Borehole Ground Penetrating Radar Research Articles

Borehole Ground Penetrating Radar attribute analysis for urban engineering

A precise and detailed knowledge and characterization of the subsurface is crucial for urban engineering planning and implementation. Drillings/boreholes are most widely used to provide the spatial distribution of blocks and boulders, but it is difficult to determine with high spatial-resolution location and distribution of the blocks and boulders in the soil stratum. We evaluate the applicability and the effectiveness of borehole Ground Penetrating Radar (GPR) for urban engineering purpose and we test attribute analysis to help the subway construction in the city center of Hangzhou, China. A multi-attribute approach exploiting amplitude-based and textural attributes is used. The results demonstrate that attribute analysis can visualize and quantitatively characterize borehole GPR features in an automatic and objective manner, maximizing the information for urban engineering. The integrated results from drillings and borehole GPR can provide an effective distribution of localized heterogeneities (such as blocks/boulders, sand/gravel lenses, cavities) in the potential target area.

Assessment of Combustion Cavern Geometry in Underground Coal Gasification Process with the Use of Borehole Ground-Penetrating Radar

In this study, the shape and size of a combustion cavity with a fracture zone in the gasified coal seam was determined with the use of control boreholes and a ground-penetrating radar (BGPR) test. The underground coal gasification (UCG) field-scale experiment was performed in Carboniferous strata in coal seam 501 at a depth of approx. 460 m in the Wieczorek hard coal mine in the Upper Silesian Coal Basin, Poland. After the termination of the UCG reactor, five coring boreholes were drilled to identify the geometry of the resulting combustion cavity and the impact of the UCG process on the surrounding rock mass. Borehole ground-penetrating radar measurements were performed using a 100 MHz antenna in three boreholes with a length of about 40–50 m. This enabled the identification of the boundaries of the combustion cavity and the fracture zone in the coal seam. The fracture zones of rock layers and lithological borders near the control borehole were also depicted. As a result, the cavity was estimated to have a length of around 32 m, a width of around 7 m and a height of around 5 m. The analyses performed with the control boreholes and the BGPR provided sufficient information to determine the geometry of the combustion cavity and the fracture zone.

Inference of geostatistical hyperparameters with the correlated pseudo-marginal method

We consider non-linear Bayesian inversion problems targeting the geostatistical hyperparameters of a random field describing hydrogeological or geophysical properties given hydrogeological or geophysical data. This problem is of particular importance in the non-ergodic setting as there are no analytical upscaling relationships linking the data to the hyperparameters, such as, mean, standard deviation, and integral scales. Full inversion of the hyperparameters and the local properties of the field (typically involving many thousands of unknowns) brings substantial computational challenges, such that simplifying model assumptions (e.g., homogeneity or ergodicity) are typically made. To prevent the errors resulting from such simplified assumptions while also circumventing the burden of high-dimensional full inversions, we use a pseudo-marginal Metropolis–Hastings algorithm that treats the random field as latent variables. In this random effects model, the intractable likelihood of observing the data given the hyperparameters is estimated by Monte Carlo averaging over realizations of the random field. To increase the efficiency of the method, low-variance approximations of the likelihood ratio are obtained by using importance sampling and by correlating the samples used in the proposed and current steps of the Markov chain. We assess the performance of this correlated pseudo-marginal method by considering two representative inversion problems involving diffusion-based and wave-based physics, respectively, in which we infer the hyperparameters of (1) hydraulic conductivity fields using apparent hydraulic conductivity data in a data-poor setting and (2) fracture aperture fields using borehole ground-penetrating radar (GPR) reflection data in a more data-rich setting. For the first test case, we find that the correlated pseudo-marginal method generates similar estimates of the geostatistical mean as classical rejection sampling, while an inversion assuming ergodicity provides biased estimates. For the second test case, we find that the correlated pseudo-marginal method estimates the hyperparameters well, while rejection sampling is computationally unfeasible and a simplified model assuming homogeneity leads to biased estimates.

Review of Ground Penetrating Radar Applications for Water Dynamics Studies in Unsaturated Zone

For water dynamics investigation in unsaturated (vadose) zones, ground penetrating radar is a popular hydro-geophysical method because it is non-invasive for soil, has high resolution and the results have a direct link with water content. Soil water content and soil hydraulic properties are two key factors for describing the water dynamics in vadose zones. There has been tremendous progress in soil water content and soil hydraulic properties estimation with ground penetrating radar. The purpose of this paper is to provide an overview of the application of ground penetrating radar for soil water dynamics studies. This paper first summarizes various methods for the determination of soil water content. including traditional methods in the surveys of surface ground penetrating radar, borehole ground penetrating radar, and off-ground ground penetrating radar, as well as relatively new methods, such as full waveform inversion, the average envelope amplitude method, and the frequency shift method. This paper further provides a review for estimating soil hydraulic properties with GPR according to the types of ground penetrating radar data. We hope that this review can provide a reference for the application of ground penetrating radar in soil water dynamics studies in the future.

Coupled full-waveform inversion of horizontal borehole ground penetrating radar data to estimate soil hydraulic parameters: A synthetic study

Coupled inversion is a promising technique for determining soil hydraulic properties from time-lapse geophysical measurements. In this inversion approach, a hydrological model is coupled with ground penetrating radar (GPR) forward modelling to avoid interpretation errors from data processing. In this study, a workflow for coupled GPR full-waveform inversion (CFWI) is proposed that combines the benefits of coupled inversion and full-waveform inversion (FWI) to estimate soil hydraulic properties from time-lapse horizontal borehole GPR measurements. In particular, a synthetic modelling study is presented that compares the performance of coupled inversion of full waveforms and first arrival time data from zero-offset profile (ZOP) borehole measurements obtained during an infiltration event for estimating hydraulic parameters and the thickness of the first layer for a 2-layer soil profile. It was found that the thickness of the first layer could be more accurately estimated by CFWI because of the additional information contained in reflected and refracted waves from the air–soil layer boundary. Furthermore, the hydraulic parameters estimated by the coupled inversion of GPR travel times slightly deviated from true values and showed a relatively large uncertainty, whereas the results of CFWI precisely matched the known water retention and hydraulic conductivity functions. Despite these promising results, several challenges should be solved before CFWI can be applied to experimental GPR data. First, conducting CFWI needs accurate conceptual setups for the hydro(geo-)logical model. Furthermore, a robust approach should be developed to accurately estimate an effective source wavelet from the experimental ZOP data, and a more sophisticated hydrological modelling approach has to be considered to better estimate the distribution of soil electrical conductivity during the infiltration event. Finally, the accuracy and efficiency of current GPR modelling methods may need to be improved.

Identification of buried historical mineshaft using ground-penetrating radar

Buried historical mineshafts constitute significant sinkhole hazards and sources of geotechnical risks. The locations of buried mineshafts on old mining maps are prone to inaccuracies. An efficient way to limit these inaccuracies is to use geophysical methods. However, their effectiveness strongly depends on the local survey conditions.The aim of the study was to identify the buried 14th century Wielki mineshaft in Bochnia Salt Mine, a UNESCO Worldwide Heritage Site, Poland. The need to locate this shaft was due to the following reasons. Firstly, the Bochnia Salt Mine had difficulties with water flowing into the workings from the supposed location of the Wielki mineshaft. The shaft area was one of the most waterlogged in the salt mine's terrain. An urgent task was to determine the shaft's exact location to implement a waterproof barrier to protect the historic mine workings. Secondly, the modernisation of the market square in Bochnia required clarification of the location of the Wielki mineshaft and checking as to whether it posed a sinkhole hazard to the terrain surface. Another important reason for locating the medieval shaft was its archaeological value.A ground surface and borehole ground-penetrating radar (GPR) study and geological-engineering recognition using boreholes were employed to indicate the location of the Wielki shaft. The anthropogenic layer of heterogeneous material and the urbanised site resulted in challenges with regard to detecting evident georadar anomalies. Several dozen intersecting GPR profiles were gathered in the research area using 250 MHz and 100 MHz antennae. The initial surface GPR results, indicative of the GPR anomaly, were consolidated by cored borehole investigation and a borehole GPR study. In particular, the presence of a void, loosened zones, and wooden fragments of the shaft lining was confirmed in the inclined control borehole. Following our classification of typical causes of the occurrence of sinkholes over buried mineshafts, the consolidation of backfill material in a mineshaft was determined as the cause of the sinkhole originated above the Wielki shaft (type B sinkhole). Destruction of the shaft's wooden cap may result in the occurrence of a sinkhole on the ground surface. Therefore, the void was backfilled and a hydraulic barrier was consequently made to stop the seepage of water into the Bochnia Salt Mine's historic workings.This case study demonstrates that surface and borehole GPR surveying integrated with borehole data offers the possibility of obtaining the high-resolution data needed to accurately locate an old buried mineshaft in an urbanised area. The GPR anomaly was indirectly detected on the basis of the deflection of the boundary between the natural ground and an anthropogenic layer with sufficient contrast in their electromagnetic properties. This approach can be useful wherever the georadar penetration range is limited by high attenuation and dispersiveness of the anthropogenic layer.

Four-dimensional tracer flow reconstruction in fractured rock through borehole ground-penetrating radar (GPR) monitoring

Abstract. Two borehole ground-penetrating radar (GPR) surveys were conducted during saline tracer injection experiments in fully saturated crystalline rock at the Grimsel Test Site in Switzerland. The saline tracer is characterized by an increased electrical conductivity in comparison to formation water. It was injected under steady-state flow conditions into the rock mass that features sub-millimeter fracture apertures. The GPR surveys were designed as time-lapse reflection GPR from separate boreholes and a time-lapse transmission survey between the two boreholes. The local increase in conductivity, introduced by the injected tracer, was captured by GPR in terms of reflectivity increase for the reflection surveys, and attenuation increase for the transmission survey. Data processing and difference imaging was used to extract the tracer signal in the reflection surveys, despite the presence of multiple static reflectors that could shadow the tracer reflection. The transmission survey was analyzed by a difference attenuation inversion scheme, targeting conductivity changes in the tomography plane. By combining the time-lapse difference reflection images, it was possible to reconstruct and visualize the tracer propagation in 3D. This was achieved by calculating the potential radially symmetric tracer reflection locations in each survey and determining their intersections, to delineate the possible tracer locations. Localization ambiguity imposed by the lack of a third borehole for a full triangulation was reduced by including the attenuation tomography results in the analysis. The resulting tracer flow reconstruction was found to be in good agreement with data from conductivity sensors in multiple observation locations in the experiment volume and gave a realistic visualization of the hydrological processes during the tracer experiments. Our methodology was demonstrated to be applicable for monitoring tracer flow and transport and characterizing flow paths related to geothermal reservoirs in crystalline rocks, but it can be transferred in a straightforward manner to other applications, such as radioactive repository monitoring or civil engineering projects.

Sequential and coupled inversion of horizontal borehole ground penetrating radar data to estimate soil hydraulic properties at the field scale

Horizontal borehole ground penetrating radar (GPR) measurements can provide valuable information on soil water content (SWC) dynamics in the vadose zone, and hence show potential to estimate soil hydraulic properties. In this study, the performance of both sequential and coupled inversion workflows to obtain soil hydraulic properties from time-lapse horizontal borehole GPR data obtained during an infiltration experiment were compared using a synthetic modelling study and the analysis of actual field data. The sequential inversion using the vadose zone flow model HYDRUS-1D directly relied on SWC profiles determined from the travel time of GPR direct waves using the straight-wave approximation. The synthetic modelling study showed that sequential inversion did not provide accurate estimates of the soil hydraulic parameters due to interpretation errors in the estimated SWC near the infiltration front and the ground surface. In contrast, the coupled inversion approach, which combined HYDRUS-1D with a forward model of GPR wave propagation (gprMax3D) and GPR travel time information, provided accurate estimates of the hydraulic properties in the synthetic modelling study. The application of the coupled inversion approach to measured borehole GPR data also resulted in plausible estimates of the soil hydraulic parameters. It was concluded that coupled inversion should be preferred over sequential inversion of time-lapse horizontal borehole GPR data in the presence of strong SWC gradients that occur during infiltration events.

Research on the Vertical Stratification Characteristics and Dielectric Constant Calculation of Geological Bodies Based on Structural Plane Information of Borehole GPR Image and Digital Panoramic Image Fusion

For Ground-penetrating radar (GPR) image and the digital panoramic image obtained from single-hole measuring, a novel and noncooperative measuring method for quantitatively calculating the relative dielectric constant of geo-bodies based on information fusion of the two types of image is proposed. Structural plane tilt angle in borehole optical image and GPR image is extracted to calculate the permittivity and quantify the geological medium properties. The vertical stratification characteristics of geo-bodies can be obtained through the analysis of changing dielectric constant. Experiment with borehole optical and GPR images in this letter reveals the continuity and discontinuity of medium change. It confirms that there are many kinds of layered structures in shallow geology, such as soil, clay, sand with different water content, crushed stone, shale, and solution cracks with varying degrees of development. On the contrary, the values of permittivity of materials in deep geology have slight variation, and it indicates that the deep geological vertical stratification characteristic is relatively simple. The method provides a new and efficient means of dielectric constant calculation and vertical stratification characteristics analysis, especially for complex shallow geology in engineering applications.

Permeability Enhancement From a Hydraulic Stimulation Imaged With Ground Penetrating Radar

AbstractWe present evidence of permeability enhancement from hydraulic stimulation experiments in fractured crystalline rock. A total of 9.49 m3 was injected in two fractured intervals of a 300 m long borehole. Repeated Ground Penetrating Radar (GPR) measurements in the same borehole were carried out prior to and following the stimulation. The initial measurements revealed fractures in the vicinity of the borehole that could be traced up to distances of 50 m away. The data measured post‐stimulation were used in a difference‐imaging approach to illuminate changes in the GPR reflections caused by the stimulations. The changes delineate the enhancement of a large and complex fracture network. These changes likely correspond to changes in local aperture, thus permeability. Our results indicate that borehole GPR yields unique information on subtle changes in hydraulic properties within a relatively large volume and provides a new perspective on the characterization and monitoring of deep geothermal reservoirs.

Characterizing a decametre-scale granitic reservoir using ground-penetrating radar and seismic methods

Abstract. Ground-penetrating radar (GPR) and seismic imaging have proven to be important tools for the characterization of rock volumes. Both methods provide information about the physical rock mass properties and geological structures away from boreholes or tunnel walls. Here, we present the results from a geophysical characterization campaign that was conducted as part of a decametre-scale hydraulic stimulation experiment in the crystalline rock volume of the Grimsel Test Site (central Switzerland). For this characterization experiment, we used tunnel-based GPR reflection imaging as well as seismic travel-time tomography to investigate the volumes between several tunnels and boreholes. The interpretation of the GPR data with respect to geological structures is based on the unmigrated and migrated images. For the tomographic analysis of the seismic first-arrival travel-time data, we inverted for an anisotropic velocity model described by the Thomsen parameters v0, ϵ and δ to account for the rock mass foliation. Subsequently, the GPR and seismic images were interpreted in combination with the geological model of the test volume and the known in situ stress states. We found that the ductile shear zones are clearly imaged by GPR and show an increase in seismic anisotropy due to a stronger foliation, while they are not visible in the p-wave (v0) velocity model. Regions of decreased seismic p-wave velocity, however, correlate with regions of high fracture density. For geophysical characterization of potential deep geothermal reservoirs, our results imply that wireline-compatible borehole GPR should be considered for shear zone characterization, and that seismic anisotropy and velocity information are desirable to acquire in order to gain information about ductile shear zones and fracture density, respectively.

Detection imaging of impulse borehole well-logging radar

Borehole ground-penetrating radar (hereinafter referred to as GPR) technology is a geophysical detection method in the well, in which, the high-frequency electromagnetic waves are transmitted to stratum and the spread properties of electromagnetic waves in the stratum are used to obtain stratum information, thereby interpreting the subsurface structures. Based on the basic principle of target detection, a set of borehole GPR was developed in this paper. Many detection experiments were conducted on this radar system, and fairly good imaging results were obtained, indicating the superior performance of impulse system-based UWB radar in geological research and mining exploration.

Measuring Soil Water Content with Ground Penetrating Radar: A Decade of Progress

Core Ideas There has been tremendous progress in GPR as a tool for soil water content determination. Numerous studies have shown the potential of GPR to detect and map SWC. We highlight new possibilities and achievements for GPR acquisition and processing strategies. Quantitative SWC detection and hydrological parameter estimation are possible using GPR. We encourages other communities to embrace GPR as a tool for SWC determination. Tremendous progress has been made with respect to ground penetrating radar (GPR) equipment, data acquisition, and processing since the establishment of GPR as a tool for soil water content determination in vadose zone hydrology about 25 yr ago. In this update, we aim to provide a critical overview of recent advances in vadose zone applications of GPR with a particular focus on new possibilities for multi‐offset and borehole GPR measurements, the development of quantitative off‐ground GPR methods, full‐waveform inversion of GPR measurements, the potential of time‐lapse GPR measurements for process investigations and hydrological parameter estimation, and recent improvements in GPR instrumentation. We hope that this update encourages the soil hydrology, groundwater, and critical zone community to embrace GPR as a viable tool for soil water content determination and the elucidation of subsurface hydrological processes.

Investigating the Surface and Subsurface in Karstic Regions – Terrestrial Laser Scanning versus Low-Altitude Airborne Imaging and the Combination with Geophysical Prospecting

Combining measurements of the surface and subsurface is a promising approach to understand the origin and current changes of karstic forms since subterraneous processes are often the initial driving force. A karst depression in south-west Germany was investigated in a comprehensive campaign with remote sensing and geophysical prospecting. This contribution has two objectives: firstly, comparing terrestrial laser scanning (TLS) and low-altitude airborne imaging from an unmanned aerial vehicle (UAV) regarding their performance in capturing the surface. Secondly, establishing a suitable way of combining this 3D surface data with data from the subsurface, derived by geophysical prospecting. Both remote sensing approaches performed satisfying and the established digital elevation models (DEMs) differ only slightly. These minor discrepancies result essentially from the different viewing geometries and post-processing concepts, for example whether the vegetation was removed or not. Validation analyses against high-accurate DGPS-derived point data sets revealed slightly better results for the DEMTLS with a mean absolute difference of 0.03 m to 0.05 m and a standard deviation of 0.03 m to 0.07 m (DEMUAV: mean absolute difference: 0.11 m to 0.13 m; standard deviation: 0.09 m to 0.11 m). The 3D surface data and 2D image of the vertical cross section through the subsurface along a geophysical profile were combined in block diagrams. The data sets fit very well and give a first impression of the connection between surface and subsurface structures. Since capturing the subsurface with this method is limited to 2D and the data acquisition is quite time consuming, further investigations are necessary for reliable statements about subterraneous structures, how these may induce surface changes, and the origin of this karst depression. Moreover, geophysical prospecting can only produce a suspected image of the subsurface since the apparent resistivity is measured. Thus, further measurements, such as borehole drillings or ground-penetrating radar are necessary for a closer analysis of the subsurface. In summary, satisfying results were achieved, which however pave the way for further studies.

Integration of electrical resistivity imaging (ERI) and ground-penetrating radar (GPR) methods to identify soil profile around Necatibey Subway Station, Ankara, Turkey

Electrical Resistivity Imaging (ERI) and Ground-Penetrating Radar (GPR) methods were utilized in this study to identify soil profiles around Necatibey Subway Station of Kizilay–Cayyolu metro line, Ankara, Turkey. The Necatibey Metro Station is located within the alluvial deposits of Dikmen stream and the so-called Ankara clay. At the metro station, a number of boreholes were drilled. However, due to the spacing of the boreholes the boundary between alluvium and Ankara clay deposits could not be separated precisely. Thus, in this study, ERI and GPR methods were utilized to distinguish soil types at the study area. GPR measurements were taken from a total of 14 profiles and total length of the profiles was about 320 m. For every ERI measurement section, Schlumberger Dipole–Dipole, Dipole–Dipole, Schlumberger and Wenner arrays were used. Results from the geophysical measurements identified that the fill materials are underlain by the Dikmen stream channel deposits, which consist of silty clay and gravelly sand units. The study also shows that the Dikmen stream channel deposits are underlain by the Ankara clay unit. The meaningful range of resistivity values was between 1 and 15 Ωm, and the GPR signals were strong in sandy units while they attenuated in clayey environments. Based on borehole logs, ERI and GPR data, three-dimensional lithological subsurface model of the survey area was constructed. The resultant three-dimensional diagrams may serve engineers as a practical tool during different construction stages, groundwater–surface water interactions within short and long term, and probable remedial measures.

Coupling ground-penetrating radar and flowmeter investigations for the characterization of a fissured aquifer

We test the integration of ground-penetrating radar (GPR) borehole investigation and electromagnetic flowmeter surveys to characterize fissured or complex aquifers. Borehole GPR investigation allows assessment of the lateral continuity of the porous and permeable zones between closely spaced boreholes, whereas flowmeter tests are adopted to identify the inflow–outflow zones in the boreholes. We apply the coupled approach in a portion (between 10 and 20 m below ground surface) of a complex aquifer hosted in a calcareous–gypsiferous formation. The hydro-geophysical characterization was aimed to detect the most conductive levels and to estimate the hydrodynamic properties of the aquifer. The integration of single-hole flowmeters and GPR measurements accurately distinguished different productive levels of high porosity and permeability in the aquifer. Two aquifer systems characterized by a complex geometry of fissures and permeable layers have been recognized, establishing the role of the drilled boreholes in the short-circuiting of the two systems. Interpretation of the GPR survey in cross-hole configuration gave values of total porosity of the order of 0.35 and 0.5 in the two more productive levels, which were detected by the flowmeter investigation.

Study on High Prestressed Anchor Beam Supporting Optimization Technology in Deep Roadway

Coal mine has stepped into deep mining period in China, in which the ground pressure is great, the stability of the surrounding rock is poor and roadway is seriously destroyed. 3302 trick roadway of Zhaolou coal mine is the kilometers depth roadway, under the condition of original supporting, the roof settling amount and the surrounding rock deformation were large, which made bad effect on safety production. The surrounding rock failure region was monitored by borehole televiewer and ground penetrating radar, and the loosening failure law was analyzed. The scheme of high prestressed anchor beam was proposed according to the detection results, the crack distribution of surrounding rock and the stress of blots and anchors of original scheme and optimized scheme were analyzed by UDEC. The results of numerical simulation and field monitoring showed that increasing prestress of blot and anchors was better for the control of roof separation in anchorage zone and maintaining the integrity of roof, and high prestressed anchor beam can effectively improve the effect of surrounding rock control in roadways.

Estimating water content in an active landfill with the aid of GPR

Landfill gas (LFG) receives a great deal of attention due to both negative and positive environmental impacts, global warming and a green energy source, respectively. However, predicting the quantity of LFG generated at a given landfill, whether active or closed is difficult due to the heterogeneities present in waste, and the lack of accurate in situ waste parameters like water content. Accordingly, ground penetrating radar (GPR) was evaluated as a tool for estimating in situ water content. Due to the large degree of subsurface heterogeneity and the electrically conductive clay cap covering landfills, both of which affect the transmission of the electromagnetic pulses, there is much scepticism concerning the use of GPR to quantify in situ water content within a municipal landfill.Two landfills were studied. The first landfill was used to develop the measurement protocols, while the second landfill provided a means of confirming these protocols. GPR measurements were initially completed using the surface GPR approach, but the lack of success led to the use of borehole (BH) GPR. Both zero offset profiling (ZOP) and multiple offset gathers (MOG) modes were tried, with the results indicating that BH GPR using the ZOP mode is the most simple and efficient method to measure in situ water content. The best results were obtained at a separation distance of 2m, where higher the water content, smaller the effective separation distance. However, an increase in water content did appear to increase the accuracy of the GPR measurements. For the effective separation distance of 2m at both landfills, the difference between GPR and lab measured water contents were reasonable at 33.9% for the drier landfill and 18.1% for the wetter landfill. Infiltration experiments also showed the potential to measure small increases in water content.

Field GPR monitoring of biostimulation in saturated porous media

Abstract Time-lapse borehole radar was used to monitor a sandy aquifer with dielectric property changes hypothesized to result from biomass growth and biodegradation of hydrocarbons. A blend of gasoline and ethanol was released below the water table at Canadian Forces Base (CFB) Borden, Canada, and a source of oxygen was used for cyclic stimulation of microbial activity over a period of two years. A dense grid of fourteen borehole ground-penetrating radar (GPR) pairs monitoring the bioactive region showed 200 MHz radar wave velocity changes of ± 4 % and signal attenuation changes of ± 25 % during three cycles of variable biostimulation. GPR signal changes correlated spatially and temporally to independent measurements of groundwater velocity flow changes and geochemical variations that occurred in response to microbial activity. Greater relative changes in radar wave velocity of propagation were observed in the region of enhanced bacterial stimulation, adjacent to the oxygen release wells, and along the path of the hydrocarbon plume where biomass growth was the greatest. In the zone of maximum biological activity, radar wave velocity decreased during two cycles of enhanced biostimulation, whereas it increased during the intervening period of low-level biostimulation. Monitored regions at the fringe of the plume and farther down-gradient of the oxygen source, where biostimulation was less or absent, exhibited GPR signal changes of lesser magnitude and consistency. Spatial and temporal variabilities of GPR observations indicate a high degree of transient heterogeneity within the relatively homogeneous Borden aquifer as a result of hydrocarbon biodegradation. Mechanisms postulated to cause the dielectric property changes observed in the radar data are biogenic gas formation, mineral dissolution/precipitation, and direct biomass growth. Geochemical data, in-situ groundwater velocity measurements and biomass quantification suggest biomass formation in the pore space as a plausible mechanism for the changes observed in the aquifer. Therefore, at 200 MHz frequency, biomass formation is observed to cause a decrease of EM wave velocity of propagation suggesting an increase of the bulk dielectric constant of the water saturated medium. This finding is in agreement with published laboratory-scale GPR monitoring of microbial growth in porous media. This study shows that time-lapse GPR along with supporting biogeochemical observations can be used for monitoring of biological activity at the field scale.

Geological disaster survey based on Curvelet transform with borehole Ground Penetrating Radar in Tonglushan old mine site

Tonglushan old mine site located in Huangshi City, China, is very famous in the world. However, some of the ruins had suffered from geological disasters such as local deformation, surface cracking, in recent years. Structural abnormalities of rock-mass in deep underground were surveyed with borehole ground penetrating radar (GPR) to find out whether there were any mined galleries or mined-out areas below the ruins. With both the multiresolution analysis and sub-band directional of Curvelet transform, the feature information of targets' GPR signals were studied on Curvelet transform domain. Heterogeneity of geotechnical media and clutter jamming of complicated background of GPR signals could be conquered well, and the singularity characteristic information of typical rock mass signals could be extracted. Random noise had be removed by thresholding combined with Curvelet and the statistical characteristics of wanted signals and the noise, then direct wave suppression and the spatial distribution feature extraction could obtain a better result by making use of Curvelet transform directional. GprMax numerical modeling and analyzing of the sample data have verified the feasibility and effectiveness of our method. It is important and applicable for the analyzing of the geological structure and the disaster development about the Tonglushan old mine site.