Refraction Traveltime Research Articles

Development of an ANN-Based Technique for Inversion of Seismic Refraction Travel Times

Non-uniqueness in the inversion of seismic data can be considered the main challenge for the application of such data. Prior information, such as downhole data, can be used to control this problem. However, in most cases, prior information is not available; accordingly, geophysicists/analysts have to suppose a primary model for the observed data and then find the final adequate layered earth model through trial and error. In this study, a new technique was developed based on the artificial neural network (ANN) for the inversion of seismic refraction data in the absence of prior information. In this regard, a sequential multilayer perceptron (SMLP) was proposed, which integrates the sequential information of the model parameters to predict a reasonable layered earth model. In fact, at first, a multilayer perceptron (MLP) (First-MLP) was trained by synthetic data; then, a layered earth model, i.e., the primary model, was predicted for the observed data. Next, using the primary model, a range for each of the model parameters, i.e., thickness and P-wave velocity, for each layer was defined. Subsequently, new synthetic samples were generated based on the determined ranges. Finally, using another MLP (Second-MLP), which was trained by the new synthetic samples, the final model for the observed data was estimated. The proposed method was also tested by employing different synthetic data with and without noise. Moreover, the SMLP inversion technique was used to analyze the experimental seismic refraction dataset at a dam construction site. The results for both synthetic and experimental data confirmed the reliability of the proposed SMLP inversion technique.

3D Refraction Travel Time Accuracy Study in High Contrasted Media

Among all the existing methods to solve the eikonal equation, three methods are chosen to verify accuracy, symmetry, reciprocity and error propagation along large offsets of refracted waves in seismic near surface exploration context. Performance is extremely highlighted nowadays and accuracy is being neglected, then an eikonal solver poorly explored in geoscience is used. A classical solver, the Fast Iterative and the modified Fast Sweeping Method are applied in three modeling schemes: a simple two layers model, a large four layers and a complex benchmark model. The three methods compute the first arrival of refracted waves in high contrast media and the results are compared to the analytical solution. A circular geometry is considered in all experiments to explore the method applicability using full azimuth angles. On the first scheme, the errors in travel time are computed among the three methods using different model sample spacing and we discuss accuracy, symmetry and reciprocity of first arrivals. On the second scheme, three circular receivers are placed in different offsets to check errors along refracted wave propagation. Finally, the third scheme, four shots are strategically positioned over the SEG/EAGE Overthrust model in order to compare the full acoustic wavefield with the eikonal solvers and then check the similarities. Although the focus is on methods accuracy, the algorithm run time is also considered and the comparison shows that the modified Fast Sweeping Method is the most accurate. The most computational efficient eikonal solver is the Fast Iterative Method, but its geoscience applicability needs to be cautious, because of its inaccurate results.

New Insights from Legacy Seismic Data regarding Basalt Elevations and Variability on the Hanford Site

Abstract Migration of groundwater contaminants in the Gable Gap area of the Hanford Site in southeastern Washington State is strongly influenced by the distribution and permeability of basalts that lie beneath an unconfined aquifer. Locally, folding and faulting of the Columbia River Basalt associated with the Yakima fold and thrust belt followed by erosion due to the Lake Missoula floods resulted in a complex basalt surface that represents either an impermeable lower boundary to the unconfined aquifer system or localized regions of increased permeability that potentially promote communication between the unconfined aquifer system and deeper, confined aquifer systems. Paleo-channels carved into the basalt by floodwaters are thought to provide preferential flow paths for groundwater contaminants. In 2011, a seismic landstreamer campaign was carried out to image the basalt surface and produced pre-stack depth migrated p-wave reflection images. The reflection images identified two large troughs that may represent paleo-channels and several areas of possible faulting. Here, the streamer data are re-analyzed using refraction travel-time and Rayleigh wave dispersion analyses to obtain images of compressional and shear wave velocities within the suprabasalt sediment sections and the upper basalt surface. The combined interpretation of reflection and seismic velocity images shows complexity in the basalt velocity and elevation, which varies by 50 m or more within the study area. These results, along with other ongoing geophysical investigations, will be used to inform the site geologic model and potentially guide placement of future boreholes needed to quantify vertical flow between the confined and unconfined aquifers.

Characterizing the shallow structure with the multimodal dispersion curves and the body wave refraction traveltimes from deep seismic sounding data

Deep seismic sounding (DSS) profiles are one of the most powerful tools for detecting crustal structures, and they have been deployed worldwide. Generally, the analysis of DSS data mainly focuses on body waves, while the surface waves are considered noise. We suggest that the surface waves in DSS data can be used to constrain subsurface structures. In this study, we use a DSS profile in the Piedmont and Atlantic Coastal Plain as an example to present the usage of the DSS surface wave. Multimodal dispersion curves were extracted from the DSS data with the Frequency-Bessel transform method, and were used in Monte Carlo joint inversions with body wave refraction traveltimes to constrain the shallow structures. Through the inversion, a horizontal stratum on the surface was identified in the Piedmont, and a two-layer sedimentary structure was identified in the Atlantic Coastal Plain. Comparisons with existing studies verified the accuracy of the shallow structures obtained in this study, demonstrating that the shallow velocity structure could be well constrained with the additional constraints provided by the multimodal dispersion curves. Thus, we believe that further research on the surface waves recorded in DSS surveys is warranted.

Seismic refraction data inversion via jellyfish search algorithm for bedrock characterization in dam sites

Seismic refraction method is an efficient tool for the investigation of dam construction sites. Velocity inversion has an essential role in an accurate seismic refraction data interpretation. This study aims to develop a new inversion algorithm to estimate P-wave velocity (Vp) structure from seismic refraction travel times. The introduced inversion algorithm is based on a recently developed nature-inspired algorithm, i.e., jellyfish search (JS) optimizer. First, the JS-based inversion algorithm was tested by several synthetic models in the presence of noise and without noise. Then, the performance of the applied inversion algorithm was evaluated by the seismic refraction travel times at a realistic dam construction site. The main objective of the actual data set analysis is the determination of Vp structure to find overburden thickness. The JS-based inversion algorithm in both synthetic models and actual data set shows acceptable performance. Results show three distinct seismic layers at the dam site. The velocities of the first, second and third layers, respectively, were estimated 400 m/s, 600 m/s and 1400 m/s. Also, the overburden thickness was estimated about 23 m, which was consistent with borehole data. The performance of the applied algorithm in the analyzing of actual data set was compared with the tomography interpretation method that the results revealed the efficiency of the JS-based inversion method.

Structure and Evolution of Northern Juan de Fuca Crust and Uppermost Mantle Over the Last 8 Ma From an Active‐Source Seismic Tomography Study

AbstractWe present results from a two‐dimensional wide‐angle controlled source seismic transect designed to characterize the velocity structure of the oceanic crust and uppermost mantle spanning the northern Juan de Fuca (JdF) plate from near Endeavor ridge to the Cascadia margin. Reflection and refraction travel time inversion is used to derive a tomographic Vp model of sediments, crust, and upper mantle. Velocity model results are compared to baseline reference Vp values for unaltered crustal and upper mantle rocks at temperatures assuming plate cooling. Effective medium theory is used to infer the degree of hydration of the crust and mantle. Results indicate a somewhat fractured and hydrated upper crust (≤2.5 wt% water), a near dry lower crust (≤0.7 wt%) and upper mantle (≤0.5 wt%) west of the deformation front, and an ∼75 km wide region of modestly lower velocity in the mid‐plate. Comparison with prior results from a complementary transect offshore Oregon indicates significant differences in Vp of the upper crust, with lower Vp‐inferred porosity along most of the Washington transects that may reflect different extents of fault‐related alteration and sediment burial histories. Approaching the deformation front, Washington transect Vp structure indicates dryer conditions than offshore Oregon, consistent with differences in extent of subduction bend faulting found in reflection imaging studies. On both transects, quasi‐abrupt changes in plate properties at ages of 8, 6, and 3.4/4 Ma are found. Distinct crustal accretion modes are recognized, aligning with changes in JdF plate motion and recent history of Cobb‐Eickelberg hotspot influence on crustal accretion at the JdF ridge.

Joint inversion of Rayleigh wave fundamental and higher order mode phase velocity dispersion curves using multi‐objective grey wolf optimization

AbstractRayleigh wave dispersion curves can be inverted to retrieve subsurface seismic velocity profiles. The inverse problem is ill‐posed, nonlinear and poorly conditioned, necessitating the application of global optimization methods. We present the application of the multi‐objective grey wolf optimization algorithm to perform joint inversion of the phase velocity dispersion curves corresponding to the fundamental and higher order modes of Rayleigh waves to obtain shear (S‐) and primary (P‐) wave velocity profiles. Multi‐objective grey wolf optimization is an extension of the grey wolf optimization algorithm for application to multi‐objective optimization and can be adapted to solve joint inversion problems. We compare the joint inversion results obtained from the multi‐objective grey wolf optimizer with those obtained from Markov chain Monte Carlo and fundamental mode inversion using the grey wolf optimizer on synthetic examples. The errors associated with phase velocity measurements are simulated by adding frequency‐dependent noise, with a higher level of noise added to the phase velocities corresponding to lower frequencies as compared to the higher frequencies. In the multimode joint inversion problem, the multi‐objective grey wolf optimizer gives a suite of solutions corresponding to each model parameter. The suite of S‐wave and P‐wave velocity profiles estimated from the multi‐objective grey wolf optimizer matches closely with the true model for the synthetic case studies even in the presence of noise. However, the suite of solutions has a greater spread for the last few layers, qualitatively indicating a higher degree of uncertainty in the predicted model parameter. The uncertainty in the solution for the deeper layers is a consequence of the uncertainty in the phase velocity at lower frequencies. We demonstrate the efficacy of the algorithm on recorded data from a shallow seismic survey conducted at the Indian Institute of Technology Bombay. The results from the multi‐objective grey wolf optimizer are in close agreement with those from Markov chain Monte Carlo, and the depth of investigation is found to be greater in comparison to results from refraction traveltime inversion.

MFAST: A MATLAB toolbox for ocean bottom seismometer refraction first-arrival traveltime tomography

First-arrival seismic traveltime tomography (FAST) is a well-established technique to estimate subsurface velocity structures. Although several existing open-source packages are available for first-arrival traveltime tomography, most were written in compiled languages and lack sufficient extendibility for new algorithms and functionalities. In this work, we develop an open-source, self-contained FAST package based on MATLAB, one of the most popular interpreted scientific programming languages, with a focus on ocean bottom seismometer refraction traveltime tomography. Our package contains a complete traveltime tomography workflow, including ray-tracing-based first-arrival traveltime computation, linearized inversion, quality control, and high-quality visualization. We design the package as a modular toolbox, making it convenient to integrate new algorithms and functionalities as needed. At the current stage, our package is most efficient for performing FAST for two-dimensional ocean bottom seismometer surveys. We demonstrate the efficacy and accuracy of our package by using a synthetic data example based on a modified Marmousi model.

Assessment of Two Refraction Traveltime Tomography Methods Applied for Statics Corrections of Land Data

ABSTRACT. Many refraction tomography algorithms for statics corrections have model parameterization, forward modeling, and inversion limitations. For instance, most traditional methods use a regular grid to parameterize the model and only invert one parameter, either velocity or thickness of the layer. Generally, these conventional techniques do not allow the simultaneous inversion of traveltimes of turning-rays and head-waves either. In this study, for the first time, we applied and evaluated the well-known 2D refraction tomography algorithm Rayinvr, which is widely used to study deep crustal models, to estimate near-surface layered velocity models and static corrections from real seismic reflection data collected for oil exploration. Here, the Rayinvr and its results are compared with another refraction tomography algorithm widely used in the petroleum industry. It employs the concept of generalized linear inversion and uses a simplified earth model composed of a layer over a half-space, which is parameterized by vertical rectangles of constant velocity. The results of both methods were applied and evaluated on two real datasets of seismic lines located in Brazilian onshore basins. The Rayinvr tomography algorithm provides precise measurement statics corrections, better velocity models, and more reliable geological structures than the standard tomography algorithm.Keywords: refraction, seismic tomography, traveltimes, near-surface geophysics. Avaliação de dois métodos de tomografia de tempos de trânsito de refração aplicados para correções estáticas de dados terrestresRESUMO. Muitos algoritmos de tomografia de refração para correções estáticas têm limitações em re- lação à parametrização do modelo, da modelagem direta e da inversão. Por exemplo, a maioria dos méto- dos tradicionais usa uma malha regular para parametrizar o modelo e apenas invertem um parâmetro, seja a velocidade ou a espessura da camada. Geralmente essas técnicas convencionais também não permitem a inversão simultânea dos tempos de trânsito de turning-rays e head-waves. Neste estudo, pela primeira vez, aplicamos e avaliamos o conhecido algoritmo de tomografia de refração 2D Rayinvr, que é amplamente usado no estudo de modelos crustais profundos, na estimativa de modelos de veloci- dade em camadas próximas à superfície e nas correções estáticas de dados reais de reflexão sísmica coletados para exploração de petróleo. Aqui, o Rayinvr e os resultados são comparados com outro al- goritmo de tomografia de refração, amplamente utilizado na indústria petrolífera. Ele emprega o conceito de inversão linear generalizada e usa um modelo terrestre simplificado composto por uma camada sobre um meio-espaço, que é parametrizado por retângulos verticais de velocidade constante. Os resultados de ambos os métodos foram aplicados e avaliados em dois conjuntos de dados reais de linhas sísmicas localizadas em bacias terrestres brasileiras. O algoritmo de tomografia Rayinvr forneceu correções es- táticas mais precisas, melhores modelos de velocidade e estruturas geológicas mais confiáveis do que o algoritmo padrão de tomografia.Palavras-chave: refração sísmica, tomografia sísmica, tempos de trânsito, geofísica rasa.

Shear-Wave Velocity Site Characterization in Oklahoma from Joint Inversion of Multimethod Surface Seismic Measurements: Implications for Central U.S. Ground-Motion Prediction

ABSTRACT We analyze multimethod shear (SH)-wave velocity (VS) site characterization data acquired at three permanent and 25 temporary seismograph stations in Oklahoma that recorded M 4+ earthquakes within a 50 km hypocentral distance of at least one of the 2016 M 5.1 Fairview, M 5.8 Pawnee, or M 5.0 Cushing earthquakes to better constrain earthquake ground-motion modeling in the region. We acquired active-source seismic data for time-averaged VS to 30 m depth (VS30) at 28 seismograph stations near the Fairview, Pawnee, and Cushing epicentral areas. The SH-wave refraction travel times coupled with Rayleigh- and Love-wave phase velocity dispersion were extracted and modeled in a nonlinear least-squares (L2) joint inversion to obtain a best-fit 1D VS versus depth profile for each site. At a subset of sites where the preferred L2 inverse model did not optimally fit each of the Love, Rayleigh, and SH travel-time datasets, we explore application of simulated annealing in a joint inversion to find a more global solution. VS30 values range from 262 to 807 m/s for the preferred measured (in situ) VS profiles, or National Earthquake Hazards Reduction Program (NEHRP) site class D to B, and are broadly comparable with estimates from previous data reports in the region. Site amplification estimates were calculated next from 1D SH transfer functions of the preferred VS profiles and then compared against observed horizontal-to-vertical spectral ratios (HVSRs) from nearby seismograph stations. We generally see good agreement between the predicted in situ model and the observed HVSR resonant frequencies, with nominal amplifications between 2 and 10 within the 2–15 Hz frequency band. Next, using 40 known in situ VS30 measurements in the region, we demonstrate that the in situ VS30 values improve the fit for selected suites of ground-motion models (GMMs) for M 4+ earthquakes within a 50 km hypocentral distance when compared with proxy methods, arguing for future development of GMMs implementing in situ VS profiles.

Subsurface space investigation jointly using seismic reflection and refraction tomography in urban area: a case study of Hangzhou, China

Seismic reflection method is one of the commonly used techniques in urban underground space exploration. However, in some areas, the effect of reflection seismic exploration is limited because of the difficulty of interpretation. In this paper, seismic reflection exploration and refraction travel time tomography are jointly used in Hangzhou to increase detection accuracy. The results show that different layers can be discerned for Quaternary sedimentary. The combined use of the two methods has obvious advantages and good practical value.

Flexible layer-based 2D refraction tomography method for statics corrections

Standard refraction traveltime tomography solutions for statics corrections do not allow irregular parameterization of the model, simultaneous inversion of velocity and depth parameters, or selection of the types of first arrival times to be inverted. An algorithm widely used to study and build crustal structure and velocity models from wide-angle seismic data was applied for the modeling and inversion of the velocity and structure of near-surface layers. This flexible tomographic solution allows the definition of complex heterogeneous models through trapezoids of irregular size to parameterize the model, and it allows the simultaneous inversion of head-waves and turning-rays, which can be defined independently for each layer interface. This work presents a study of the application of this flexible tomographic inversion algorithm to obtain the velocity model and geological structure of the near-surface weathering zone. The proposed workflow was evaluated on two synthetic models built with real topography, and geophysical and geological information was obtained from real cases. The first is a model with three layers with constant velocities, and the second is a model with two heterogeneous layers with lateral and vertical velocity variations in the weathering zone. For the second model, turning-rays crossing the weathering layer are considered in the tomography process. Synthetic seismic data were generated, and an initial model was built using traditional seismic refraction interpretation methods. Thereafter, inversion was performed using a 2D ray-based tomography algorithm to minimize first-arrival traveltime misfits by inverting the seismic velocities and the vertical position of the refractors. It was verified that the proposed workflow obtained the smallest relative errors in most parameters of the velocity model and consequently, for the statics corrections.

Cavity effect on Rayleigh wave dispersion and P-wave refraction

Sinkholes and cavities can represent serious hazards to human safety and urban infrastructures, cause roadbed subsidence, and so on. It is therefore essential to evaluate various sinkholes in different depths and sizes to assess the risk of collapse. This paper evaluates the effect of different cavities on Rayleigh-wave propagation and body (P) wave refraction. Rayleigh (R) wave propagation is analyzed according to the classical multi-channel analysis of surface waves (MASW) method also considering the R-wave backscattering. Synthetic seismic traces are computed by means of finite element modeling (FEM) for cavity and intrusion at different depths and sizes. Furthermore, field acquisition data is used to verify the detection effect of a cavity on R- and P-waves. The results show that the presence of backscattered R-waves and the changes in the R-wave velocity spectrum can help in cavity identification. Additional possible evidence is represented by significant changes in the refraction travel times over the cavity location. It can be concluded that the field data are in good agreement with the synthetic, and it could be effective to consider the results of both R- and P-wave analysis in order to efficiently identify the cavities.

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.

Joint refraction traveltime tomography and migration for multilayer near-surface imaging

In near-surface velocity structure estimation, first-arrival traveltime tomography tends to produce a smooth velocity model. If the shallow structures include a weathering layer over high-velocity bedrock, first-arrival traveltime tomography may fail to recover the sharp interface. However, with the same traveltime data, refraction traveltime migration proves to be an effective tool for accurately mapping the refractor. The approach downward continues the refraction traveltime curves and produces an image (position) of the refractor for a given overburden velocity model. We first assess the validity of the refraction traveltime migration method and analyze its uncertainties with a simple model. We then develop a multilayer refraction traveltime migration method and apply the migration image to constrain traveltime tomographic inversion by imposing discontinuities at the refraction interfaces in model regularization. In each subsequent iteration, the shape of the migrated refractors and the velocity model are simultaneously updated. The synthetic tests indicate that the joint inversion method performs better than the conventional first-arrival traveltime tomography method with Tikhonov regularization and the delay-time method in reconstructing near-surface models with high-velocity contrasts. In application to field data, this method produces a more accurately resolved velocity model, which improves the quality of common midpoint stacking by making long-wavelength static corrections.

Towards large‐scale stochastic refraction tomography: a comparison of three evolutionary algorithms

ABSTRACTThe main goal of this study is to assess the potential of evolutionary algorithms to solve highly non‐linear and multi‐modal tomography problems (such as first arrival traveltime tomography) and their abilities to estimate reliable uncertainties. Classical tomography methods apply derivative‐based optimization algorithms that require the user to determine the value of several parameters (such as regularization level and initial model) prior to the inversion as they strongly affect the final inverted model. In addition, derivative‐based methods only perform a local search dependent on the chosen starting model. Global optimization methods based on Markov chain Monte Carlo that thoroughly sample the model parameter space are theoretically insensitive to the initial model but turn out to be computationally expensive. Evolutionary algorithms are population‐based global optimization methods and are thus intrinsically parallel, allowing these algorithms to fully handle available computer resources. We apply three evolutionary algorithms to solve a refraction traveltime tomography problem, namely the differential evolution, the competitive particle swarm optimization and the covariance matrix adaptation–evolution strategy. We apply these methodologies on a smoothed version of the Marmousi velocity model and compare their performances in terms of optimization and estimates of uncertainty. By performing scalability and statistical analysis over the results obtained with several runs, we assess the benefits and shortcomings of each algorithm.

Joint Body‐ and Surface‐Wave Tomography of Yucca Flat, Nevada, Using a Novel Seismic Source

AbstractIn preparation for the next phase of the Source Physics Experiments, we acquired an active‐source seismic dataset along two transects totaling more than 30 km in length at Yucca Flat, Nevada, on the Nevada National Security Site. Yucca Flat is a sedimentary basin which has hosted more than 650 underground nuclear tests (UGTs). The survey source was a novel 13,000 kg modified industrial pile driver. This weight drop source proved to be broadband and repeatable, richer in low frequencies (1–3 Hz) than traditional vibrator sources and capable of producing peak particle velocities similar to those produced by a 50 kg explosive charge. In this study, we performed a joint inversion of P‐wave refraction travel times and Rayleigh‐wave phase‐velocity dispersion curves for the P‐ and S‐wave velocity structure of Yucca Flat. Phase‐velocity surface‐wave dispersion measurements were obtained via the refraction microtremor method on 1 km arrays, with 80% overlap. Our P‐wave velocity models verify and expand the current understanding of Yucca Flat’s subsurface geometry and bulk properties such as depth to Paleozoic basement and shallow alluvium velocity. Areas of disagreement between this study and the current geologic model of Yucca Flat (derived from borehole studies) generally correlate with areas of widely spaced borehole control points. This provides an opportunity to update the existing model, which is used for modeling groundwater flow and radionuclide transport. Scattering caused by UGT‐related high‐contrast velocity anomalies substantially reduced the number and frequency bandwidth of usable dispersion picks. The S‐wave velocity models presented in this study agree with existing basin‐wide studies of Yucca Flat, but are compromised by diminished surface‐wave coherence as a product of this scattering. As nuclear nonproliferation monitoring moves from teleseismic to regional or even local distances, such high‐frequency (>5 Hz) scattering could prove challenging when attempting to discriminate events in areas of previous testing.

2D inversion of refraction travel-time curves using homogeneous functions: a case study from Wadi El-Nakheil, Eastern Desert, Egypt

The area of study is located at Wadi El-Nakheil to the west of Quseir district along the Egyptian Red Sea Coast. The present work is a case study of applying the 2D homogeneous function method. This function automatically inverts the first arrival refractions to derive a 2D velocity distribution involving the seismic boundaries. The present study aims at investigating the near-surface geological conditions of Wadi El-Nakheil. These conditions include the following: the different stratigraphic units, structural configuration, and the groundwater occurrence. Authors achieved this aim by carrying out shallow seismic refraction measurements along three profiles using direct and reverse shooting. A newly developed field methodology is used in the data acquisition. The interpretation of the seismic data is performed using GODOGRAF software which represents a 2D refraction tomographic inversion algorithm. The interpretation revealed the presence of seven geo-seismic zones, overlain by low-velocity surface layer, with a velocity range of less than 800 to 6800 m/s. The detected zones are dissected by several normal faults with NE and SW dipping directions and maximum throw of about 20 m. An angular unconformity is noticed between the overlying horizontal layers and the dipping (towards SW) underlying layers. One of the interpreted zones is identified as the water-bearing zone where the velocity is between 2200–2400 and 2600 m/s. This was emphasized by the correlation with the nearest borehole in the area. The water-bearing zone is interpreted as the Nakheil Formation. The thickness of the water-bearing zone ranges from 20 to 55 m, while its depth ranges from 35 to 65 m.

Non-uniqueness with Refraction Inversion ? A Syncline Model Study

Non-uniqueness occurs with the 1D parameterization 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 nonuniqueness is the inversion algorithm used to generate the starting model. This study generates tomograms with wavepath eikonal traveltime (WET) tomography from starting models obtained with a smooth velocity gradient, the tau-p algorithm and the generalized reciprocal method (GRM) to traveltime data for a syncline (2D) model. The seismic velocity at the region of maximum gradient, taken to be refractor interface, was both ambiguous and anomously low for the smooth velocity gradient and the tau-p starting tomograms. These starting models also produced an artefact with a high seismic velocity in the main refractor. The GRM tomograms accurately reproduced the syncline, together with narrow regions at the thalweg with seismic velocities that are less than and greater than, as well as the true seismic velocities. It is concluded that inversion algorithms, which explicitly identify forward and reverse traveltime data, such as those of the GRM, 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 GRM 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 even the most probable.

Generating Density Models with Seismic Refraction Data

Seismic refraction traveltime data can be inverted to obtain a depth model of the weathered layer in terms of the seismic velocities. Furthermore, the head wave amplitudes can be inverted to generate a model of the scaled density ratios in the near surface. These seismic refraction models of the near-surface can be used in two modes. The first is to derive a representative model of the in-situ rock densities in the sub-weathering. The second is to fully compensate for the weathered layer, in order to detect any prospective gravity anomalies originating in the sub-weathering. The extensive sets of seismic data recorded for regional investigations by Geoscience Australia are well suited to deriving detailed models of the depth of weathering and scaled density contrasts. These models can be usefully employed as starting models for model-based inversion of coincident gravity data.