Multi-azimuth ground penetrating radar surveys to improve the imaging of complex fractures

Ground Penetrating Radar (GPR) images are affected, to some degree, by the relative orientation of antennas and subsurface targets. This is particularly true not only for targets that show a significant directivity, but also for inclined planes, such as fractures and faults. Depending on the relative geometry between the antennas and the orientation of the target, radar waves can be preferentially scattered, which causes changes in the reflected signal amplitude. Therefore, traditional single polarization and single azimuth surveys may produce inadequate results. The work presented here examines the use of a multi-azimuth GPR survey to increase the imaging performance of inclined fractures, showing the shortcomings of single-profile surveying and highlighting the benefits that such a strategy has on detection and characterization

A two-step multifrequency imaging technique for ground penetrating radar

In the present paper, a combined method for ground penetrating radar imaging is presented. The proposed technique has a first step in which the electric field scattered by the buried structure is estimated and a qualitative reconstruction is obtained, and a second quantitative inversion step for reconstructing the dielectric properties of the buried targets. The full-wave multifrequency inexact-Newton inversion approach used in the second step uses the information about the target position extracted by the qualitative procedure and takes the scattered field data estimated by a time-domain filtering method. Numerical simulations are presented to prove the effectiveness of the proposed technique

Multi-azimuth ground penetrating radar surveys to improve the imaging of complex fractures

Ground Penetrating Radar (GPR) images are affected, to some degree, by the relative orientation of antennas and subsurface targets. This is particularly true not only for targets that show a significant directivity, but also for inclined planes, such as fractures and faults. Depending on the relative geometry between the antennas and the orientation of the target, radar waves can be preferentially scattered, which causes changes in the reflected signal amplitude. Therefore, traditional single polarization and single azimuth surveys may produce inadequate results. The work presented here examines the use of a multi-azimuth GPR survey to increase the imaging performance of inclined fractures, showing the shortcomings of single-profile surveying and highlighting the benefits that such a strategy has on detection and characterization

GPR Method for the Detection and Characterization of Fractures and Karst Features: Polarimetry, Attribute Extraction, Inverse Modeling and Data Mining Techniques

The presence of fractures, joints and karst features within rock strongly influence the hydraulic and mechanical behavior of a rock mass, and there is a strong desire to characterize these features in a noninvasive manner, such as by using ground penetrating radar (GPR). These features can alter the incident waveform and polarization of the GPR signal depending on the aperture, fill and orientation of the features. The GPR methods developed here focus on changes in waveform, polarization or texture that can improve the detection and discrimination of these features within rock bodies. These new methods are utilized to better understand the interaction of an invasive shrub, Juniperus ashei, with subsurface flow conduits at an ecohydrologic experimentation plot situated on the limestone of the Edwards Aquifer, central Texas. First, a coherency algorithm is developed for polarimetric GPR that uses the largest eigenvalue of a scattering matrix in the calculation of coherence. This coherency is sensitive to waveshape and unbiased by the polarization of the GPR antennas, and it shows improvement over scalar coherency in detection of possible conduits in the plot data. Second, a method is described for full-waveform inversion of transmission data to quantitatively determine fracture aperture and electromagnetic properties of the fill, based on a thin-layer model. This inversion method is validated on synthetic data, and the results from field data at the experimentation plot show consistency with the reflection data. Finally, growing hierarchical self-organizing maps (GHSOM) are applied to the GPR data to discover new patterns indicative of subsurface features, without representative examples. The GHSOMs are able to distinguish patterns indicating soil filled cavities within the limestone. Using these methods, locations of soil filled cavities and the dominant flow conduits were indentified. This information helps to reconcile previous hydrologic experiments conducted at the site. Additionally, the GPR and hydrologic experiments suggests that Juniperus ashei significantly impacts infiltration by redirecting flow towards its roots occupying conduits and soil bodies within the rock. This research demonstrates that GPR provides a noninvasive tool that can improve future subsurface experimentation

IVGPR: A New Program for Advanced End-To-End GPR Processing

Ground penetrating radar (GPR) processing workflows commonly rely on techniques developed particularly for seismic reflection imaging. Although this practice has produced an abundance of reliable results, it is limited to basic applications. As the popularity of GPR continues to surge, a greater number of complex studies demand the use of routines that take into account the unique properties of GPR signals. Such is the case of surveys that examine the material properties of subsurface scatterers. The nature of these complicated tasks have created a demand for GPR-specific processing packages flexible enough to tackle new applications. Unlike seismic processing programs, however, GPR counterparts often afford only a limited amount of functionalities. This work produced a new GPR-specific processing package, dubbed IVGPR, that offers over 60 fully customizable procedures. This program was built using the modern Fortran programming language in combination with serial and parallel optimization practices that allow it to achieve high levels of performance. Within its many functions, IVGPR provides the rare opportunity to apply a three-dimensional single-component vector migration routine. This could be of great value for advanced workflows designed to develop and test new true-amplitude and inversion algorithms. Numerous examples given through this work demonstrate the effectiveness of key routines in IVGPR. Additionally, three case studies show end-to-end applications of this program to field records that produced satisfactory result well-suited interpretatio

An evaluation of the performance of multi-static handheld ground penetrating radar using full wave inversion for landmine detection

This thesis presents an empirical study comparing the ability of multi-static and bi-static, handheld, ground penetrating radar (GPR) systems, using full wave inversion (FWI), to determine the properties of buried anti-personnel (AP) landmines. A major problem associated with humanitarian demining is the occurrence of many false positives during clearance operations. Therefore, a reduction of the false alarm rate (FAR) and/or increasing the probability of detection (POD) is a key research and technical objective. Sensor fusion has emerged as a technique that promises to significantly enhance landmine detection. This study considers a handheld, combined metal detector (MD) and GPR device, and quantifies the advantages of the use of antenna arrays. During demining operations with such systems, possible targets are detected using the MD and further categorised using the GPR, possibly excluding false positives. A system using FWI imaging techniques to estimate the subsurface parameters is considered in this work.A previous study of multi-static GPR FWI used simplistic, 2D far-field propagation models, despite the targets being 3D and within the near field. This novel study uses full 3D electromagnetic (EM) wave simulation of the antenna arrays and propagation through the air and ground. Full EM simulation allows the sensitivity of radio measurements to landmine characteristics to be determined. The number and configuration of antenna elements are very important and must be optimised, contrary to the 2D sensitivity studies in (Watson, Lionheart 2014, Watson 2016) which conclude that the degree (number of elements) of the multi-static system is not critical. A novel sensitivity analysis for tilted handheld GPR antennas is used to demonstrate the positive impact of tilted antenna orientation on detection performance. A time domain GPR and A-scan data, consistent with a commercial handheld system, the MINEHOUND, is used throughout the simulated experiments which are based on synthetic GPR measurements.Finally, this thesis introduces a novel method of optimising the FWI solution through feature extraction or estimation of the internal air void typically present in pressure activated mines, to distinguish mines from non-mine targets and reduce the incidence of false positives

Structural Investigation of the Odessa Meteorite Crater Using High Resolution Geophysics in a Complex Environment

From its discovery in 1921, the Odessa Meteorite Crater has interested researchers and mining companies who had initially hoped to locate a buried mass of meteoric iron. To find the impactor, a major geologic study of the crater was conducted in 1941. Even though the impactor has not been found, the thorough geologic constraints make the crater an excellent location to test the application of near-surface geophysical methods to complex environments. Recently, researchers have focused on determining the age of the crater, the environmental effects of the impact, and the size, incident angle, and direction of the impactor responsible for the crater. However, the heavily eroded and anthropogenically modified state of the exposed crater presents several challenges to impactor attribute estimation. The exposed rim is irregular in shape such that the original size and shape of the crater is indeterminate, only ~3 m of the estimated 30 m of original crater depth remain unfilled by post impact sediment, and previous geologic studies have left the remains of several large trenches transecting the crater rim. To more accurately determine the original size of the crater, ERT and GPR geophysical methods were used to image the exposed and unexposed rim strata. However, the geologic complexity of the crater and the presence of anthropogenic or “cultural” noise posed problems to both ERT and GPR data acquisition and processing. Geophysical results point to a main crater of ~120 m in diameter and ~35 m depth. Additionally, the eastern non-circular portion of the expose crater rim is hypothesized to have form from the simultaneous impact of a small meteorite broken from the main meteorite during atmospheric entry. Further ERT study is recommended to investigate the secondary crater further

Spectral Analysis of Thinning Beds Using Ground Penetrating Radar

Ground Penetrating Radar (GPR) is a near surface geophysical method that has been used for applications including archaeological sites, groundwater contamination, and geological mapping. Though GPR has been used extensively, advancements on data processing had a great impact on data resolution. GPR is frequently used for shallow investigations because of the high resolution near the surface; however, it has limited depth of penetration and vertical bed resolution. Vertical resolution is proportional to frequency. The thickness of beds in the subsurface is conventionally resolved to one-fourth the wavelength of the central frequency. The vertical resolution at a central frequency of 200 MHz in a beach environment is approximately 17 cm; however, that value does not accurately represent fine-scale lamina and pinching out of beds, which can be an order magnitude or more than the current resolution. Complex trace analysis and spectral analysis have been used in seismic reflection for characterizing structures and stratigraphy. These "attributes" have been used to indicate hydrocarbon presence in industry. The same concept was applied to a theoretical GPR model and tested against actual data. The theoretical GPR model was created to simulate a case in which two ideal 0 degree phase Ricker wavelets merge. The wavelets constructively "add" together to create a composite wavelet with double amplitude. Applying a spectral analysis reveals that an attribute in the form of instantaneous phase and instantaneous frequency can be used to image the beds merging. The spectral analysis was applied to field data from North Padre Island National Seashore, Texas, to image "pinch-outs". Multiple survey arrays were collected using a 200 MHz frequency antenna to image internal dune structures. The results showed anomalous features at merging beds and contacts between interfaces. The results directly influence sedimentological and geomorphological interpretations of internal dune structure and can be used to better understand erosional processes in coastal sedimentary environments