Travel Time Tomography Using Frequency Responses Collected By Cwr Experimentation

Soil disturbance evaluation can be used as an indirect tool to detect buried objects, which is in turn of interest in forensic and security applications to local police departments, the Federal Bureau of Investigation and the Department of Homeland Security. Heterogeneity and variations in soil stratigraphy due to finite size rocks and boulders, etc. may be mistaken with disturbed zones. Besides, even uniform precipitation can affect the nonuniform environment unevenly and locally. The moisture difference is the key to creation of dielectric property contrast between disturbed and undisturbed zones. The research on the sandy soil in the pilot scale facility of Northeastern University (referred to as SoilBED) has shown that the disturbance effect may fade by vibrations or after cyclic drainage and recharge due to precipitation over time and up to some extent, but this effect is minimal. Cross-Well Radar (CWR) and Ground Penetrating Radar (GPR) are among the most promising potential detection techniques. The SoilBED facility is used to experimentally simulate the problem along with other numerical and analytical simulations. To study and evaluate the feasibility of disturbance detection using radar-based geophysical techniques, PVC-cased borehole monopole antennas were installed on a circle around the area suspected to be disturbed. Then, cross-tomography measurements are collected and images of the disturbed zone were reconstructed. Obviously, CWR may not be the perfect tool to evaluate soil disturbance over a large area, but can be employed to study the potential of using radarbased techniques to detect soil disturbance. The results can be used for detection of other anomalies, such as tunnels, landmine, etc. Introduction GPR and CWR are radar-based geophysical techniques that can be implemented in a vast variety of applications such as forensic, safety, security, civil infrastructure, and geo-environmental engineering. One application in the forensic field is to detect bodies or objects buried underground. During the experimental application of traveltime tomography to detect DNAPLs (dense nonaqueous phase liquids) in soil, the background soil was removed and then replaced back using the same compaction effort and moisture content, to place objects in soil. Precise provisions were employed to assure achieving the same soil density and moisture content. The background soil was poorly graded uniform silty sand, with minimal dependency of mechanical strength on the grain-size and packing structure. Despite these provisions, a strong scattering effect on the electromagnetic (EM) field by the soil disturbance was observed. The aim of this study is to experimentally and theoretically evaluate the disturbance in the soil. Then, the outcome will be used to evaluate different techniques to eliminate this disturbance and replicate the same soil background for various experimental applications. Besides, detecting the disturbance in soil using radar-based techniques can still be helpful in different forensic or homeland and national security to lead to indirectly detecting difficult to detect buried objects (due to weak property contrast with the background). The disturbance can fade gradually due to rainfalls, drainage, and recharge of groundwater, but to some extent and not completely.

Widespread contamination of underground environments with dense non-aqueous phase liquids (DNAPLs) is of great concern to the public, military, and industrial sectors. Proper management of contaminated sites requires detection and monitoring of the contaminants, and accurate knowledge of their transport behavior in underground environments. Over the last years we have done great efforts to develop and integrate technologies that serve to locate contamination and monitor transport mechanism underground. In this paper, we describe a two-dimensional multiphase flow experiment to develop and evaluate two modes of concurrent detection and monitoring technologies: Cross Well Radar (CWR) and Image Analysis (IA). Loop antennas preset at specific locations in the tank are used to evaluate wave scattering properties of the soil under different conditions, while color images are acquired. The electromagnetic response in the CWR antennas and IA are used to establish the relation between electrical soil properties variations and changes spatial and temporal mass of water and contaminants. The technologies used in this research are both in development, but they can be successful tools for the detection, monitoring and imagining of underground contaminants and process. Once develop, the technology may be applied for detecting and monitoring other buried objects.

Simulations of nonpartitioning and partitioning tracer tests were used to parameterize the equilibrium stream tube model (ESM) that predicts the dissolution dynamics of dense nonaqueous phase liquids (DNAPLs) as a function of the Lagrangian properties of DNAPL source zones. Lagrangian, or stream‐tube‐based, approaches characterize source zones with as few as two trajectory‐integrated parameters, in contrast to the potentially thousands of parameters required to describe the point‐by‐point variability in permeability and DNAPL in traditional Eulerian modeling approaches. The spill and subsequent dissolution of DNAPLs were simulated in two‐dimensional domains having different hydrologic characteristics (variance of the log conductivity field = 0.2, 1, and 3) using the multiphase flow and transport simulator UTCHEM. Nonpartitioning and partitioning tracers were used to characterize the Lagrangian properties (travel time and trajectory‐integrated DNAPL content statistics) of DNAPL source zones, which were in turn shown to be sufficient for accurate prediction of source dissolution behavior using the ESM throughout the relatively broad range of hydraulic conductivity variances tested here. The results were found to be relatively insensitive to travel time variability, suggesting that dissolution could be accurately predicted even if the travel time variance was only coarsely estimated. Estimation of the ESM parameters was also demonstrated using an approximate technique based on Eulerian data in the absence of tracer data; however, determining the minimum amount of such data required remains for future work. Finally, the stream tube model was shown to be a more unique predictor of dissolution behavior than approaches based on the ganglia‐to‐pool model for source zone characterization.

Detection of dense non-aqueous phase liquids (DNAPLs) is a practical interesting problem in geophysics. This problem involves forward modeling, subsurface sensing and object reconstruction. An analytical forward model - HSBA - in the frequency domain containing dyadic Green's function solution to describe the cross-well radar sensing in infinite soil media is developed. First order Born approximation is employed to linearize this inverse scattering problem. The forward model is validated by both frequency domain computational models and laboratory experiments that are collected using cross-well radar method that uses broadband antennas in subsurface to illuminate the inhomogeneous field and receive scattered electromagnetic (EM) waves. A shape-based inversion algorithm is proposed for contaminant pool localization and reconstruction, in which the shape of the object is assumed priori and represented by a low- order differentiable parametric function. The characteristics of the object - location, size and dielectric contrast to the background medium - are obtained by an iterative non-linear optimization. The inversion method using synthesized data by numerical experiments gives promising preliminary results.

Seismic bulletins, with trustworthy phase picks, origin times, and source locations are key for regional seismic studies, such as travel-time (TT) tomography, attenuation tomography, and anisotropy studies. To lay the groundwork for such studies in Israel, we revised the seismic bulletin of Israel and the surrounding area and obtained a trustworthy TT data set. From the earthquake and explosion bulletins of the Geophysical Institute of Israel, we compiled a starting data set of about 123,000 earthquakes and explosions that occurred during the past 40 yr. After screening out the poorly recorded events, we were left with a data set of ∼38,000 well-recorded events. We then revised the remaining data set in two consecutive steps. In the first, we reviewed and updated station metadata, including changes in station metadata parameters over time. In the second step, we jointly relocated a list of selected seismic events, using the Bayesian hierarchical location software package (BayesLoc) of Myers et al. (2007) that performs joint relocation of multiple events. We observed striking dissimilarities between the spatial distributions of the newly relocated catalog and the initial locations. Although the depth distribution of the starting catalog is trimodal with peaks at 0, 5, and 10 km, the distribution in this study is unimodal, with a broad peak between 7.5 and 12.5 km. By differencing the observed arrival times and the origin times obtained through relocation with BayesLoc, we obtained a revised TT database that consists of 261,336 Pg, 132,876 Pn, 114,816 Sg, and 60,394 Sn arrivals, from a set of 30,458 jointly relocated seismic sources. We compared prerevision and postrevision TTs as a function of epicentral distance and concluded that the revised data set contains far fewer outliers and inconsistencies than the original data set. The revised TT data set may be used for seismic studies, such as TT tomography, attenuation tomography, and anisotropy studies.

Velocity anisotropy caused by fine layering has a strong effect on the travel paths at large vertical offsets in crosshole seismic data. This effect must be considered in seismic waveform tomography, so that the inversion procedure can match the waveforms in real data. In this paper, we propose to properly take into account this anisotropic effect in two stages. First, we invert for the anisotropy parameter simultaneously with the (horizontal) velocity in travel time tomography. Then we incorporate this estimated anisotropy parameter model in the waveform simulation during waveform tomography, to refine the velocity image. When considering this anisotropic effect, far-offset data are properly used in the inversion, then both travel time and waveform tomography may have a better ray coverage, producing a velocity image with a higher resolution.

A fast marching algorithm for the factored eikonal equation

Ray perturbation theory is concerned with the change in ray paths and travel times due to changes in the slowness model or the end‐point conditions of rays. Several different formulations of ray perturbation theory have been developed. Even for the same physical problem different perturbation equations have been derived. The reason for this is that ray perturbation theory contains a fundamental ambiguity. One can move a point along a curve without changing the shape of the curve. This means that the mapping from a reference curve to a perturbed curve is not uniquely defined, because one may associate a point on the reference curve with different points on the perturbed curve. The mapping that is used is usually defined implicitly by the choice of the coordinate system or the independent parameter. In this paper, a formalism is developed where one can specify explicitly the mapping from the reference curve to the perturbed curve by choosing a stretch factor that relates increments in arc length along the reference curve and the perturbed curve. This is incorporated in a theory that is accurate to first order in the ray position and to second order in the travel time. The second order travel time perturbation describes the effect of changes in the position of the ray on the travel time. For first arrivals, rays are paths of minimum travel time. The travel time along a ray estimate is therefore necessarily longer than the travel time along the true ray. The resulting travel time bias is described to leading order by the second order perturbation of the travel time. This quantity may be of great importance in nonlinear travel time tomography. Several existing perturbation equations are shown to correspond to the general equation derived in this paper for specific choices of the stretch factor. Depending on the stretch factor, analytical solutions for the ray perturbation can be found for different models of the reference slowness. For two‐point ray tracing problems, the arc length (measured in terms of the independent parameter) may change with the ray perturbation. It is shown both numerically and analytically that for two‐point ray tracing problems, one must introduce a tuning parameter in the perturbation problem. Leaving out such a tuning parameter, as is done in current Hamiltonian ray perturbation theory, may lead to erroneous solutions. In the formulation of this paper, paraxial ray perturbations, slowness perturbations, and pure ray bending are treated in a uniform fashion. This may be very useful in nonlinear tomographic inversions which include earthquake relocation.

In the deep ocean scenario, the sound propagates by typical models, such as convergence zone model or bottom bounce model. When the sound propagates in the bottom bounce model, there are four typical paths which the sound travels by. The difference between four typical paths conducts to four different travel time. The signal travel time is in a certain structure for a vertical array, which has been proved by a vertical array data. Utilizing the feature of the certain temporal arrival structure, a cost function is proposed to locate the source. Both the numerical simulations and experimental results proved the temporal arrival structure and show that the proposed localization method can work well.

In shallow water acoustic tomography, broadband mid-frequency acoustic waves (1 to 5 kHz) follow multiple ray-like paths to travel through the ocean. Travel-time (TT) variations associated to these raypaths are classically used to estimate sound speed perturbations of the water column using the ray theory. In this shallow water environment, source and receiver arrays, combined with adapted array processing, provide the measurement of directions-of-arrival (DOA) and directions-of-departure (DOD) of each acoustic path as new additional observables to perform ocean acoustic tomography. To this aim, the double-beamforming technique is used to extract the TT, DOA, and DOD variations from the array-to-array acoustic records. Besides, based on the first order Born approximation, we introduce the time-angle sensitivity kernels to link sound speed perturbations to the three observable variations. This forward problem is then inversed by the maximum a posteriori method using both the extracted-observable variations and the proposed sensitivity kernels. Inversion results obtained on numerical data, simulated with a parabolic equation code, are presented. The inversion algorithm is performed with the three observables separately, namely TT, DOA, and DOD. The three observables are then used jointly in the inversion process. The results are discussed in the context on ocean acoustic tomography.

In seismic refraction exploration, travel time tomography is the most widely used method in engineering and environmental geophysical exploration. In this paper, we mainly optimize the travel time tomography of refraction. First, with respect to the forward algorithm, we introduce a new travel time calculation method to improve the accuracy and efficiency of forward calculation. Based on the fast marching method (FMM), we introduce an improved forward calculation method called the multi-stencil fast marching method (MSFM). In the process of inversion, we propose a dynamic prior model composite constraint (DPMCC) method based on the T0 difference method from the idea of multi-scale inversion. Meanwhile, we use the prior information to improve the accuracy of inversion. Furthermore, we use the dynamic regularization factor selection method to make the inversion solution more stable and reliable. Finally, we test and analyze the synthetic data and the measured data to verify the effectiveness of the optimized travel time tomography algorithm.

A number of different geodynamic models have been proposed to explain the extension that occurred during the Miocene in the Alboran Sea region of the western Mediterranean despite the continued convergence and shortening of northern Africa and southern Iberia. In an effort to provide additional geophysical constraints on these models, we performed a local, regional, and teleseismic tomographic travel time inversion for the lithospheric and upper mantle velocity structure and earthquake locations beneath the Alboran region in an area of 800×800 km2. We picked P and S arrival times from digital and analog seismograms recorded by 96 seismic stations in Morocco and Spain between 1989 and 1996 and combined them with arrivals carefully selected from local and global catalogs (1964–1998) to generate a starting data set containing over 100,000 arrival times. Our results indicate that a N‐S line of intermediate‐depth earthquakes extending from crustal depths significantly inland from the southern Iberian coast to depths of over 100 km beneath the center of the Alboran Sea coincides with a W to E transition from high to low velocities imaged in the uppermost mantle. A high‐velocity body, striking approximately NE‐SW, is imaged to dip southeastwards from lithospheric depths beneath the low‐velocity region to depths of ∼350 km. Between 350 and 500 km the imaged velocity anomalies become more diffuse. However, pronounced high‐velocity anomalies are again imaged at 600 km near an isolated cluster of deep earthquakes. In addition to standard tomographic methods of error assessment, the effects of systematic and random errors were assessed using block shifting and bootstrap resampling techniques, respectively. We interpret the upper mantle high‐velocity anomalies as regions of colder mantle that originate from lithospheric depths. These observations, when combined with results from other studies, suggest that delamination of a continental lithosphere played an important role in the Neogene and Quaternary evolution of the region.

<p>The Ordos block is located on the west side of the North China Carton, adjacent to the northeastern part of the Tibetan Plateau. Affected by two tectonic movements, Ordos block internal structure remains relatively stable structure, but surrounded by active tectonic belts. With the development of the second and third part of the “China Seismological Science Array”, the distribution of seismic observation stations in Ordos region has been greatly improved. This study will use the new seismic observation data of "Array III", and combined with the phase observation data of "Array II" to form a more complete seismic phase travel time data set. The regional seismic body-wave travel time tomography will figure out a more reliable three-dimensional velocity structure of P waves in Ordos.</p><p>Our study area spans from 32°N to 42°N and 108°E to 114°E , which includes the Ordos block and its adjacent structures . The seismic data we used for inversion were recorded by 1244 stations including: 198 permanent stations and 1043 temporary stations (ChinArray II and III), from November 2013 to August 2017. After manual labeled the seismic phase, we select events with more than ten phase records of individual seismic events. The epicentral distance is less than 200km. Finally, we obtained about 22,500 phase records of 1882 local seismic events.</p><p>The preliminary results are consistent with previous studies and surface structures of a wide range of velocity distributions. However, in the middle-upper crust under the Liupan Mountain west, the low-speed anomaly extending downward is shown, which may be caused by the shallow crustal damage caused with the continuous eastward compression of asthenosphere in the northeastern margin of the Qinghai-Tibet Plateau during the Cenozoic. It is worth noting that there is an EW-trending low-velocity zone under the Dingbian-Suide fault beneath the Ordos Basin, with a depth form lower crust to 50 km in upper mantel. This low-velocity anomaly divides the high-speed disturbance in the Ordos block into two parts，indicate the depth of the fault can reach the upper mantel. In the Taihang Mountains in the west of the study area, low-velocity anomalies extending to the upper layer of the mantle are shown. We initially believe that this anomaly is related to the volcanic thermal motion that once existed on the area.</p>

Using waveforms and travel times from deep earthquakes, we constructed 16 seismic profiles, each of which constrains the radial variation in Vp over a small area beneath the northern Philippine Sea. Taken together, the azimuthal coverage of these profiles also places tight bounds on the lateral extent of a region of anomalously high Vp (up to 3% faster than average Earth models) originally suggested by travel time tomography. Unlike travel time tomography, which relies heavily on arrival times of the direct P phase, we utilize the waveforms and move‐out of later arrivals that mainly sample the mantle transition zone of interest. Our results identify three important characteristics of the northern Philippine Sea anomaly that are distinct from previous results. First, being approximately a subhorizontal, laterally uniform feature, the anomaly is localized beneath the northwestern corner of the Philippine Sea, within a region of approximately 500×500 km2 immediately east of the Ryukyu arc. Second, the anomaly is well constrained to occur in the lower portion of the transition zone, extending all the way down to the 660‐km discontinuity. Third, the presence of such a distinct anomaly reduces the contrast in Vp across the 660‐km discontinuity from approximately 6% to 3%. Such a configuration is consistent with the interpretation that the anomaly is caused by a remnant of subducted slab, as negative buoyancy should rest the slab just above the 660‐km discontinuity where resistance to subduction is expected from a negative Clapeyron slope during the spinel—Mg‐Fe‐perovskite transition.

Seismic attenuation tomography and its application to rock mass evaluation