Acoustic Rapid Detection Technology and Its Application for Rare Earth Element (REE)-Rich Sediments in the Pigafetta Basin of the Western Pacific

Abstract

This study aims to investigate the stratigraphic features and rare earth element (REE) mechanisms of deep-sea REE-rich sediments in the West Pacific Pigafetta Basin using acoustic rapid detection technology. Through an analysis of sub-bottom profile data and synthesis of existing studies, this study reveals the acoustic properties and thickness distribution of the REE-rich sediments. Acoustic spectral records identify three distinct acoustic facies: opaque (O), transparent (T), and laminated (L). This study maps the thickness and spatial distribution of the REE-rich sediment layer in the research area, ranging from approximately 6 to 36 m in thickness. Regions with REE-rich sediments exceeding 30 m in depth are identified, showing concentrated distribution along the northwest–southeast axis and a contiguous zone in the southwest corner of the study area. The method employed in this study can determine the potential bottom boundary of the REE-rich layer by assessing the thickness range of the sedimentary layer, overcoming limitations of traditional sampling methods. Furthermore, the thickness distribution characteristics of the REE-rich sedimentary layer in the study area provide valuable insights for future research on resource evaluation and estimation.

1. Introduction

Rare earth elements (REE) refer to a group of 17 elements located in the periodic table, encompassing lanthanide elements, scandium (Sc), and yttrium (Y). The majority of China’s rare earth reserves, over 97%, are situated in carbonate rare earth deposits distinguished by the prevalence of light rare earths. Conversely, there is a notable scarcity of medium to heavy rare earths in these deposits [1]. As the utilization of medium to heavy rare earth resources continues to rise, there is a growing need to discover new resources abundant in medium to heavy rare earth elements [2].

“Deep-sea REY-rich sediments”, also referred to as “deep-sea rare earth” or “REY-rich mud”, are rare earth elements (lanthanide elements plus yttrium, collectively known as REY (REY = REE + Y)) exceeding 400 ppm (${\displaystyle \sum \mathrm{REY}}$ > 400 × 10⁻⁶) that are found in deep-sea basins and are particularly abundant in medium to heavy rare earth elements. This type of sediment represents the fourth deep-sea metal mineral identified following polymetallic nodules, cobalt-rich crusts, and polymetallic sulfides [3]. In recent times, deep-sea rare earths have emerged as a novel and significant marine mineral resource, notably abundant in medium and heavy rare earth elements. The quantity of these resources exceeds that of terrestrial reserves, underscoring their substantial potential [4]. Consequently, the exploration of deep-sea rare earth resources and the identification of high-quality mining sites for deep-sea rare earth resources hold crucial strategic and practical importance [5].

Current surveys of deep-sea rare earths predominantly utilize a combination of sediment core sampling and sub-bottom profiling techniques. However, traditional methods like box, grab, and gravity piston core sampling prove insufficient for sampling the deep-sea sediments exceeding 10 m in depth, often failing to reach the bottom of the high-grade enrichment layer. Deep-sea drilling necessitates specialized drilling vessels incurring high costs, thus underscoring the pressing need for the development of viable deep-sea rare earth sub-bottom exploration technologies [6]. Globally, the exploration of deep-sea sediments has predominantly relied on multibeam and sub-bottom exploration technologies. Sub-bottom profiling, in conjunction with single-channel analysis, stands as a well-established method in marine sub-bottom research, extensively employed to investigate sediment distribution characteristics in deep-sea basins and around seamounts. This method serves as a standard for sub-bottom detection of seabed sediments [7]. Noteworthy is the July 2011 report by Japanese researchers led by Kato in Nature Geoscience, which discussed rare earth resources in the Pacific deep-sea clay at depths ranging from 3500 to 6000 m; the Pacific seabed alone is estimated to harbor reserves of up to 88 billion tons [1].

In 2013, a research expedition conducted by Japanese geologists in the vicinity of Minamitorishima Island revealed significant findings: rare earth concentrations reaching up to 0.66% in the sub-bottom seabed sediments, located just 3 m below the ocean floor. This discovery represents one of the world’s most concentrated and commercially valuable seabed rare earth deposits, with an estimated reserve of approximately 6.8 million tons [4]. In 2017, Wang Fenlian highlighted that deep-sea sediments rich in rare earth elements found in the Pacific and Indian Ocean basins could serve as a potential rare earth mineral resource. Metalliferous sediments, zeolite clay, and pelagic clay are identified as the primary types of REY-rich sediments, exhibiting high total REY content ranging from 400 × 10⁻⁶ to 2000 × 10⁻⁶, with the highest concentration reaching 6600 × 10⁻⁶ [5]. Despite ongoing exploration efforts, the utilization of sub-bottom exploration technology for rare earth resource investigations in China has not yet yielded practical results. Shi Xuefa emphasized that the deep-sea REY enrichment layer may extend hundreds of meters below the seabed; however, existing sub-bottom profilers and other geophysical methods lack precision in delineating the three-dimensional distribution boundaries of these REY-rich sediment layers [8]. There is a pressing need for the advancement of sophisticated sub-bottom fine detection technology to facilitate cost-effective and efficient exploration and evaluation of high-quality deep-sea REY enrichment zones that hold significant development potential.

In recent years, there has been a growing interest in the application of lithology identification technology for deep-sea shallow surface sediments using sub-bottom profile characteristics in ocean surveys. This technology has also been utilized in the investigation of deep-sea rare earth-rich sediments. A study by Nakamura et al. (2016) focused on conducting a survey of deep-sea rare earth-rich sediments in the Japanese Exclusive Economic Zone (EEZ) around Minamitorishima Island in the Western Pacific Ocean. The researchers utilized sub-bottom profiles and multibeam bathymetric data collected within a 20 m depth contour around Minamitorishima to categorize the sediments into three groups: opaque (O-type), transparent (T-type), and layered (L-type) [4]. Through a comparison of different acoustic facies with the lithological and geochemical characteristics of sediment core samples, Nakamura et al. found that the total REY concentrations (${\displaystyle \sum \mathrm{REY}}$) in the cores from Ti-type and Ts-type facies areas exceeded 400 ppm within 10 m below the seafloor. This observation clearly indicated that T-type acoustic facies are associated with REY-rich mud. In contrast, the L-type facies corresponded to sediments that are not rich in REY [9,10].

To effectively address the challenges associated with identifying the top and bottom interfaces of deep-sea REY enrichment layers and to map the vertical extent and spatial distribution of these REE-rich sediment layers, this study utilized sub-bottom profile data collected by the Guangzhou Marine Geological Survey in the Pigafetta Basin of the Western Pacific. By integrating these data with relevant research findings from both domestic and international sources, this study examines the characteristics of the sub-bottom profiles, the depositional thickness, and the spatial distribution of the REY-rich sediment layers in the region. Through the analysis of acoustic spectral records, this study identifies three distinct acoustic facies: opaque (O), transparent (T), and laminated (L). By defining the transparent layer as the target enrichment horizon for deep-sea REY sediment layers and the opaque layer as its lower boundary, this study determines the thickness and spatial distribution of the REY-rich sediment layers within the surveyed area. This spatial distribution characteristic of the REY-rich sediment layer holds significant implications for identifying potential zones of deep-sea REY enrichment resources, offering valuable guidance for the exploration and potential exploitation of deep-sea rare earth resources.

2. Geological Setting

The study area is situated in the Pigafetta Basin of the Western Pacific Ocean, located to the east of Guam (Figure 1b). The sub-bottom data utilized in this research comprises seven survey lines, with four primary survey lines oriented in the NE–SW direction and three connecting survey lines oriented in the NW–SE direction (Figure 1a). The bathymetric depth within the study area varies from 5000 m to 6500 m. The Pigafetta Basin is predominantly flat, encompassed by a series of seamounts, and extends in a southeastward direction. This basin represents a stable sedimentary region from the Jurassic period, characterized by the presence of developed seamounts, with the shallowest depth recorded at 1200 m [11,12].

In recent years, significant rare earth deposits have been found in the Western Pacific Ocean, ${\displaystyle \sum \mathrm{REY}}$ with the largest deposit reaching nearly 6000 $\mathsf{\mu }\mathrm{g}/\mathrm{g}$. The amount of ${\displaystyle \sum \mathrm{REY}}$ discovered in this region surpasses that found in other parts of the world. These deposits are primarily located at depths of approximately 2 m below the seabed surface, characterized by their high concentration and relatively deep burial depth [13].

3. Methods

3.1. Field Collection and Fine Processing

The Guangzhou Marine Geological Survey conducted expeditions to investigate deep-sea rare earth resources in the Pigafetta Basin of the Western Pacific Ocean, employing sub-bottom profiling techniques (illustrated in Figure 1). The survey vessel utilized for these missions was the “Ocean No. 6” with the advanced Parasound P70 sub-bottom profiler (Teledyne Marine, Houston, TX, USA); the vessel operated consistently during the expeditions, collecting reflection data from the upper sedimentary layers.

When utilizing the Parasound P70 for data collection purposes, researchers commonly employ the parametric sub-bottom section/single-beam depth measurement mode (P-SBP/SBES). This mode involves selecting the first high-frequency (PHF) transmission frequency at 20 KHz, which offers the highest concentration of transmission energy, and the second low-frequency (SLF) one at 4 KHz [14]. In areas characterized by significant seabed topography, the profiler initially operates in the single-pulse transmission mode (Single Pulse) to ensure robust bottom-tracking capabilities for stabilizing seabed tracking. As the seabed topography becomes less pronounced, the system switches to the pulse train (Pulse Train) transmission mode, which enhances horizontal resolution. The chosen recording methodology involves a full-profile recording strategy, enabling the capture of comprehensive water depth data. This approach includes a low-frequency recording span of 60 ms and a high-frequency span of 300 ms.

The operation of multiple acoustic devices during survey line activities can lead to interference and mutual influence among the devices. The noise level of the data acquisition interface is significantly affected by sea conditions, which can complicate subsequent data processing. This study concentrates on two main areas: the precise processing of deep-sea rare earth sub-bottom profiles and attribute inversion. The fine processing involves denoising and enhancing the signal-to-noise ratio to ensure data fidelity. Attribute inversion aims to identify the upper boundary of deep-sea REE-rich sedimentary layers and calculate sediment thickness. The fine processing of deep-sea rare earth sub-bottom profiles consists of several steps (Figure 2), including (1) reordering the segmented survey lines to maintain data organization, (2) preparing segmented survey line data for further analysis, (3) adjusting the length of segmented survey line records for accurate data alignment, (4) integrating and quality-controlling segmented survey line data to ensure data integrity, (5) correcting amplitude to account for signal strength variations, (6) eliminating sudden noise spikes to reduce their impact, (7) using predictive deconvolution to enhance signal sharpness and resolution, (8) converting coordinates to align data with appropriate geographical or spatial references, and (9) calculating the vertical thickness of deep-sea rare earth-rich sedimentary layers to determine the extent of these valuable deposits [15,16]. These steps are critical for accurately interpreting sub-bottom profiles and effectively applying attribute inversion techniques, which are vital for exploring and evaluating deep-sea rare earth resources.

The comparison of sub-bottom profile lines before and after fine processing is presented in Figure 3. The left image displays the raw data obtained from the sub-bottom profiler, which may exhibit noise or lack of clarity due to factors like equipment interference or sea conditions. In contrast, the right image illustrates the refined data resulting from the fine processing steps, including denoising, signal enhancement, and predictive deconvolution. The processed data demonstrate enhanced clarity, improved signal-to-noise ratio, and clearer layer boundaries, thereby aiding in the easier identification and analysis of the sedimentary layers.

3.2. Sample Analysis

This study involved columnar sampling from two stations (GC1 and GC2) situated in the deep-water region of the Western Pacific (Figure 1), with water depths ranging from 5000 to 6500 m [17]. A total of 66 sediment samples were collected at regular 25 cm intervals from stations GC1 and GC2 for a sediment smear analysis to differentiate lithological variations in the sediments. The smear analysis was conducted following the guidelines outlined in the “The Expertise for Oceanic Polymetallic Nodules Survey” (GB/T17229.34-1998) [18]. Subsequently, the samples were dried, crushed, and ground to a 200-mesh size for testing. The sediment samples underwent analyses for major and rare earth elements, with a major-element analysis conducted in accordance with the “Methods for Chemical Analysis of Silicate Rocks—Part 28” (GB/T14506.28-2010) [19] and a rare earth element analysis following the “Chemical Analysis Methods for Marine Sediment” (GB/T20260-2006) [20].

4. Discussion

4.1. Classification and Characterization of Acoustic Facies

Based on the echogram records, Nakamura et al. [4] identified three acoustic facies: opaque (O), transparent (T), and layered (L) (Figure 4). The study, building on prior research, employed acoustic rapid detection technology to investigate the sub-bottom profile characteristics of rare earth-rich sediments in the Pigafetta Basin of the Western Pacific Ocean.

Opaque facies (O-type) is characterized acoustically by its impenetrable nature and intense reflectivity, as illustrated in Figure 5. This facies is predominantly found in or around seamounts; Figure 4C and Figure 5 exhibit similar acoustic features. It lacks a distinct sedimentary structure, and its acoustic characteristics, along with its spatial distribution, suggest that the opaque facies indicates exposed bedrock without a covering of soft sediments. In the case of O-type facies, these reflections would be indicative of strong reflectivity and limited sound penetration.

The transparent facies (T-type) is acoustically characterized by its ability to allow sound penetration, influenced by the acoustic basement reflection interface. It is further classified into two subtypes based on the configuration of its upper boundary: irregular (Ti) and smooth (Ts). The Ti subtype typically exhibits an irregular upper boundary, which, although generally parallel to the acoustic basement topography, may not consistently adhere to this pattern (as illustrated in Figure 6, Figure 7 and Figure 8). In contrast, the Ts subtype is distinguished by a smooth upper boundary. Despite these differences in upper surface morphology, both subtypes share similar acoustic properties (as shown in Figure 6). Echograms (Figure 6a,b) can help us visualize the transition zone between the Ti and Ts subtypes. Figure 4A and Figure 6a demonstrate comparable acoustic features, indicating a gradual shift in acoustic properties from those associated with an irregular upper boundary (Ti) to a smooth upper boundary (Ts).

The layered facies (L-type) is characterized by the existence of numerous reflection interfaces, usually overlaying the T-type phase and primarily composed of non-enriched hemipelagic sediments. This facies is typically continuous and runs parallel to the seafloor, although it may not always correspond precisely to the basement topography. It is commonly observed above the Ts type, displaying a layered configuration that mirrors the sediment deposition chronology (as depicted in Figure 8). Figure 4A and Figure 8 share similar acoustic characteristics.

Profile 8a demonstrates a transition in the uppermost layer from L-type to Ts-type, as illustrated in Figure 8a. This transition is characterized by the gradual thinning of the L-type facies as it transitions into the Ts-type facies, eventually leading to its disappearance. Simultaneously, the distinct layered structure of the L-type facies becomes less defined as it nears the boundary with the Ts-type facies. The interface shapes of both the L-type and Ts-type facies are predominantly smooth, resulting in minimal changes in seafloor topography at the transition point between these two acoustic facies, as depicted in Figure 8a. In contrast, Profile 8b showcases the transition from L-type to Ti-type. Similar to the transition to Ts-type, the L-type facies thins out, and its layered structure becomes less distinct as it interfaces with the Ti-type facies, as shown in Figure 8b. However, the interface shapes between the Ti-type and L-type facies exhibit notable differences. During the transition from Ti-type to L-type, as the L-type facies decreases in thickness, there is a gradual morphological shift in seafloor topography from the smooth configuration typical of the L-type region to the more irregular pattern associated with the Ti-type region.

Through a comparison of the acoustic spectral records from both regions, it was determined that the sub-bottom profiles of the two areas exhibit similar acoustic characteristics.

From the analysis of the processed sub-bottom profile data, it is evident that areas with soft seabed sediment exhibit better penetration capabilities, reaching a maximum vertical depth of approximately 50 m. Beneath the seabed, there exists an acoustic transparent layer containing relatively weak wave impedance reflection interfaces, suggesting minor physical variances within the layer. The presence of seamounts in the study area significantly alters the terrain. Along the primary survey line (oriented from northwest to southeast), the seabed terrain initially appears relatively flat with minor fluctuations (100 ms–500 ms), transitioning to a flat terrain with significant undulations (500 ms–1000 ms). The maximum fluctuation range of the seabed along survey lines Line 1–Line 2–Line 3 is 500 ms–1000 ms–1500 ms, respectively, indicating a shift from relatively flat to rugged terrain along this survey line (from northeast to southwest). Line 4 and Line 5 exhibit substantial underwater undulations, with maximum undulation ranges of 2500 ms and 3750 ms, respectively. Conversely, Line 6 and Line 7 display a relatively flat and slightly undulating seabed terrain trend (100 ms–500 ms). The classification of acoustic phases reveals that Line 4 and Line 5 feature significantly undulating submarine topography, characterized by O-shaped and T-type facies. In contrast, Line 1, Line 2, Line 3, Line 6, and Line 7 predominantly exhibit O-type, L-type, and T-type facies characteristics.

4.2. The Corresponding Relationship between Sub-Bottom Profile Measurement Results and Different Types of Sediments

Nakamura et al. [4] conducted a comprehensive analysis of the correlation between the acoustic facies of the T and L types and the lithological and geochemical characteristics of sediment core samples. Their study revealed that T-type facies are associated with REY-rich mud, while L-type facies are linked to non-REY-rich sediment overlaying REY-rich mud. In a study by Wang Haifeng et al. [21], comparisons were made between sub-bottom profiles and photos of sediment cores containing chert from core S50 in the Central Pacific Basin. The researchers observed a continuous acoustic transparent layer located several meters below the seafloor in the sub-bottom profile, which corresponded to a deep-sea clay layer approximately 2 m thick at the top of the sediment core. Beneath this layer, a dark acoustic opaque layer was identified, corresponding to a high acoustic impedance flint mixed layer.

Recent research findings indicate that rare earth elements are predominantly found in the sedimentary layers of ocean clay and zeolite clay. The distinction between zeolite clay and oceanic clay primarily pertains to the proportion of clay minerals and zeolite present. Zeolite clay typically contains 50–70% clay and 25–40% zeolite, while oceanic clay is predominantly composed of clay, constituting more than 75% of its composition [22]. Both zeolite clay and oceanic clay are categorized as deep-sea clays. Deep-sea clay formations are commonly found in remote oceanic regions at depths exceeding 4000 m, comprising calcareous, siliceous biogenic shells, and terrestrial detrital materials. The REY content in these layers is typically higher than 400 ppm and can reach up to about 7000 ppm. In contrast, the REY content in calcareous/siliceous soft mud/clay with a high biological component content is generally not higher than 400 ppm, and below the opaque flint layer lies early-deposited calcareous sediment. Consequently, the transparent layer can be identified as the target occurrence layer of deep-sea rare earth sediments, while the opaque layer can be considered as the lower boundary of the sedimentary layer rich in rare earth elements [21].

Two cores (GC1 and GC2) are from the Ti-type facies region [17]. The GC1 station is characterized by a water depth of 5652 m and a core length of 8 m. The sample obtained from this station has been segmented into two distinct sections based on color and lithology. The upper section spans from 0 to 2.8 m, displaying a brown coloration and comprising deep-sea clay lithology. Notably, no microfossils or microorganisms have been identified within this stratum. Drawing from the established stratigraphic classifications in the surrounding region, it is hypothesized that this layer corresponds to a Quaternary stratum. Moving to the lower section, which extends from 2.8 m to 8 m, it is characterized by a dark brown color and exhibits a high viscosity. The on-site smear analysis has revealed the presence of zeolite-containing clay, indicating that the formation in this segment aligns with the Upper Neogene series. The GC2 station is characterized by a water depth of 5163 m and a core length of 8 m. The sample obtained from this station has been segmented into two distinct sections based on color and lithology. The upper section spans from 0 to 3.5 m, displaying a brown coloration and comprising deep-sea clay lithology. Notably, no microfossils or microorganisms have been detected within this stratum. Drawing upon the established stratigraphic classifications in the surrounding region, it is hypothesized that this layer corresponds to a Quaternary stratum. Moving to the lower section, which extends from 3.5 m to 8 m, it is characterized by a dark brown coloration with a high viscosity. Upon microscopic examination, a notable presence of fish bone fragments is observed. The analysis of this section indicates the presence of zeolite-containing clay, suggesting an Upper Neogene origin for this particular formation.

From Figure 9, it is evident that the total REY concentrations (${\displaystyle \sum \mathrm{REY}}$) in the cores retrieved from the Ti-type facies regions exceed 400 ppm within 10 m below the seafloor. This observation strongly indicates that the T-type acoustic facies are associated with REY-rich mud. The rare earth grade of deep-sea REY-rich sediments is typically categorized into different grades. ${\displaystyle \sum \mathrm{REY}}$ is usually referred to as the rare earth grade of deep-sea sediments. Further, the from 400–700 ppm is classified as low-grade (or Grade III), 700–1000 ppm as medium-grade (or Grade II), and ≥1000 ppm as high-grade (or Grade I). The threshold ${\displaystyle \sum \mathrm{REY}}$ for deep-sea sediments is set at 400 ppm. The REY content of the GC1 core is relatively modest, with an average value of 730 ppm, thus placing the rare earth grade of the GC1 core in the medium-grade category. Conversely, the average REY content of the GC2 core, at 1038 ppm, qualifies it as high-grade (or Grade I).

In recent years, research has indicated that a high phosphorus (P) content in high rare earth clay plays a crucial role in the high content of REY in zeolite clay. Additionally, the content of rare earth elements in oceanic clay is closely linked to the presence of phosphate [23]. The total rare earth element content exhibits a pattern of initial decrease followed by an increase (Figure 9). In layers with low rare earth element content, the predominant sediment lithology consists of deep-sea clay and zeolite-containing clay. The content of ${\mathrm{SiO}}_2$, ${\mathrm{Fe}}_2{\mathrm{O}}_3$, and ${\mathrm{TiO}}_2$ increases initially but then displays a significant decreasing trend. The trend curve of the total rare earth element content aligns with the overall content trend curve, suggesting a close relationship between rare earth element content and phosphorus.

4.3. Distribution of Rare Earth Element-Rich Sediment Thickness

Utilizing the characteristics of sub-bottom profiles to distinguish the types of deep-sea sediments is of great significance in the investi of REY-rich deep-sea sediments. Recent surveys and studies have shown that REEs are primarily present in Pacific pelagic clays and zeolite clay sedimentary layers [24,25,26,27,28,29,30,31]. Wang Haifeng et al. [21] have proposed that the transparent layer, distinguished by its acoustic properties, can serve as the target horizon for the REY-rich sediment layer. Conversely, siliceous and cobalt-rich layered strata are less favorable for REY concentration. The opaque layer, characterized by its high reflectivity and impermeability, is suggested to demarcate the lower boundary of the REY-rich sediment layer.

To improve the effectiveness of identifying the upper and lower boundaries of REY-rich sediment layer and to monitor the vertical thickness and arrangement of this sediment layer, a manual picking procedure was carried out on the upper and lower interfaces of the deep-sea REY-rich sediment layer (the transparent layer) along sub-bottom survey lines in the research area. In Figure 10, the green line denotes the selected upper interface of the REY-rich sediment layer, whereas the blue line corresponds to the lower interface. The statistical analysis of the selected data indicates that the time interval for the appearance of the upper and lower boundaries of the REY-rich sediment layer falls approximately within the range of 3300 ms and 7800 ms. This timeframe serves as a significant point of reference for comprehending the vertical scope of the REY-rich sediment layer and can facilitate focused exploration and evaluation of these valuable resources.

Conducting subtraction operations on the identified upper and lower interfaces of the REY-rich sediment layers allows for the calculation of the time difference between these two horizons. This time disparity constitutes a critical piece of information that, when combined with the known seismic velocity of the sediments in the study area, can be utilized to estimate the actual thickness of the sediment layer. Employing the empirical seismic velocity of 1.74 km/s as documented by Heezen et al. [32], the time disparity between the upper and lower interfaces can be transformed into a depth or thickness measurement. This conversion is founded on the principle that the travel time of a seismic wave through a medium is directly proportional to the distance traveled and the velocity of the wave in that medium.

The thickness of the rare earth enrichment horizon can be computed [33]. As depicted in Figure 11, the thickness of the REY-rich sediment layer varies, ranging approximately from 6 m to 36 m. The thinnest regions are determined at 6 m, while the thickest regions reach up to 36 m. Notably, the areas where the REY-rich sediment layer has a thickness of 30 m show a regionally concentrated and contiguous distribution, mainly oriented in the northwest–southeast direction. This pattern is particularly distinct at the northwest corner of the survey lines Line 1 and Line 7, as illustrated in Figure 12.

As depicted in Figure 12, Line 1 possesses the thickest REY-rich sediment layer documented in the study area, attaining a thickness of up to 36 m. This implies a highly potential zone for REY enrichment and concentration. Parallel to Line 1 and extending from northwest to southeast, Line 2 showcases a variable thickness of the REY-rich sediment layer, ranging from 18 m to 12 m and then back to 18 m. Such variation indicates a fluctuation in the concentration of REY enrichment along the line. Positioned at the southeast corner of the study area, Line 4 reveals a change in thickness from 18 m to 24 m from southwest to northeast on both sides of a seamount. The sediments within these basin areas are characterized by the Ts-type acoustic facies, which could be associated with specific geological conditions favorable for REY enrichment. Perpendicular to Line 1, Line 7 has a concentrated area of rare earth-rich sediment at its northeast corner, with a thickness of 36 m. This concentration, similar to that of Line 1, indicates another area of high potential for REY enrichment. The thickness of the REY-rich sediment layer demonstrates a concentrated and contiguous distribution at the northwestern corner of the research area, with a thickness of 36 m. Additionally, a continuous layer with a thickness of up to 36 m is identified at the southwestern corner of the research area.

The REY-rich sedimentary layer in the research area exhibits a distinct east–west stratification, characterized by thinner layers at the periphery and a pronounced central thickening. In the west, the layer is notably thicker compared to the east. The thickness varies from 12 m to 36 m, with the maximum thickness reaching up to 36 m. The northwestern strip, particularly, is a zone of interest, with thicknesses predominantly between 30 m and 36 m, marking it as a pivotal region for future exploration of rare earth resources.

Nakamura and colleagues [4] have identified T-type facies (REY-rich mud) predominantly in the southern and southeastern regions of the Minamitorishima Exclusive Economic Zone (EEZ). By juxtaposing the geographic positions of the EEZ and our research area (as depicted in Figure 1) alongside the zones of rich REY accumulation, it can be inferred that a REY-rich belt may extend from the southeastern edge of the EEZ to the northwestern boundary of our study area.

5. Conclusions

Our analysis of the sub-bottom profile data from the rare earth element-rich sediments in the Pigafetta Basin of the Western Pacific has provided the following insights:

• Acoustic Facies Classification: We have classified the sub-bottom profile data into three distinct acoustic facies, differentiated by the morphology and pattern of the reflection profiles: opaque facies (O-type), transparent facies (T -type), and layered facies (L-type).
• Profile Characteristics: The survey lines with significant seabed topography undulations—ranging up to 2500 ms and 3750 ms—predominantly exhibit features of O-type and T-type facies. In contrast, where the seabed undulation is less than 1500 ms, the survey line profiles display characteristics of O-type, L-type, and T-type facies.
• Sediment Analysis: Examination of the depth variations in sediment lithology, element content, and mineral composition at the GC1 and GC2 cores from the T-type facies region has led to the conclusion that the rare earth element content is closely associated with phosphorus (P). The transparent facies (T-type), corresponding to rare earth-rich mud, is identified as a potential target horizon for the REY-rich sediment layers.
• Spatial Distribution of REY-Rich Sediments: The REY-rich sedimentary layer in our research area exhibits a distinct east–west zoning pattern. It is thinner at the peripheries, with the western part being thicker than the eastern part, and the central region showing the greatest thickness. The layer’s thickness generally varies from 12 m to 36 m, reaching a maximum of 36 m. A particularly thick strip in the northwestern area, ranging from 30 m to 36 m, is identified as a pivotal region for future exploration of rare earth resources. By comparing the geographic locations of the two regions depicted in Figure 1 (the EEZ and our research area) with known areas of rich rare earth accumulation, it is inferred that the area extending from the southeastern edge of the EEZ to the northwestern boundary of our study area likely forms part of a REY-rich accumulation zone.
• Method for Identifying Enriched Layers: The methodology presented in this article enables the identification of the potential lower boundary of the enriched layer by assessing the thickness range of the REY-rich sedimentary layer. However, further research is essential for the precise delineation of enrichment layers that may extend beyond the core’s length. This could involve analyzing the acoustic characteristics of the enriched layers within sediment cores, conducting attribute analysis along the top and bottom boundaries of deep-sea REY-rich sedimentary layers, and identifying the attribute signatures of enriched layers in sediment cores. Ultimately, this process aims to determine the possible REY-enriched layers within the broader sedimentary context through comparative analyses.