were distinguished: Holocene Marine facies sediment layer (Q₄^m), Pleistocene alluvial and pluvial facies sediment (Q₃^al+pl), and Pleistocene Marine facies sediment (Q₃^m). Sequences A₁ and B₁ represent Q₄^m; Sequence C₁ as well as sequence B₁ correspond to Q₃^al+pl, which are among the main occurring sand layers in this region; Sequence E₁ represents marine sedimentary facies consisting mainly of silty clay and rounded gravel that corresponds to Q₃^m. Two distinct marine placers were discovered with a total resource estimate of approximately 46 million m³ and an average thickness around 11 m. These findings of marine sand hold significant economic potential for local society.

The approach we use requires a high degree of experience in seismic data interpretation and the choice of drilling location. In the future, the operation process will be further standardized to further improve the method.