Refraction Inversion Research Articles

Sub-surface configuration in the northern part of Lembang groundwater basin recharge area

Lembang groundwater basin has an area of 209 km2. Increased tourism activities in the Lembang Groundwater Basin require sufficient groundwater resources to support the sustainability of these tourism activities. The purpose of this study was to analyze the hydrostratigraphy of the groundwater recharge zone of the Lembang groundwater basin in the Cikole-Lembang. Electrical Resistivity Tomography (ERT) with dipole–dipole electrode array and seismic refraction was used to analyze the hydrostratigraphy. The results of ERT and seismic refraction inversion show that Tangkubanparahu Pyroclastic Fall 2 can be characterized as aquifers. This layer consists of low (123–292 Omega m) and intermediate (293–700 Omega m) resistivity value and also has low (300–1350 m/s) to medium (2700–1350 m/s) velocity. Tangkubanparahu lava (Tl) layer has a high resistivity value (701–3875 Omega m) and high (1350–2999 m/s) velocity. The Tangkubanparahu Pyroclastic Fall 1 has a resistivity of 300–700 ohms. Tangkubanparahu (Tl) lava deposits can be characterized as fracture aquifer and impermeable layers in the Tangkubanparahu volcanic hydrogeological system depend on the historical structural geology event.

Bayesian Seismic Refraction Inversion for Critical Zone Science and Near‐Surface Applications

AbstractThe critical zone (CZ) is the region of the Earth’s surface that extends from the bottom of the weathered bedrock to the tree canopy and is important because of its ability to store water and support ecosystems. A growing number of studies use active source shallow seismic refraction to explore and define the size and structure of the CZ across landscapes. However, measurement uncertainty and model resolution at depth are generally not evaluated, which makes the identification and interpretation of CZ features inconclusive. To reliably resolve seismic velocity with depth, we implement a Transdimensional Hierarchical Bayesian (THB) framework with reversible‐jump Markov Chain Monte Carlo to generate samples from the posterior distribution of velocity structures. We also perform 2D synthetic tests to explore how well THB traveltime inversion can resolve different subsurface velocity structures. We find that THB recovers both sharp changes in velocity as well as gradual velocity increases with depth. Furthermore, we explore the velocity structure in a series of ridge‐valley systems in northern California. The posterior velocity model shows an increasing thickness of low velocity material from channels to ridgetops along a transect parallel to bedding strike, implying a deeper weathering zone below ridgetops and hillslopes than below channels. The THB method enhances the ability to reliably image CZ structure, and the model uncertainty estimates it yields provides an objective way to interpret deep CZ structure. The method can be applied across other near‐surface studies, especially in the presence of significant surface topography.

Anisotropic P-wave travel-time tomography implementing Thomsen's weak approximation in TOMO3D

Abstract. We present the implementation of Thomsen's weak anisotropy approximation for vertical transverse isotropy (VTI) media within TOMO3D, our code for 2-D and 3-D joint refraction and reflection travel-time tomographic inversion. In addition to the inversion of seismic P-wave velocity and reflector depth, the code can now retrieve models of Thomsen's parameters (δ and ε). Here, we test this new implementation following four different strategies on a canonical synthetic experiment in ideal conditions with the purpose of estimating the maximum capabilities and potential weak points of our modeling tool and strategies. First, we study the sensitivity of travel times to the presence of a 25 % anomaly in each of the parameters. Next, we invert for two combinations of parameters (v, δ, ε and v, δ, v⊥), following two inversion strategies, simultaneous and sequential, and compare the results to study their performance and discuss their advantages and disadvantages. Simultaneous inversion is the preferred strategy and the parameter combination (v, δ, ε) produces the best overall results. The only advantage of the parameter combination (v, δ, v⊥) is a better recovery of the magnitude of v. In each case, we derive the fourth parameter from the equation relating ε, v⊥ and v. Recovery of v, ε and v⊥ is satisfactory, whereas δ proves to be impossible to recover even in the most favorable scenario. However, this does not hinder the recovery of the other parameters, and we show that it is still possible to obtain a rough approximation of the δ distribution in the medium by sampling a reasonable range of homogeneous initial δ models and averaging the final δ models that are satisfactory in terms of data fit.

Robust refraction statics solution and near-surface velocity model building using feedback from reflection data

An accurate near-surface velocity model is critical for weathering statics correction and initial model building for depth migration and full-waveform inversion. However, near-surface models from refraction inversion often suffer from errors in refraction data, insufficient sampling, and over-simplified assumptions used in refraction algorithms. Errors in refraction data can be caused by picking errors resulting from surface noise, attenuation, and dispersion of the first-arrival energy with offset. These errors are partially compensated later in the data flow by reflection residual statics. Therefore, surface-consistent residual statics contain information that can be used to improve the near-surface velocity model. We have developed a new dataflow to automatically include median and long-wavelength components of surface-consistent reflection residual statics. This technique can work with any model-based refraction solution, including grid-based tomography methods and layer-based methods. We modify the cost function of the refraction inversion by adding model and data weights computed from the smoothed surface-consistent residual statics. By using an iterative inversion, these weights allow us to update the near-surface velocity model and to reject first-arrival picks that do not fit the updated model. In this nonlinear optimization workflow, the refraction model is derived from maximizing the coherence of the reflection energy and minimizing the misfit between model arrival times and the recorded first-arrival times. This approach can alleviate inherent limitations in shallow refraction data by using coherent reflection data.

Near-surface Seismic Velocity Model Building from First Arrival Travel-times - A Case Study from an Onshore, Niger Delta Field

Seismic refraction was used to image the near-surface, using a3D seismic data acquired from an onshore Niger Delta field. The objective of the study was to build a near-surface model of the earth in terms of sub-weathering layer thicknesses and seismic wave velocities and highlight the near-surface seismic wave velocity variation in the region. A velocity-depth model consisting of the weathering layer and three consolidated (sub-weathering) layers was first estimated. The first-breaks were picked and used as input for this study. The models were generally consistent in terms of their large-scale features, demonstrating the robustness of refraction data inversion in general. Results support confidence in the reliability and robustness of the refraction inversion method. The method of analysis and resolution for 3-layer models adopted in this study may be used in resolving such cases observed during processing of seismic reflection data from the Niger-Delta Basin.

Geophysical Evidence for a San Andreas Subparallel Transtensional Fault along the Northeastern Shore of the Salton Sea

Abstract The southern San Andreas fault (SSAF) accommodates a significant amount of strain between the Pacific and North American plates; thus, the fault represents a major geohazard to the populated areas of southern California, in particular the larger Los Angeles metropolitan area. Paleoseismic chronology of ruptures along the SSAF segment suggests this fault is near the end of its interseismic period (∼180 years), because it has not ruptured in historic times (∼320 years). A recent active‐source seismic experiment performed in the Salton Sea west of the SSAF provides evidence for extensional deformation along the northeastern shore of the Salton Sea. This study posits that the extensional deformation is due to a previously unmapped fault, here named the Salton trough fault (STF). The seismic reflection data image a divergent sediment package that dips toward the northeast with thicknesses up to at least 2 km. Refraction inversion produces a southwestward‐dipping velocity discontinuity that crops out east of the SSAF surface trace, consistent with the existence of a southwest to northeast gradient in lithology. If present, the existence of the STF has scientific and societal relevance. First, the STF appears to control the recent Salton trough architecture north of Bombay Beach. Second, from a seismological hazards perspective, the presence of this structure could alter the current understanding of stress transfer and rupture dynamics in the region, as well as community fault models and ground‐motion simulations on the SSAF. Online Material: Uninterpreted seismic‐reflection profiles.

Blind Test of Methods for Obtaining 2-D Near-Surface Seismic Velocity Models from First-Arrival Traveltimes

Seismic refraction methods are used in environmental and engineering studies to image the shallow subsurface. We present a blind test of inversion and tomographic refraction analysis methods using a synthetic first-arrival-time dataset that was made available to the community in 2010. The data are realistic in terms of the near-surface velocity model, shot-receiver geometry and the data's frequency and added noise. Fourteen estimated models were determined by ten participants using eight different inversion algorithms, with the true model unknown to the participants until it was revealed at a session at the 2011 SAGEEP meeting. The estimated models are generally consistent in terms of their large-scale features, demonstrating the robustness of refraction data inversion in general, and the eight inversion algorithms in particular. When compared to the true model, all of the estimated models contain a smooth expression of its two main features: a large offset in the bedrock and the top of a steeply dipping low-velocity fault zone. The estimated models do not contain a subtle low-velocity zone and other fine-scale features, in accord with conventional wisdom. Together, the results support confidence in the reliability and robustness of modern refraction inversion and tomographic methods.

Uncertainty in Near-Surface Refraction Inversion

Uncertainty in the tomographic inversion of near-surface seismic refraction data can be separated into aleatory variability, which describes the misfit errors and epistemic uncertainty, which describes the suite of acceptable models. Common implementations of refraction tomography usually focus on reducing aleatory variability and frequently disregard epistemic uncertainty. In this study, the tomograms generated with three models of the seismic velocities in both the weathering and in the sub-weathering, using the generalized reciprocal method (GRM), are consistent with the traveltime data. However, only one tomogram is consistent with the optimum XY value and the attributes derived from the head wave amplitudes and seismic velocities. This study demonstrates that epistemic uncertainty can be explicitly addressed with the GRM, because the most probable tomogram can be selected objectively from a number of acceptable alternatives. The GRM-based tomogram successfully detects, defines and differentiates narrow regions with low seismic velocities which represent shear zones and a massive sulphide ore body. None of these zones is detected with the tomogram generated with the default starting model using smooth vertical velocity gradients. It is concluded that minimizing epistemic uncertainty through the use of the most appropriate starting model is more important than minimizing aleatory variability.

Resolution and ambiguity studies for a series expansion based multilayer refraction inversion method

The paper discusses the characteristics and feasibility of a multilayer kinematic refraction inversion method that is applicable to estimate the — even laterally changing — parameters of a geological model in the simultaneous evaluation of refraction data. The studied method has been developed at the Department of Geophysics of the University of Miskolc for the interpretation of parameters (layer thicknesses and seismic velocities) using series expansion based on adequately chosen basis functions. Besides revealing its resolution properties, ambiguity studies are also presented on synthetic data sets. In the terminology of refraction inversion uncertain and ambiguous results are frequently encountered problems in multilayer cases when all parameters (or all coefficients describing the parameters) are estimated simultaneously in the same inversion procedure. Results of resolution and ambiguity analyses of synthetic data sets and field examples are shown in this study.

Joint Shear‐Wave Analysis Using MASW and Refraction Traveltime Tomography

We use the multichannel analysis of surface waves (MASW) and SH-wave refraction traveltime tomography to constrain each method’s starting layer-model at a site located at the Yuma Proving Ground, AZ. Our treatment resulted in a more stratified, higher-resolution MASW S-wave velocity (Vs) model that deviated from the tomographic SH-wave solution by approximately 15%. The top 12 m were our target at the test site, which is comprised of roughly 30 m of unconsolidated sediments overlying bedrock. A bungee-assisted drop-weight source was used for MASW and a hammer and block were used for the refraction survey. A preliminary MASW profile served as the starting model for refraction inversion. The resultant Vs tomogram contained slightly higher velocity ranges and imaged a high-velocity layer not apparent with initial MASW results. Using this a priori information, along with density values from well logs, more layers were added to an updated Vs model for MASW inversion. The subsequent higher-resolution MASW profile moved toward the tomographic solution. The use of two inversion schemes allowed a more constrained Vs model of the site, while limiting resolution uncertainties for the MASW Vs section. Moreover, the possible over-parameterization problem seen with increased layers is defended through refraction comparison. We recommend the combined approach as a means to constrain MASW inversion and increase confidence in future Vs appraisals.

Response to comments by Robert J. Whiteley on: Palmer, D., 2010. Is visual interactive ray trace an efficacious strategy for refraction inversion? Exploration Geophysics, 41, 260-267

I thank Whiteley for his comments on my critique of visual interactive ray trace (VIRT). Whiteley has disputed virtually every aspect of my critique, including the title. This is not surprising, gi...

Non-Uniqueness with Refraction Inversion – The Mt Bulga Shear Zone

Non-Uniqueness with Refraction Inversion – The Mt Bulga Shear Zone

Non‐uniqueness with refraction inversion – the Mt Bulga shear zone

ABSTRACTThe tau‐p inversion algorithm is widely employed to generate starting models with many computer programs that implement refraction tomography. However, this algorithm can frequently fail to detect even major lateral variations in seismic velocities, such as a 50 m wide shear zone, which is the subject of this study.By contrast, the shear zone is successfully defined with the inversion algorithms of the generalized reciprocal method. The shear zone is confirmed with a 2D analysis of the head wave amplitudes, a spectral analysis of the refraction convolution section and with numerous closely spaced orthogonal seismic profiles recorded for a later 3D refraction investigation.Further improvements in resolution, which facilitate the recognition of additional zones with moderate reductions in seismic velocity, are achieved with a novel application of the Hilbert transform to the refractor velocity analysis algorithm. However, the improved resolution also requires the use of a lower average vertical seismic velocity, which accommodates a velocity reversal in the weathering. The lower seismic velocity is derived with the generalized reciprocal method, whereas most refraction tomography programs assume vertical velocity gradients as the default.Although all of the tomograms are consistent with the traveltime data, the resolution of each tomogram is comparable only with that of the starting model. Therefore, it is essential to employ inversion algorithms that can generate detailed starting models, where detailed lateral resolution is the objective.Non‐uniqueness can often be readily resolved with head wave amplitudes, attribute processing of the refraction convolution section and additional seismic traverses, prior to the acquisition of any borehole data. It is concluded that, unless specific measures are taken to address non‐uniqueness, the production of a single refraction tomogram that fits the traveltime data to sufficient accuracy does not necessarily demonstrate that the result is either correct, or even the most probable.

Non‐uniqueness with refraction inversion – a syncline model study

ABSTRACTNon‐uniqueness occurs with the 1D parametrization of refraction traveltime graphs in the vertical dimension and with the 2D lateral resolution of individual layers in the horizontal dimension. The most common source of non‐uniqueness is the inversion algorithm used to generate the starting model. This study applies 1D, 1.5D and 2D inversion algorithms to traveltime data for a syncline (2D) model, in order to generate starting models for wave path eikonal traveltime tomography.The 1D tau‐p algorithm produced a tomogram with an anticline rather than a syncline and an artefact with a high seismic velocity. The 2D generalized reciprocal method generated tomograms that accurately reproduced the syncline, together with narrow regions at the thalweg with seismic velocities that are less than and greater than the true seismic velocities as well as the true values.It is concluded that 2D inversion algorithms, which explicitly identify forward and reverse traveltime data, are required to generate useful starting models in the near‐surface where irregular refractors are common. The most likely tomogram can be selected as either the simplest model or with a priori information, such as head wave amplitudes.The determination of vertical velocity functions within individual layers is also subject to non‐uniqueness. Depths computed with vertical velocity gradients, which are the default with many tomography programs, are generally 50% greater than those computed with constant velocities for the same traveltime data. The average vertical velocity provides a more accurate measure of depth estimates, where it can be derived.Non‐uniqueness is a fundamental reality with the inversion of all near‐surface seismic refraction data. Unless specific measures are taken to explicitly address non‐uniqueness, then the production of a single refraction tomogram, which fits the traveltime data to sufficient accuracy, does not necessarily demonstrate that the result is either ‘correct’ or the most probable.

Exploiting lateral resolution of near-surface seismic refraction methods

The 1D τ-p inversion algorithm is widely employed to generate starting models with most computer programs that implement refraction tomography. However, this algorithm emphasizes the vertical resolution of many layers, and as a result, it frequently fails to detect even large lateral variations in seismic velocities, such as the decreases that are indicative of shear zones. This study presents a case that demonstrates the failure of the 1D τ-p inversion algorithm to define or even detect a major shear zone that is 50 m or ten stations wide. Furthermore, the majority of refraction tomography programs parameterize the seismic velocities within each layer with vertical velocity gradients. By contrast, the 2D generalized reciprocal method (GRM) inversion algorithms emphasize the lateral resolution of individual layers. This study demonstrates the successful detection and definition of the 50-m wide shear zone with the GRM inversion algorithms. The existence of the shear zone is corroborated by a 2D analysis of the head wave amplitudes and by numerous closely spaced orthogonal seismic profiles carried out as part of a later 3D refraction investigation. Furthermore, a 1D analysis of the head wave amplitudes indicates that a reversal in the seismic velocities, rather than vertical velocity gradients, occurs in the weathered layers. While all seismic refraction operations should aim to provide as accurate depth estimates as is practical, the major conclusion reached in this study is that refraction inversion algorithms that emphasize the lateral resolution of individual layers generate more useful results for geotechnical and environmental applications. The advantages of the improved lateral resolution are obtained with 2D profiles in which the structural features can be recognized from the magnitudes of the variations in the seismic velocities. Furthermore, the spatial patterns obtained with 3D investigations facilitate the recognition of structural features that do not display any intrinsic variation or “signature” in seismic velocities.

Overview of high resolution seismic prospecting in pre-Alpine and Alpine basins

Several high resolution seismic surveys were conducted in Alpine and pre-Alpine Lombardia (Northern Italy) valleys in order to reconstruct the geometries of the sediments filling the valleys, their seismo-stratigraphy and the rock basement depth. The acquisitions were made mainly in the framework of the CARG (Geological CARtography) project for the new geological map of Italy and under different research programs between the National Research Council and Regione Lombardia. The research firstly shows the capability of seismic methods in support of Quaternary research, and secondly demonstrates the response of different geomorphological features in particular: glacial valley, terminal moraine, deeply eroded valley and alluvial plain. Different tests were carried out in the seismic surveys in order to optimize the acquisition techniques for shallow prospecting (100–1000 m depth). The data were processed by applying a seismic refraction inversion technique, in addition to standard reflection processing. The integration of the two data sets improves the seismic migration of the reflection sections and the seismic interpretation, producing well-constrained seismo-stratigraphic sections coupled with an accurate seismic velocity section. The quality of the reflected events is generally higher in the glacial domain than in the fluvioglacial eroded valley, probably due to a stronger impedance contrast between soft sediments and hard rock. The terminal moraine sections show an intermediate quality of reflection record.

A Comparison of Electrical Resistivity, Ground Penetrating Radar and Seismic Refraction Results at a River Terrace Site

Electrical resistivity imaging (ERI), ground penetrating radar (GPR) and seismic refraction (SRF) profiles were repeated over three lines on a terrace of the Bow River. The site had a resistive gravel layer overlying mudstone bedrock with horizontal transitions to lacustrine and overbank deposits. Electrical resistivity results were best for determining changes in sediment types and detecting boundaries, but the ERI smoothness constraint blurred the location of the boundaries. The GPR gave the most resolution and showed internal structures that the other methods did not image. The GPR signal was severely attenuated in several areas where the surficial sediments became too conductive because of a fine grained component. The seismic refraction inversion provided good reproduction of the bedrock interface, but it did not detect changes in the composition of the surficial sediments. It also required the introduction of a low velocity surficial layer not indicated by the other methods that may be related to the increase in effective stress with depth. Jointly interpreting the three data sets gives a more reliable and less ambiguous interpretation than any single method. The data may be useful to test joint inversion algorithms and are available for download.

Case studies of shallow marine investigations in Australia with advanced underwater seismic refraction (USR)

Underwater seismic refraction with advanced interpretation approaches makes important contributions to shallow marine exploration and geotechnical investigations in Australia’s coastal areas. A series of case studies are presented to demonstrate the recent applications of continuous and static USR methods to river crossing and port infrastructure projects at various sites around Australia.In Sydney, static underwater seismic refraction (USR) with bottom-placed receivers and borehole seismic imaging assisted the development of improved geotechnical models that reduced construction risk for a tunnel crossing of the Lane Cove River.In Melbourne, combining conventional boomer reflection and continuous USR with near-bottom sources and receivers improved the definition of a buried, variably weathered basalt flow and assisted dredging assessment for navigation channel upgrades at Geelong Ports.Sand quality assessment with continuous USR and widely spaced borehole information assisted commercial decisions on available sand resources for the reclamation phase of development at the Port of Brisbane.Buried reefs and indurated layers occur in Australian coastal sediments with the characteristics of laterally limited, high velocity, cap layers within lower velocity materials. If these features are not recognised then significant error in depth determination to deeper refractors can occur. Application of advanced refraction inversion using wavefront eikonal tomography to continuous USR data obtained along the route of a proposed offshore pipeline near Fremantle allowed these layers and the underlying bedrock refractor to be accurately imaged. Static USR and the same interpretation approach was used to image the drowned granitic regolith beneath sediments and indurated layers in the northern area of Western Australia at a proposed new berthing site where deep piling was required. This allowed preferred piling sites to be identified, reducing overall pile lengths.USR can be expected to find increased application to shallow marine exploration and geotechnical investigations in Australia’s coastal areas as economic growth continues and improved interpretation methods are developed.

Shallow marine investigations in Australia with advanced underwater seismic refraction (USR)

Underwater seismic refraction (USR) with advanced interpretation approaches makes important contributions to shallow marine exploration and geotechnical investigations in Australia?s coastal areas. We present a series of recent case studies to demonstrate the application of continuous and static USR methods to river crossing and port infrastructure projects around Australia. In Sydney, static USR and borehole seismic imaging assisted the development of improved geotechnical models that reduced construction risk for a tunnel crossing of the Lane Cove River. In Victoria, combining conventional boomer seismic reflection and continuous USR improved the definition of a submerged, buried basalt flow and dredging assessment for navigation channel upgrades at Geelong Ports. Sand quality assessment with continuous USR and widely spaced borehole information led to improved commercial decisions on available sand resources for the reclamation phase of development at the Port of Brisbane. Buried reefs and indurated layers occur in Australian coastal sediments with the characteristics of laterally limited, high velocity, cap layers within lower velocity materials. If these features are not recognised then significant error in depth determination to deeper refractors can occur. Application of advanced seismic refraction inversion using wavefront eikonal tomography and continuous USR data obtained along the route of a proposed offshore pipeline near Fremantle allowed these layers and the underlying bedrock refractor to be accurately imaged. Static USR and the same interpretation approach was used to image the drowned granitic regolith beneath sediments and indurated layers in the northern area of Western Australia at a proposed new berthing site where deep piling was required. This accurately mapped the regolith and identified preferred piling sites that minimised pile lengths. USR can be expected to find increased application to shallow marine exploration and geotechnical investigations in Australia?s coastal areas as economic growth continues.

Refraction traveltime and amplitude corrections for very near‐ surface inhomogeneities

ABSTRACTThe performance of refraction inversion methods that employ the principle of refraction migration, whereby traveltimes are laterally migrated by the offset distance (which is the horizontal separation between the point of refraction and the point of detection on the surface), can be adversely affected by very near‐surface inhomogeneities. Even inhomogeneities at single receivers can limit the lateral resolution of detailed seismic velocities in the refractor.The generalized reciprocal method ‘statics’ smoothing method (GRM SSM) is a smoothing rather than a deterministic method for correcting very near‐surface inhomogeneities of limited lateral extent. It is based on the observation that there are only relatively minor differences in the time‐depths to the target refractor computed for a range of XY distances, which is the separation between the reverse and forward traveltimes used to compute the time‐depth. However, any traveltime anomalies, which originate in the near‐surface, migrate laterally with increasing XY distance. Therefore, an average of the time‐depths over a range of XY values preserves the architecture of the refractor, but significantly minimizes the traveltime anomalies originating in the near‐surface. The GRM statics smoothing corrections are obtained by subtracting the average time‐depth values from those computed with a zero XY value. In turn, the corrections are subtracted from the traveltimes, and the GRM algorithms are then re‐applied to the corrected data. Although a single application is generally adequate for most sets of field data, model studies have indicated that several applications of the GRM SSM can be required with severe topographic features, such as escarpments.In addition, very near‐surface inhomogeneities produce anomalous head‐wave amplitudes. An analogous process, using geometric means, can largely correct amplitude anomalies. Furthermore, the coincidence of traveltime and amplitude anomalies indicates that variations in the near‐surface geology, rather than variations in the coupling of the receivers, are a more likely source of the anomalies.The application of the GRM SSM, together with the averaging of the refractor velocity analysis function over a range of XY values, significantly minimizes the generation of artefacts, and facilitates the computation of detailed seismic velocities in the refractor at each receiver. These detailed seismic velocities, together with the GRM SSM‐corrected amplitude products, can facilitate the computation of the ratio of the density in the bedrock to that in the weathered layer. The accuracy of the computed density ratio improves where lateral variations in the seismic velocities in the weathered layer are known.