Seismic Refraction Data Research Articles

Quantitative water content estimation in landfills through joint inversion of seismic refraction and electrical resistivity data considering surface conduction

The disposal of municipal solid waste (MSW) in landfills is the prevalent method of waste management at the global scale. However, the production of landfill gases due to the methanogenic fermentation of wet MSW is a possible threat to human health and accounts for a substantial contribution to the global greenhouse gas emissions. Accordingly, information regarding water content is critical as it is an important factor triggering methane production in MSW landfills. In this study, we propose a petrophysical joint inversion scheme to quantitatively solve for the water content (WC) in landfills based on seismic refraction as well as electrical resistivity data collected at two different frequencies. In this way, we also take into account the contribution of the surface conductivity to the observed electrical response, which is crucial for a reliable quantification of the WC. Our results reveal a high water content within the MSW unit (WC > 20%) for areas characterized by a strong polarization response (normalized chargeability > 5 Mn mS/m). Such areas can be related to an increased biogeochemical activity as evidenced by the detected methane production. We observe consistent estimates between the water content resolved through the proposed joint inversion scheme and values measured in waste samples with a median percentage error of 17%. Our study demonstrates the possibility to obtain reliable estimates for the WC in MSW landfills through the petrophysical joint inversion of seismic and electrical data when surface conductivity is explicitly considered.

A 3D Snapshot of Crustal Breakup Deduced From Seismic Analysis of the Tip of the NW South China Sea

AbstractBased on a high‐resolution seismic reflection data set, we describe and map rift domains at the tip of the V‐shaped NW‐South China Sea (NW‐SCS) Basin. We compare our rift domain map with seismic refraction, magnetic and gravity field data, and previously mapped Continent‐Ocean Boundaries (COB). We also define and map syn‐breakup sedimentary and magmatic sequences across the sediment‐rich NW‐SCS, which enables us to reconstruct the tectono‐stratigraphic evolution of the area. This work enables us to describe and discuss the 3D architecture and nature of the crust at the tip of the NW‐SCS, investigate how extensional and magmatic processes interacted during the breakup, and propose a kinematic model for late rifting, breakup, and finally stalling of the seafloor spreading system. This study provides exceptionally well‐documented insights into the spatial and temporal evolution at the tip of a so‐called “rift propagator,” and enables us to show that breakup in the NW‐SCS may rather correspond to a “retrogrador,” in which the extension zone retreats due to the reorganization of a failing rift system.

Hydrogeochemical, Geophysical and Geological Assessment of the Recharge Zone of Guarani Aquifer System in Alegrete, southern Brazil

The state of Rio Grande do Sul has a huge part of the Guarani Aquifer, having a recharge area in its central and western region and, therefore, this investigation was focused on the city of Alegrete. There is no annual plan for water resources in the state and, hence, it is required to evaluate the hydrogeochemical, geophysical and geological characteristics where the aquifer has its recharge regions. Thus, some data were acquired, having chemical analysis of the water in different wells (mostly in rural areas) and refraction seismic and geoelectrical surveys. In addition, the geological information was obtained from the public database SIAGAS (Sistema de Informações de Águas Subterrâneas – ground water information system). Then, different maps of chemical parameters and a Piper diagram were made. Moreover, the refraction seismic and geoelectrical data were processed and interpreted, concatenating these results with the geological data obtained from the SIAGAS wells, mapping the different formations and static levels of the aquifer. A high concentration of lead, chromium and cadmium along with phosphate was found. Additionally, by means of geophysical surveys, a line was pointed out as a possible aquifer, which could be used for human, agropastoral or industrial activities after treatments of altered values.

Critical Zone Structure by Elastic Full Waveform Inversion of Seismic Refractions in a Sandstone Catchment, Central Pennsylvania, USA

AbstractSeismic imaging provides key information for revealing structures within Earth's critical zone (CZ) and quantifying subsurface fluid properties. The P‐wave velocity (Vp) models estimated by seismic refraction tomography or acoustic full waveform inversion (FWI) are useful to delineate the thicknesses of weathered bedrock but ambiguously map fluid properties in CZ. Considering the complementary sensitivity of S‐wave to subsurface fluid saturation, we explore advanced elastic full waveform inversion (EFWI) to estimate both Vp and S‐wave velocity (Vs) models simultaneously. Several strategies are proposed for robust EFWI implementation of noisy single (vertical) component refraction data: (a) we window seismic data to preserve early arrivals mainly including P‐wave, converted waves, and S‐wave refractions; (b) we use a correlative misfit rather than the classic L2 misfit to alleviate the interference of unreliable data amplitudes; and (c) we perform inversion using a multiscale frequency strategy with the iterative constraint with the rule of Vp/Vs > 1. We validate that the EFWI approach reliably reconstructs Vp and Vs models using synthetic data. Then, we apply our EFWI approach to seismic refraction data acquired at the Garner Run site of the Susquehanna Shale Hills Critical Zone Observatory. The inverted Vp and Vs models indicate three distinguished layers and with significant lateral and depth heterogeneities (e.g., low Vp and Vs zones). Joint analyses of Vp, Vs, and Vp/Vs with rock‐physics knowledge reveal potential gas or water gathering zones.

Evolution of excavation damaged zones in Opalinus Clay shale inferred from seismic investigations

We present the results of new and repeat gallery- and borehole-scale seismic experiments carried out in two excavation damaged zones (EDZs) of 10 (gallery 08) and 20 (gallery 98) years of age at the Mont Terri Underground Rock Laboratory in St. Ursanne, Switzerland. The initial geometry and rock mass properties of the EDZs were studied during and shortly after excavation—allowing for a direct comparison of measurements collected in 2018 to those completed in the past.In gallery 08 parts of the EDZ remain unchanged over ten years, whilst other zones show improvement (i.e., self-sealing) or considerable degradation. In addition, velocity tomograms from 2018 measure extensive areas of p-wave velocity reduction in comparison to the surrounding rock mass at depths shallower than 3.5 m. Isolated extensile fractures are still found in boreholes up to a maximum radial extent of 4.2 m. Data from borehole interval velocity measurements and cross-hole seismic surveys from this gallery suggest the damaged zone still extends up to 1.9 m and 3.1 m radial depth.Refraction seismic results in gallery 98, in contrast, suggest a less pervasive, shallower (<1.0 m) EDZ with a lower frequency of extensile fractures. The smaller circumference of this gallery can in part explain this; however, when compared to various measurements collected during and after excavation, some reduction in EDZ extent is inferable. Both borehole interval velocity measurements and seismic refraction data record p-wave velocity values higher on average than those in gallery 08 and without obvious areas of velocity reduction. Most interval velocity measurements likewise do not measure zones of decreased velocity (e.g., corresponding to rock mass damage) at this location. This suggests more self-sealing has occurred over the twenty-year post-excavation period in gallery 98 in comparison to the ten-year period in gallery 08. As repository safety depends upon the integrity of the rock mass surrounding an excavation, these results are relevant for mid-term assessments of safety (i.e., especially prior to repository closure).

VELOCITY LAYERING IN THE MIDDLE BENUE TROUGH NIGERIA USING SEISMIC REFRACTION DATA

Seismic refraction data was acquired in the Middle Benue Trough which is located in the north central Nigeria with latitude 07.5-08.5oN and longitude 08.00-09.30oE. The aim was to find out how velocity vary in the near-surface layers in the area using seismic refraction data. The seismic survey was investigated at 14 acquisition stations. Two overlapping reversed profiles were shot into 14 stations. Each acquisition station was primed with 0.20kg dynamite loaded at 1.5m below the surface. The recording equipment was McSeis-160MXTM, and monitor record was processed with Microsoft Excel software to determine the velocities from both the forward and reversed shot-points. The results show two distinct layers cases. The thickness of the topmost layer varies between 2.75m and 5.45m with a mean of 4.1m. The velocity of this topmost layer varies between 399.0 ms-1 and 767.0ms-1 with a mean of 604.8ms-1. The velocity of the underlying layer (V1) varies between 1212.5ms-1 and 3257.0ms-1 with a mean of 1757.9ms-1. The velocity increases with depth into the subsurface. The results are useful in locating groundwater aquifer, and in the design of source-receiver arrays for reflection seismic.

Hydrogeochemical analysis, geophysical assessment and geological mapping of the recharge zone of the Guarani Aquifer system in Santa Cruz do Sul, Brazil

A large part of the Guarani Aquifer is in the state of Rio Grande do Sul (southern Brazil). Aquifer?s recharge area is in the central and western regions of the state. So, this work was focused on the municipality of Santa Cruz do Sul. Because there is no water resources plan in the state by 2022, the hydrogeochemical, geophysical properties, and geological characteristics of the aquifer recharge region were evaluated. Field data were gathered, including chemical analysis of water in different wells and geophysical surveys in refraction seismic and geoelectrical methods. In addition, geological information was obtained from the Groundwater Information System (SIAGAS) public database. The different chemical parameters were mapped to perceive some elements? concentrations in Santa Cruz do Sul according to the SIAGAS data. In addition, the seismic refraction data and geoelectric method were processed and interpreted, concatenating all these results with the geology obtained from SIAGAS wells. The existing formations and the static level of the Guarani Aquifer were mapped using geological data. A high concentration of lead in the region which is a possible aquifer was found using geophysical methods. If located in this area aquifer could be used for human, agropastoral, or industrial after treatments.

Reinforcement learning in optimization problems. Applications to geophysical data inversion

&lt;abstract&gt; &lt;p&gt;In this paper, we introduce a novel inversion methodology that combines the benefits offered by Reinforcement-Learning techniques with the advantages of the Epsilon-Greedy method for an expanded exploration of the model space. Among the various Reinforcement Learning approaches, we applied the set of algorithms included in the category of the Q-Learning methods. We show that the Temporal Difference algorithm offers an effective iterative approach that allows finding an optimal solution in geophysical inverse problems. Furthermore, the Epsilon-Greedy method properly coupled with the Reinforcement Learning workflow, allows expanding the exploration of the model-space, minimizing the misfit between observed and predicted responses and limiting the problem of local minima of the cost function. In order to prove the feasibility of our methodology, we tested it using synthetic geo-electric data and a seismic refraction data set available in the public domain.&lt;/p&gt; &lt;/abstract&gt;

Seismic refraction data analysis using machine learning and numerical modeling for characterization of dam construction sites

Seismic refraction is a cost-effective tool to reveal subsurface compressional wave (P-wave) velocity. Inversion of traveltimes for estimating a realistic velocity model is a significant step in the processing of seismic refraction data. The results of the seismic data inversion are stochastic; thus, using prior information or complementary geophysical data can have a significant role in estimating the structural properties based on the observed data. Nevertheless, sufficient prior information or auxiliary data are not available in many geophysical sites. In such situations, developing advanced computational modeling is a vital step in providing primary information and improving the results. To this aim, a new inversion framework through hybrid committee artificial neural networks (CANNs) and the flower pollination (FP) optimization algorithm is introduced for inversion of refracted seismic traveltimes. Synthetic models generated by a forward-modeling approach are used to train the machine-learning model. Then, model parameters, such as the number of layers, thicknesses, and P-wave velocities, are predicted using a committee machine constructed based on several neural networks, which is achieved by averaging and stack generalization methods in which the latter method provides a better result. Then, the CANN results are used in the FP inversion algorithm to estimate the final model because it provides essential prior information on the number of layers and model parameters, which can be used in the FP searching algorithm. Our inversion procedure is tested on different synthetic data sets and applied at a dam site to determine the number of layers and their thicknesses. Our findings indicate a successful performance on synthetic and real data for automatic inversion of seismic refraction data.

Refrapy: A Python program for seismic refraction data analysis

The development and use of open-source software in geophysics over the last decade have grown considerably. However, there are few open-software alternatives for seismic methods and even fewer regarding seismic refraction data processing and inversion. We present Refrapy, an open-source package for seismic refraction data analysis written in Python, using some of the main libraries for scientific computing and geosciences. Divided into two main programs (Refrapick and Refrainv), it is possible to perform basic waveform processing, picking of first breaks, and inversion through time-terms analysis or traveltimes tomography, all with GUIs interaction. To evaluate and validate the products obtained using Refrapy, we analyzed sets of synthetic data and real field data. The calculated models recovered the geometry and velocities values of the actual synthetic models satisfactorily. The joint interpretation of the results obtained with both available inversion techniques permitted a more detailed interpretation. As for the analysis of real data, the results presented a compatible response compared to the model obtained by a well-established commercial software, satisfactorily representing the geological context interpreted initially. The software presented in this work is expected to contribute to the activities of researchers, practitioners, and students regarding the analysis of seismic refraction data as a free, open, and user-friendly alternative.

Determination of bedrock depth in Universitas Indonesia using the seismic refraction method

The seismic refraction method is used to determine the exact bedrock depth for placing a foundation pole. The study was conducted in Universitas Indonesia precisely at the Fasilkom Universitas Indonesia complex. The seismic survey configuration consists of 24 geophone channels with a length of 67.5 m, geophone intervals of 2.5 m, and near offset of 10 m. The wave source was generated using a hammer, and the distance between blows was 5 m. The secondary data used was geological data from SPT (Soil Penetration Test) borehole as a reference for comparison of seismic survey results. Seismic refraction data was processed using traditional techniques, namely the Hagedoorn’s Plus-Minus Method and tomographic inversion using Rayfract software. The correlation between the results of the process with the geological data from SPT drill point shows good results. However, the Plus-Minus Haggedorn method results are only able to show one refractor because of the data limitation, in contrast to the inversion method, which was able to show more than one refractor. There are two main refractors at a depth of 6 meters and 12 meters, and the adequate depth obtained only reaches 15 m. The maximum speed obtained is also around 900 m/s. It can be concluded up to a depth of 15 meters, and there is no recommended rock layer for placement of deep foundations for high rise buildings. A seismic survey with a longer seismic line is needed to get an underground picture exceeding 15 meters.

Insights from synthetic seismogram modelling study of oceanic lower crust and Moho Transition Zone

The Moho discontinuity is one of the fundamental boundaries on Earth, separating the crust from the upper mantle. Arrival times of seismic reflection and refraction data are used to estimate the depth of the Moho, but the nature of the Moho remains illusive. We present a synthetic seismogram modelling study for a suite of possible velocity models of the oceanic lower crust and Moho transition zone (MTZ). For modelling, we have used a 45-km long multichannel streamer (MCS) geometry, to get insight about waveforms and amplitudes of seismic phases like the crustal refraction (Pg), Moho reflection (PmP) and mantle refraction (Pn), and the interactions between these phases. We find that the nature of PmP reflection not only depends on the velocity structure of the MTZ but also on the velocity structure of the lower crust and upper mantle. The triplication resulting from these three seismic arrivals and its lateral extent mainly depend on the velocity structure in the lower crust. Apart from the velocity in the crust, the critical distance for the PmP arrival depends on the velocity gradient in the MTZ. The amplitude versus offset analysis of the wide-angle PmP arrivals indicates that there are two broad high amplitude peaks for the MTZ thickness less than or equal to the dominant seismic wavelength and only one large narrow peak for a thicker MTZ. Consequently, strong PmP arrivals for a thick MTZ lie in a narrow offset range around the critical offset whereas they lie over a large offset range for a sharp Moho. We also have modelled the synthetic seismograms for velocity structures from the Oman and the Bay of Islands ophiolites. Our modelling results indicate that the quantitative analysis of amplitudes and waveforms of ultra-long offset MCS data could provide detailed information about the lower crust and MTZ.

Application of the Stacked Refraction Convolution Section to 2D Ocean Bottom Seismometer Wide-angle Seismic Data Along the Tamayo Through Basin, Gulf of California

The stacked refraction convolution section can be used as an interpretation tool in wide-angle refraction seismic data generated by air gun shooting and recorded by Ocean Bottom Seismometers (OBS). The refraction convolution section is a full-wave extension of the Generalized Reciprocal Method (GRM), a method frequently used in shallow refraction seismic interpretation, but not applied to deep crustal-scale studies. The sum of the travel times of the waves refracted in the same interface and recorded in a pair of forward and reverse profiles, time-corrected by the reciprocal time, is an estimation close to the two-way travel times of the multichannel seismic reflection sections, but with seismic rays illuminating the interfaces upwards. The sum of seismic traces is obtained with the convolution section. Furthermore, several pairs of convolved forward-reverse refraction recordings of the same area can be stacked together to improve the signal to noise ratio. To show the applicability of the refraction convolution section in OBS deep data, we interpreted the basement structure of the Tamayo Through Basin in the southern Gulf of California, offshore Mexico. We compared the results with both, a multichannel seismic section recorded in the same profile, and the previous interpretations of the same wide-angle seismic data modeled with ray tracing and tomography methods. The basement imaged by the stacked refraction convolution section is similar in geometry to that obtained by seismic reflection processing. The stacked refraction convolution section identifies the full extent of the basement and confirms the location of a nearly constant thickness volcanic layer in the northwestern half of the basin. However, only a small area of volcanic deposits is found in the shallower parts of the southwestern margin. We also show that the convolution process can be used to estimate the occurrence of lateral variations of seismic velocities in the basement, as a further application of the GRM to deep refraction data.

Permian rifting processes in the NW Junggar Basin, China: Implications for the post-accretionary successor basins

The Junggar Basin, bounded by the West Junggar along late Carboniferous strike-slip faults in the west, is a post- orogenic successor basin within the Neoproterozoic to Paleozoic Central Asian Orogenic Belt (CAOB). The Junggar Basin is a key region for understanding the Permian rifting processes since it is located in an extensional step-over zone between the Irtysh and North Tianshan strike-slip fault zones that were most active in the Permian. Based on new seismic reflection profiles, detrital petrology and zircon U-Pb ages, together with regional geological data and a refraction seismic profile, this study investigates the tectonic evolution and the geodynamic mechanics of the basin. The early Permian phase of syn-rift mechanical subsidence in the western Junggar Basin formed half grabens. Volcanic detritus is abundant, and detrital zircon U-Pb ages show sediment supply was entirely local. During the middle Permian post-rift phase with thermal subsidence, sediment supply exceeded subsidence, and there are increasingly distant sources of sediment. The Permian regional extensional stress field between major post- accretionary strike-slip fault zones triggered the reactivation of the pre-existing late Carboniferous strike-slip faults in West Junggar and produced thinning of the crust beneath the Junggar Basin, with a β crustal stretching factor of 1.74. Seismic refraction data reveals asthenospheric upwelling resulted in underplated mafic lower crust (6–10 km). The location of initial rift axis in the Junggar Basin was controlled by the juxtaposition of thin and weak crust beneath the basin and relatively strengthened thick crust in the West Junggar attributed to the cooling of A-type granites, resulting from localized strain. Similar location of post-orogenic rifting in southwestern CAOB, in areas of high heat flow following collision, suggests that Junggar Basin is an example of a more general process.

A comparative study of neural network methods for first break detection using seismic refraction data over a detrital iron ore deposit

Accurate basement delineation is crucial for mineral deposits where formation is associated with a paleo-basement topography, such as in detrital iron ore deposits. In such environments seismic surveys provide useful data to assist the mapping of the basement interface. A key stage of the process is identification of the first arrival of elastic waves at a recording station, or so-called ‘first break’. Although a variety of automated first break detection algorithms were proposed previously for use in different seismological environments, first break detection in hard rock environments remains challenging due to low amplitude arrivals and the presence of non-stationary noise within the seismic traces. This study explores the use of three types of supervised neural networks for first break detection for a seismic refraction dataset of 15,000 traces, over a detrital iron ore deposit. A fully connected neural network (NN), a Convolutional Neural Network (CNN), and a Long-Short Term Memory recurrent neural network (LSTM) are evaluated and their predicted first breaks are compared to manually picked first breaks and predictions from the widely used Coppens’ method. The results showed that the CNN and LSTM were particularly effective for the first break detection, achieving mean square errors of 0.051 ms (with a statistical bias of 2.2 ms) and 0.058 (with a statistical bias of 21.2 ms) respectively against manual interpretation, compared to –35 ms for Coppens’ method and x ms for the NN −122 ms.

Integration of gravity, magnetic, and seismic data for subsalt modeling in the Northern Red Sea

Rifts and rifted passive margins are often associated with thick evaporite layers, which challenge seismic reflection imaging in the subsalt domain. This makes understanding the basin evolution and crustal architecture difficult. An integrative, multidisciplinary workflow has been developed using the exploration well, gravity and magnetics data, together with seismic reflection and refraction data sets to build a comprehensive 3D subsurface model of the Egyptian Red Sea. Using a 2D iterative workflow first, we have constructed cross sections using the available well penetrations and seismic refraction data as preliminary constraints. The 2D forward model uses regional gravity and magnetic data to investigate the regional crustal structure. The final models are refined using enhanced gravity and magnetic data and geologic interpretations. This process reduces uncertainties in basement interpretation and magmatic body identification. Euler depth estimates are used to point out the edges of high-susceptibility bodies. We achieved further refinement by initiating a 3D gravity inversion. The resultant 3D gravity model increases precision in crustal geometries and lateral density variations within the crust and the presalt sediments. Along the Egyptian margin, where data inputs are more robust, basement lows are observed and interpreted as basins. Basement lows correspond with thin crust ([Formula: see text]), indicating that the evolution of these basins is closely related to the thinning or necking process. In fact, the Egyptian Northern Red Sea is typified by dramatic crustal thinning or necking that is occurring over very short distances of approximately 30 km, very proximal to the present-day coastline. The integrated 2D and 3D modeling reveals the presence of high-density magnetic bodies that are located along the margin. The location of the present-day Zabargad transform fault zone is very well delineated in the computed crustal thickness maps, suggesting that it is associated with thin crust and shallow mantle.

Tectonostratigraphic evolution of the Orphan Basin and Flemish Pass region – Part 1: Results from coupled kinematic restoration and crustal area balancing

The Orphan Basin and Flemish Pass region on the Newfoundland continental margin is a frontier area in terms of oil and gas exploration and remains poorly understood in terms of structural evolution and rift development. The area has few exploration wells and, until recently, sparse seismic data coverage. Existing gravity inversion and seismic refraction data from the area suggest that stretched continental crust in the Orphan Basin is highly attenuated (locally to < 5 km) but previously published structural restorations have been unable to fully restore the continental crust below the Orphan Basin to pre-deformed thicknesses (30–32 km). Here we perform a structural restoration of a regional, WNW-ESE oriented cross-section to investigate crustal structure, rifting style, and structural evolution of the Orphan Basin and Flemish Pass region. Interpretation is constrained by modern regional depth converted broadband 2D and 3D seismic reflection data, well data, and published gravity inversion results. Present-day crustal thicknesses are used as constraints for crustal area balancing and estimation of crustal thinning across the basin. We find that low-angle extensional detachments are widespread in the area, with offsets on individual structures of up to 42 km. Zones of attenuated continental crust are coincident with the presence of low-angle detachment surfaces displaying isostatic uplift of detachment footwalls. Comparison of total extension derived from crustal area balancing vs. structural restoration suggests approximately 39% of extension across the cross-section is unresolvable in seismic data. We attribute this to (i) ductile deformation prior to brittle faulting, (ii) subseismic-scale faulting, (iii) uncertainties in detachment breakaway positions, and (iv) unresolvable later offset of detachment surfaces. Widespread and relatively evenly distributed extension across the Orphan Basin and Flemish Pass region from Middle-Late Jurassic to Early Cretaceous influences sediment distribution across the region. Calibrated regional seismic mapping of the Orphan Basin suggests that significant thicknesses of Jurassic strata exist throughout the area, including in the northwestern part of the study area, where Kimmeridgian and Tithonian strata reach combined thicknesses of up to 4.9 km.

Three-dimensional supervirtual seismic refraction interferometry: A case study in western Saudi Arabia

The semiautomatic seismic refraction supervirtual interferometry (SVI) algorithm has been developed to improve the conventional SVI method. The conventional SVI method uses convolution techniques and involves the raw trace, which reintroduces noise back into the enhanced trace. However, the semiautomatic method uses a first-arrival reference picked from a raw trace to compute the arrival times of all enhanced virtual traces. The semiautomatic SVI method has been extended recently from 2D to 3D geometry and applied on a synthetic 3D seismic data set using the raw traces of only one inline. We have developed a case study of the semiautomatic 3D SVI method by applying the algorithm on an active seismic refraction data set that consists of 82,944 raw traces from 288 shot gathers that use an accelerated weight drop source. Due to possible differences in the source wavelet among shots, the semiautomatic 3D SVI method is applied on the 288 raw traces from each shot gather separately. The SVI technique generates 41,328 distinct correlograms from one shot, which results in the production of a trace with a much better signal-to-noise ratio.

Seismic observations of a complex firn structure across the Amery Ice Shelf, East Antarctica

AbstractWe use seismic refraction data to investigate the firn structure across a suture zone on the Amery Ice Shelf, East Antarctica, and the possible role of glacier dynamics in firn evolution. In the downstream direction, the data reveal decreasing compressional-wave velocities and increasing penetration depth of the propagating wave in the firn layer, consistent with$\sim$1 m firn thickening every 6 km. The boundary between the Lambert Glacier unit to the west and a major suture zone and the Mawson Escarpment Ice Stream unit to the east, is marked by differences in firn thicknesses, compressional-wave velocities and seismic anisotropy in the across-flow direction. The latter does not contradict the presence of a single-maximum crystal orientation fabric oriented 45–$90^{\circ }$away from the flow direction. This is consistent with the presence of transverse simple shear governing the region's underlying ice flow regime, in association with elevated strain along the suture zone. The confirmation and quantification of the implied dynamic coupling between firn and the underlying ice requires integration of future seismic refraction, coring and modelling studies. Because firn is estimated to cover$\sim$98% of the Antarctic continent any such coupling may have widespread relevance to ice-sheet evolution and flow.

Imaging the P‐Wave Velocity Structure of Arctic Subsea Permafrost Using Laplace‐Domain Full‐Waveform Inversion

AbstractClimate change in the Arctic has recently become a major scientific issue, and detailed information on the degradation of subsea permafrost on continental shelves in the Arctic is critical for understanding the major cause and effects of global warming, especially the release of greenhouse gases. The subsea permafrost at shallow depths beneath the Arctic continental shelves has significantly higher P‐wave velocities than the surrounding sediments. The distribution of subsea permafrost on Arctic continental shelves has been studied since the 1970s using seismic refraction methods. With seismic refraction data, the seismic velocity and the depth of the upper boundary of subsea permafrost can be determined. However, it is difficult to identify the lower boundary and the internal shape of permafrost. Here, we present two‐dimensional P‐wave velocity models of the continental shelf in the Beaufort Sea by applying the Laplace‐domain full‐waveform inversion method to acquired multichannel seismic reflection data. With the inverted P‐wave velocity model, we identify anomalous high seismic velocities that originated from the subsea permafrost. Information on the two‐dimensional distribution of subsea permafrost on the Arctic continental shelf area, including the upper and lower bounds of subsea permafrost, are presented. Also, the two‐dimensional P‐wave velocity model allows us to estimate the thawing pattern and the shape of subsea permafrost structures. Our proposed P‐wave velocity models were verified by comparison with the previous distribution map of subsea permafrost from seismic refraction analyses, geothermal modeling, and well‐log data.