High-resolution Seismic Methods Research Articles

Two-dimensional complex wavelet transform for linear noise attenuation and image decomposition

Abstract For developing a high-fidelity, high-resolution seismic denoising method, we use the two-dimensional complex wavelet transform (2D CWT) to analyze noise and signals. By investigating a surface wave's features and evaluating factors affecting the fidelity of the method, the best practice for the wavelet transform-based denoising has been established. First, static and normal moveout correction are applied on shot gathers. Then, 2D CWT is used to attenuate linear noises. The results demonstrate that the proposed method and practice significantly attenuate noises and preserve the signal's amplitudes and frequency band. In addition to denoising, we also apply the 2D CWT to decompose a seismic image into multiscale images with different resolutions. Multiscale decomposed images derive more detailed information for subsurface structures and fault networks. The decomposed images depict sharper structures and reveal detailed features of faults more significantly than the original images.

Evaluation of Compressive Geophysical Prospecting Method for the Identification of the Abandoned Goaf at the Tengzhou Section of China’s Mu Shi Expressway

Subsidence deformation of abandoned goafs can induce cracking, distortion and even collapse of surface buildings (structures), and thus, subsidence deformation poses a great threat. Accurate detection of the abandoned goaf location and overburden morphology is an important prerequisite for stability evaluation and scientific management of surface buildings (structures), and effective detection methods are bottlenecks for accurate detection. Taking the abandoned goaf in the Tengzhou section of China’s Mu Shi expressway as an engineering example, step-by-step detection, traditional detection and combination methods are used to determine the location of the underlying abandoned goaf and overburden morphology. First, we conduct disaster investigation on the expressway and surface within the affected area of the abandoned goaf and initially determine the detection area. Then, according to the principle that the detection range can be examined step-by-step from large to small, the high-density resistivity method is used for detection, and the high-resolution seismic method is further selected to analyze the target area. Then, based on the results of the resistivity method, the position of the abandoned goaf is evaluated with the high-resolution seismic method, and the distributions of the overburden subsidence, the water-filled fractured zone and the caving zone (the three belts) are determined. Finally, boreholes are drilled deep into the bottom of the abandoned goaf at specific locations and the distributions of the abandoned goaf and three belts are verified and corrected with drilling data, acoustic detection and borehole TV imaging technology, thereby providing accurate data on abandoned goafs for highway stability evaluation.

Fine detection of water-bearing collapse column based on information fusion of seismic exploration and TEM

When exploring water-bearing collapse columns (WBCCs) in coal mines, a high-resolution seismic method can roughly locate the collapse column manually but cannot identify the collapse column water abundance. Comparatively, the transient electromagnetic method (TEM) is sensitive to the low resistivity of WBCCs, but it is difficult to delineate the boundary due to the volume effect. Therefore, the fusion processing combined with seismic wave field and transient electromagnetic field is necessary for automatic fine detection. A multifield and multiattribute information fusion model based on a neural network was constructed. The geometry, kinematics, dynamics, and statistical features contained in seismic data are often described by seismic attributes, and the resistivity of the water in TEM is relatively low. Therefore, we first extracted the TEM inversion resistivity and optimized eight seismic attributes sensitive to the target geologic structure. Then, a backpropagation neural network (BPNN) optimized using particle swarm optimization (PSO-BPNN) was selected as the fusion method for model test and engineering applications. By the test model, we verified the feasibility and effectiveness of using PSO-BPNN to predict different water-bearing collapse columns in coal mine. In engineering applications, the attributes in borehole were used for PSO-BPNN training, and a good fusion result was achieved. Results show that the nine attributes can be used as the main feature set for the information fusion of the WBCC based on seismic method and TEM. Combined with the features, the PSO-BPNN can detect the boundary and water abundance of the collapse column in coal mine, which can greatly improve detection accuracy.

Analysis of High-Resolution Seismic Exploration in Coalfield Geological Exploration Methods

In the field of coalfield geological research, the era of traditional geological research has basically ended. At present, it is in the period of earth system scientific research. Starting from the main factors affecting high-resolution seismic exploration and combining with some examples of high-resolution seismic exploration in coal exploration,.this paper analyzes and discusses the middle and high-resolution seismic exploration methods in coal exploration, mainly discusses several key links of high-resolution seismic exploration in the field data acquisition process, and makes a brief summary and induction of seismic wave excitation, seismic wave reception, demonstration of acquisition methods and noise suppression.

Geophysical investigations in the Radovna River Spring area (Julian Alps, NW Slovenia)

vodeAbstractThe Radovna River Valley is located in the north-western part of Slovenia in the Julian Alps, where there is an extensive intergranular aquifer whose depth to pre-Quaternary bedrock is unknown. Therefore, to obtain information about the depth of the valley and the geometry of the aquifer two geophysical methods were used in our study; ground penetrating radar (GPR) and seismic reflection method. The low-frequency GPR method has shown to be useful for determining the depth of the groundwater and the predominant groundwater recharge. Also, the high-resolution seismic method provided an insight about the morphology of the pre-Quaternary basement with the deepest point at 141 meters below surface. Measurements of hydrogeological parameters such as groundwater level and river discharge measurements were carried out in the study area. Both data analyses showed that groundwater level and river discharge are highly fluctuating and rapidly changing, indicating a well-permeable aquifer, implying that such an aquifer is extremely sensitive and vulnerable to extreme climate events. Both the geophysical methods and the hydrogeological information have provided important information about the morphology of the valley and the alluvial aquifer, as well as increasing the knowledge about the Radovna springs system, which will contribute very important information for future hydrogeological studies.

Can no-bubble methane seeps affect biological traits of benthic macroinvertebrates in coastal systems?

Methane seep studies often target sites where methane gas is actively bubbling from the sea floor to the water column. However, gas bubbles often remain trapped in surficial seabed sediments, especially in coastal systems; notably few ecological studies exist for these systems. These environments form patches on the seafloor in the Bahia Blanca estuary that, based on taxonomic analysis, represent a distinct benthic habitat. Here we examine whether taxonomic changes in benthic communities associated with trapped methane gas bubbles are related to functional changes. We found significant differences in biological traits of macroinvertebrates weighted by abundance and by species between two sites differing in gas presence and between spring and winter seasons. Compared with the control (no gas) site, the gas site had subsurface-deposit feeders and large sessile animals partially calcified with suspension-feeding mode. In contrast, at the control site the small and non-calcified animals are discretely motile, building burrows and tubes and feeding on surface deposit material. Trapped methane gas, detected with seismic methods, changes biological traits in benthic fauna and can lead to alteration of some ecosystem functions. This suggests the utility of high-resolution seismic methods for detect these benthic habitats with high spatial accuracy, and a need to further understanding of ecosystem processes associated with gas presence in the context of both direct anthropogenic and climate-change pressures.

On cycle-skipping and misfit function modification for full-wave inversion: Comparison of five recent approaches

Full-waveform inversion, a high-resolution seismic imaging method, is known to require sufficiently accurate initial models to converge toward meaningful estimations of the subsurface mechanical properties. This limitation is due to the nonconvexity of the least-squares distance with respect to the kinematic mismatch. We propose a comparison of five misfit functions promoted recently to mitigate this issue: adaptive waveform inversion, instantaneous envelope, normalized integration, and two methods based on optimal transport. We explain which principles these methods are based on and illustrate how they are designed to better handle kinematic mismatch than a least-squares misfit function. By doing so, we can exhibit specific limitations of these methods in canonical cases. We further assess the interest of these five approaches for application to field data based on a synthetic Marmousi case study. We illustrate how adaptive waveform inversion and the two methods based on optimal transport possess interesting properties, making them appealing strategies applicable to field data. Another outcome is the definition of generic tools to compare misfit functions for full-waveform inversion.

Crystal preferred orientations, deformation mechanisms and seismic properties of high pressure metamorphic rocks from the Central Qiangtang metamorphic belt, Tibetan Plateau

Crystal preferred orientation (CPO) of high pressure metamorphic rocks is an important contributor to seismic anisotropy in the subduction interface and continental crust. However, the mechanisms of deformation and CPO development, as well as the effect of CPO patterns on the seismic anisotropies remain not fully solved. By analyzing the microstructures, CPOs and seismic properties of the high pressure rocks from the Central Qiangtang metamorphic belt in the northern Tibetan Plateau, we found that the mineral CPOs (e.g., omphacite and amphibole) are developed dominantly by non-dislocation based deformation mechanisms in the eclogites and amphibolites, while quartz and phengite CPOs are primarily formed by dislocation creep in the garnet-quartz-mica schists. The variations of CPO patterns are mainly controlled by the strain geometry and/or strain magnitude, which also regulate the first-order symmetry patterns of seismic anisotropies among samples. Besides, amphibole enrichment can remarkably increase the anisotropies and decrease the velocities of seismic waves in the eclogite‒amphibolite suite. A combination of seismic parameters such as Vp, Vs, Vp/Vs ratio, intensities and symmetries of P-wave anisotropy (AVp) and shear wave splitting (AVs), and P- or S-wave reflection coefficients can be used to discriminate eclogites, amphibolites and their garnet-quartz-mica schists country rocks at depth when high resolution seismic methods can be applied. Notably, amphibolite and garnet-quartz-mica schists showing moderate-to-large AVp and max. AVs can make considerable contributions to field-observed crustal anisotropy near the strike-slip continental shear zones or mantle wedge anisotropy in the subduction zones.

High resolution seismic waveform migration location method and its applications to induced seismicity

Locating seismic events is a central task for earthquake monitoring. Compared to arrival-based location methods, waveform-based location methods do not require picking phase arrivals and are more suitable for locating seismic events with noisy waveforms. Among waveform-based location methods, one approach is to stack different attributes of P and S waveforms around arrival times corresponding to potential event locations and origin times, and the maximum stacking values are assumed to indicate the correct event location and origin time. In this study, to obtain a high-resolution location image, we improve the waveform-based location method by applying a hybrid multiplicative imaging condition to characteristic functions of seismic waveforms. In our new stacking method, stations are divided into groups; characteristic functions of seismic waveforms recorded at stations in the same group are summed, and then multiplied among groups. We find that this approach can largely eliminate the cumulative effects of noise in the summation process and thus improve the resolution of location images. We test the new method and compare it to three other stacking methods, using both synthetic and real datasets that are related to induced seismicity occurring in petroleum/gas production. The test results confirm that the new stacking method can provide higher-resolution location images than those derived from currently used methods.

Characterizing flow in the first hundred-meter depth of a fractured aquifer using hybrid seismic methods, acoustic logging, and flow-log measurements

Understanding subsurface flow, especially in fractured rocks only housing water through a few preferential pathways, is still challenging. The point is mainly associated with the poor accessibility of the subsurface and the lack of accurate representations for both heterogeneity and spatial distribution of water bearing bodies. This notwithstanding, highly-resolved geophysical investigations bring new images of the subsurface. This is exemplified over a fractured limestone aquifer at the site scale (for example, that of the radius of influence of an extraction well). On an experimental site, situated in the Cher region (France), two boreholes have been drilled for field experiments. Full Waveform Acoustic Logging (FWAL) and seismic experiments were conducted. Hybrid seismic imaging, which consists in combining refraction and reflection seismic results, has been carried out. Based on a four-step procedure, the processing of refracted and reflected waves provided two sections. After assemblage, these sections produced in a first step an extended time reflectivity section starting from the surface and, in a second step, a section over depth after calibration with Vertical Seismic Profile (VSP) and acoustic data. However, even the Very High Resolution (VHR) seismic methods do not have a sufficient vertical resolution to describe accurately the geological formation. The acoustic sections were processed to separate the different wave fields, to extract the criss-cross events and to build a criss-cross index log. A log of fracturation index, based on both criss-cross index and P-wave velocity measurements, was computed to detect the presence of fractures. After calibration, and under the assumption that the slower the P-wave velocity, the higher the permeability – porosity, a 3D seismic block of reflection can inform on preferential areas where flow should occur. At the scale of an open wellbore, acoustic loggings that measure wave velocities over a short distance within the well also inform on open features crosscut by the well. Finally, flow log measurements confirm the occurrence of flowing horizons that were previously marked by both seismic and acoustic data. Seismic and acoustic data are therefore suited to image contrasted hydraulic properties over fractured subsurface systems usually poorly documented.

ПРОЯВЛЕНИЯ СОВРЕМЕННОЙ ФЛЮИДОДИНАМИЧЕСКОЙ АКТИВНОСТИ НА СЕВЕРНОМ КАСПИИ (ГЕОФИЗИЧЕСКИЕ ДАННЫЕ)

The paper presents analysis of geological-geophysical data, both from archives and collected in recent expeditions by Shirshov Institute of Oceanology, RAS (SIO RAS) together with satellite data. The interpretation shows modern rise of fluid dynamic activity on North Caspian. Based on summary of previous data, it is possible to recognize ryithmostratigraphic and seismostratigraphic complexes. Side sonar data shows many reflexing objects on the sea bottom. One of the most prominent features of Northern and Central Caspian is great number of pipe-like vertical bodies, they were discovered by SIO RAS expeditions with high-resolution seismic methods. These “pipes” go down beyond seismic section data. All specified seismic anomalies and bottom microrelief can be considered as evidence of vertical migration of gas and water via sediments and its escaping to water by gas and water grifons. Satellite images of North and Central Caspian show great amount of sliks and oil signs on the sea surface, and some of them present constantly on the same place, so it is possible to claim the sub-bottom fluid escape theory. The paper shows connection between bottom structures and grifon activity via satellite images (spatial-time grouping of oil slicks) of 2011–2019.

Optimal transport for mitigating cycle skipping in full-waveform inversion: A graph-space transform approach

Optimal transport distance has been recently promoted as a tool to measure the discrepancy between observed and seismic data within the full-waveform-inversion strategy. This high-resolution seismic imaging method, based on a data-fitting procedure, suffers from the nonconvexity of the standard least-squares discrepancy measure, an issue commonly referred to as cycle skipping. The convexity of the optimal transport distance with respect to time shifts makes it a good candidate to provide a more convex misfit function. However, the optimal transport distance is defined only for the comparison of positive functions, while seismic data are oscillatory. A review of the different attempts proposed in the literature to overcome this difficulty is proposed. Their limitations are illustrated: Basically, the proposed strategies are either not applicable to real data, or they lose the convexity property of optimal transport. On this basis, we introduce a novel strategy based on the interpretation of the seismic data in the graph space. Each individual trace is considered, after discretization, as a set of Dirac points in a 2D space, where the amplitude becomes a geometric attribute of the data. This ensures the positivity of the data, while preserving the geometry of the signal. The differentiability of the misfit function is obtained by approximating the Dirac distributions through 2D Gaussian functions. The interest of this approach is illustrated numerically by computing misfit-function maps in schematic examples before moving to more realistic synthetic full-waveform exercises, including the Marmousi model. The better convexity of the graph-based optimal transport distance is shown. On the Marmousi model, starting from a 1D linearly increasing initial model, with data without low frequencies (no energy less than 3 Hz), a meaningful estimation of the P-wave velocity model is recovered, outperforming previously proposed optimal-transport-based misfit functions.

The Application of Broadband Remodeling Inversion in Saertu Oil Bed of the First Northeast Area

There is a problem in forecasting thin sandstone in development with the earthquake in the area with dense well network. The research was done on Saertu oil bed in the first northeast area of Lamadian reservoir Make use of the fuzzy system as recovery of the low frequency of the wave impedance inversion, and use the parawave independence remodeling inversion as the compensation of the high frequency, the broadband remodeling inversion came true. The remodeling inversion result not only the thick sandstone more than 5 meters and the thin sandstone less than 2 metres could be recognized. As a result, sedimentary phase modified by well to seismic integration integration after the vertical contact and spatial distribution of sandstone accurately locating, provided the reliable analysis inference for the research of the remaining oil in the thin sandstone. With the constant improvement of the degree of oilfield development,Daqing oilfield has entered the stage of high watercut stage mining of Changyuan,caused by factors such as reservoir heterogeneity in residual oil highly fragmented distribution, that is easy to develop the potential for residual oil in thick oil layer decreasing year by year,and more and more proportion of remaining oil in the oil sheet,remaining recoverable reserves of effective development more and more difficult,seriously affect the oilfield development,so looking for thin oil reservoir remaining oil problems aroused people's more and more attention. According to the company "build a new Changyuan oilfield geological understanding system, looking for determining further improve oil recovery potential" the instruction spirit, in 2006 decided to carry out full cover in Daqing Changyuan fine three dimensional seismic work development, further defined the fine development of Changyuan earthquake "315" project research goal in 2008. Completed in 2008 in Lamadian oil field 100km2 digital detection of Changyuan three-dimensional threecomponent seismic acquisition and Saertu 690km2 figure oilfield high density 3D seismic acquisition work, since then reveal the prelude of seismic reservoir development work. With the deepening of the reservoir prediction work, in thin reservoirs in Lamadian oilfield remaining oil research is becoming more and more attention.The writer use the north of the east area Saertu figure reservoir as the goal, to take advantage of high resolution seismic inversion method to carry out the thin sandstone reservoir inversion, objectively explained reservoir spatial distribution characteristics of dense well pattern.

An optimal transport approach for seismic tomography: application to 3D full waveform inversion

The use of optimal transport distance has recently yielded significant progress in image processing for pattern recognition, shape identification, and histograms matching. In this study, the use of this distance is investigated for a seismic tomography problem exploiting the complete waveform; the full waveform inversion. In its conventional formulation, this high resolution seismic imaging method is based on the minimization of the L2 distance between predicted and observed data. Application of this method is generally hampered by the local minima of the associated L2 misfit function, which correspond to velocity models matching the data up to one or several phase shifts. Conversely, the optimal transport distance appears as a more suitable tool to compare the misfit between oscillatory signals, for its ability to detect shifted patterns. However, its application to the full waveform inversion is not straightforward, as the mass conservation between the compared data cannot be guaranteed, a crucial assumption for optimal transport. In this study, the use of a distance based on the Kantorovich–Rubinstein norm is introduced to overcome this difficulty. Its mathematical link with the optimal transport distance is made clear. An efficient numerical strategy for its computation, based on a proximal splitting technique, is introduced. We demonstrate that each iteration of the corresponding algorithm requires solving the Poisson equation, for which fast solvers can be used, relying either on the fast Fourier transform or on multigrid techniques. The development of this numerical method make possible applications to industrial scale data, involving tenths of millions of discrete unknowns. The results we obtain on such large scale synthetic data illustrate the potentialities of the optimal transport for seismic imaging. Starting from crude initial velocity models, optimal transport based inversion yields significantly better velocity reconstructions than those based on the L2 distance, in 2D and 3D contexts.

Near surface geophysics: Investigating the dynamic coastal environment — Introduction

The coastal zone is a habitat for much of the world’s population, a key economic driver for fishing, shipping, recreation and tourism, and a critical wetlands ecology. However, the coastal zone is also a dynamic setting undergoing significant change at many different time scales. Large storms, for example, can restructure coastal morphology on the scale of hours or days. Rising sea level or changes in sediment supply will alter the coastline over decades to centuries and longer. These are human-relevant time scales, and such changes can have major impacts on coastal infrastructure, habitability, and ecology. Human activity also impacts the coast; e.g., hardening of coastlines and inlets, reduction in sediment supply due to damming, artificial beach replenishment, extraction of ground water for human consumption and agriculture, and the introduction of pollutants. Both land- and marine-based near-surface geophysical methods can play a significant role in understanding the many processes that impact …

Seismic imaging to help understand and manage water quality in coastal Bénin, West Africa

We collected seismic data along 15 transects to characterize the geometry of a coastal aquifer in Bénin, West Africa, that is being contaminated by saltwater. We used standard high-resolution seismic methods to image the upper [Formula: see text] using a sledgehammer source and a 120-channel recording system. Three transects were processed with an iterative updating flow that includes prestack depth migration, residual moveout analysis, and reflection tomography, and the remaining 12 transects were processed with routine processing flows and poststack time migration. We identified one unconfined aquifer and three confined aquifers separated by reflective confining clay layers. Some transects showed areas of missing reflectors, which we interpreted as sand-filled channels that could provide potential high-permeability conduits for saltwater flow to the Godomey well field.

The Technologic Research and Application of Frequency-Divided Inversion in SL Oilfield

Differing from the current inversion methods, the frequency-divided inversion is depend on well logging and seismic data, using the methods of support vector computer to study the amplitude response in different detective frequencies (amplitude versus frequency, simply called AVF), taking AVF as independent information of inversion and creating the non-linear mapping relationship between wave impedance curves of logging data and seismic waveform, fully using the information of low, medium and high frequencies in seismic data to reduce the inversion uncertainty and get a higher resolution result. Frequency-divided inversion is also a high resolution seismic inversion method which no needs wavelet extraction and prior model.

Optimising seismic data acquisition parameters for open-cut coal exploration using a shallow high resolution reflection seismic test survey

A shallow high-resolution seismic reflection test survey was conducted in the Maryborough Basin in an area near Gundiah, Queensland. The purpose of the seismic reflection test survey was to evaluate the application of using high resolution seismic data acquisition methods to detect thin coal seams at between 30m to 150m depth for open-cut coal exploration. The seismic test survey tested three seismic sources which included a 50kg weight-drop from 2m height, a 7kg sledgehammer, and a 12g (blank cartridge) in-hole shotgun. Seismic forward modelling indicated that thin coal seams (1 to 2m thickness) should provide seismic reflections at shallow depths of 30m, using 72 channel recording with 2m geophone intervals. The seismic source tests showed that the 12g in-hole shotgun produced the strongest seismic signal at the furthest geophone offsets. The seismic recording spread consisted of 96 seismic channels with single spiked 30hz geophones spaced at 2m geophone intervals. Based on the seismic source tests, the 12g in-hole shotgun was used to record a seismic test line with shot-points recorded from one end of the 96 channel spread through to the other end of the spread. The recording arrangements provided seismic data to both test data quality with shot to receiver offset, and examine the optimum common depth point (CDP) fold coverage to use for a larger seismic survey. Based on the seismic data processing results, a larger seismic survey could be conducted using a 72 channel recording spread with 4m geophone and shot-point intervals and achieve production rates of 400m per day with a 3-man crew.

Seismic processing, inversion, and AVO for gold exploration — Case study from Western Australia

We investigate the potential of using high-resolution seismic methods for rock characterization and for targeting of gold deposits at the St. Ives gold camp. The application of seismic methods in hard-rock environments is challenged by complex structures, intrinsically low signal-to-noise ratio, regolith distortions, and access restrictions. If these issues can be addressed, then the unparalleled resolving power of reflection seismic can be used for mineral exploration. Appropriate spatial sampling of the wavefield combined with a survey geometry design and rigorous data processing to incorporate high fold and long offsets are necessary for creation of high-quality seismic images. In the hard-rock environment of Western Australia, accurate static corrections and multiphase velocity analysis are essential processing steps. This is followed by a rigorous quality control following each processing step. In such a case, we show that the role of reflection seismic could be lifted from mere identification of first-order structures to refined lithological analyses. Five deep boreholes with sonic logs and core sample test data were used to calibrate 2D seismic images. Despite seismic images were produced with relatively robust scaling it was possible to achieve reasonably high seismic-log correlation across three of the tightly spaced boreholes using a single composite wavelet. Amplitude-versus-offset (AVO) analysis indicated that gold-bearing structures may be related to elevated AVO effect and increased reflectivity. Consequently, partial stack analysis and acoustic and elastic inversions were conducted. These results and impedance crossplots were then evaluated against known gold occurrences. While still in the preliminary stages, hard-rock seismic imaging, inversion, and the application of AVO techniques indicated significant potential for targeting mineral reserves.

Near-surface Seismic Investigation of Barringer (Meteor) Crater, Arizona

We investigated the shallow subsurface of Barringer (Meteor) Crater, Arizona using high-resolution seismic methods. The seismic surveys were conducted in May, 2010 during a joint expedition by the University of Houston, the University of Texas at Austin, and the Lunar and Planetary Institute (LPI). We performed compressional (P)-wave refraction analysis on the seismic data and found P-wave velocities of 450–2,500 m/s for a 55-m deep model. Away from the crater rim (toward the south), the shallow P-wave low-velocity layers thin. We also estimated a near-surface, shear (S)-wave velocity structure using a surface-wave inversion method. S-wave velocities vary from 200–700 m/s for the top 16–20 m, increasing to 900–1,000 m/s at 38-m depth. We interpret a prominent change in S-wave velocity (at around 500–600 m/s) as the transition from the ejecta blanket (a sheet of debris thrown out of the crater during the impact) to the bed-rock Moenkopi sandstone. The ejecta is characterized as unconsolidated, low velocity, and low density. This S-wave transition takes place at a depth range of 12–20 m near the crater rim with a thinning away from the crater rim. This consistent P-wave and S-wave structure is interpreted as the ejecta blanket. Ultrasonic measurements on hand samples collected during the expedition give a range of P-wave velocities of 800–1,600 m/s for the Moenkopi. Predicted bulk densities from estimated S-wave velocities using modified Gardner's equation fall in the range of 1.8–2.5 gm/cm3, with low-density materials (ejecta) underlain by high-density materials (bedrock). These density results, along with available drilling information and residual gravity anomalies, also support the thinning of the ejecta blanket.