Inversion of multiple intersecting high-resolution crosshole GPR profiles for hydrological characterization at the Boise Hydrogeophysical Research Site

The integration of geophysical data into the subsurface characterization problem has been shown in many cases to significantly improve hydrological knowledge by providing information at spatial scales and locations that is unattainable using conventional hydrological measurement techniques. In particular, crosshole ground-penetrating radar (GPR) tomography has shown much promise in hydrology because of its ability to provide highly detailed images of subsurface radar wave velocity, which is strongly linked to soil water content. Here, we develop and demonstrate a procedure for inverting together multiple crosshole GPR data sets in order to characterize the spatial distribution of radar wave velocity below the water table at the Boise Hydrogeophysical Research Site (BHRS) near Boise, Idaho, USA. Specifically, we jointly invert 31 intersecting crosshole GPR profiles to obtain a highly resolved and consistent radar velocity model along the various profile directions. The model is found to be strongly correlated with complementary neutron porosity-log data and is further corroborated by larger-scale structural information at the BHRS. This work is an important prerequisite to using crosshole GPR data together with existing hydrological measurements for improved groundwater flow and contaminant transport modeling.     

Application of the virtual refraction to near‐surface characterization at the Boise Hydrogeophysical Research Site‡

Seismic interferometry is a relatively new technique to estimate the Green's function between receivers. Spurious energy, not part of the true Green's function, is produced because assumptions are commonly violated when applying seismic interferometry to field data. Instead of attempting to suppress all spurious energy, we show how spurious energy associated with refractions contains information about the subsurface in field data collected at the Boise Hydrogeophysical Research Site. By forming a virtual shot record we suppress uncorrelated noise and produce a virtual refraction that intercepts zero offset at zero time. These two features make the virtual refraction easy to pick, providing an estimate of refractor velocity. To obtain the physical parameters of the layer above the refractor we analyse the cross‐correlation of wavefields recorded at two receivers for all sources. A stationary‐phase point associated with the correlation between the reflected wave and refracted wave from the interface identifies the critical offset. By combining information from the virtual shot record, the correlation gather and the real shot record we determine the seismic velocities of the unsaturated and saturated sands, as well as the variable relative depth to the water‐table. Finally, we discuss how this method can be extended to more complex geologic models.     

Aquifer Imaging with Oscillatory Hydraulic Tomography: Application at the Field Scale

Modeling and laboratory experiments have demonstrated the ability of oscillatory hydraulic tomography (OHT) to characterize heterogeneity in aquifer hydraulic properties. In OHT, a location is stressed via periodic pumping/injection at a set frequency, and the resulting head signal is measured at a number of monitoring locations. The source of oscillations is repeatedly moved, allowing tomographic imaging of aquifer properties. Changing the period of oscillation also results in observations with additional information. In theory, OHT is comparable to other hydraulic tomography methods in that distributed pressure change measurements provide characterization information. In practice, OHT has several benefits including: (1) little to no water injected into or extracted from the aquifer; and (2) an observational signal at a set period that can be easily extracted in the presence of noise. We report the first field application of OHT, carried out at the Boise Hydrogeophysical Research Site (BHRS) using an oscillating signal generator with a very small cycling volume of <2 L, and a period range of 5 to 70 s. For these tests, signals were detected at distances of over 15 m. After processing to extract periodic signal properties, we perform tomography using a frequency-domain numerical model for groundwater flow. In comparing results against prior characterization results from the BHRS, we find moderate to strong positive correlations between K profiles estimated via different methods at multiple wells, with moderate overall correlation between estimated three-dimensional (3D) K volumes.     

Simulation for ground penetrating radar (GPR) study of the subsurface structure of the Moon

Ground penetrating radar (GPR) is currently within the scope of China's Chang-E 3 lunar mission, to study the shallow subsurface of the Moon. In this study, key factors that could affect a lunar GPR performance, such as frequency, range resolution, and antenna directivity, are discussed firstly. Geometrical optics and ray tracing techniques are used to model GPR echoes, considering the transmission, attenuation, reflection, geometrical spreading of radar waves, and the antenna directivity. The influence on A-scope GPR echoes and on the simulated radargrams for the Sinus Iridum region by surface and subsurface roughness, dielectric loss of the lunar regolith, radar frequency and bandwidth, and the distance between the transmit and receive antennas are discussed. Finally, potential scientific return about lunar subsurface properties from GPR echoes is also discussed. Simulation results suggest that subsurface structure from several to hundreds of meters can be studied from GPR echoes at P and VHF bands, and information about dielectric permittivity and thickness of subsurface layers can be estimated from GPR echoes in combination with regolith composition data.     

Recursive impedance inversion of ground-penetrating radar data in stochastic media

The travel time and amplitude of ground-penetrating radar (GPR) waves are closely related to medium parameters such as water content, porosity, and dielectric permittivity. However, conventional estimation methods, which are mostly based on wave velocity, are not suitable for real complex media because of limited resolution. Impedance inversion uses the reflection coefficient of radar waves to directly calculate GPR impedance and other parameters of subsurface media. We construct a 3D multiscale stochastic medium model and use the mixed Gaussian and exponential autocorrelation function to describe the distribution of parameters in real subsurface media. We introduce an elliptical Gaussian function to describe local random anomalies. The tapering function is also introduced to reduce calculation errors caused by the numerical simulation of discrete grids. We derive the impedance inversion workflow and test the calculation precision in complex media. Finally, we use impedance inversion to process GPR field data in a polluted site in Mongolia. The inversion results were constrained using borehole data and validated by resistivity data.     

Improvements in crosshole GPR full-waveform inversion and application on data measured at the Boise Hydrogeophysics Research Site

Crosshole ground penetrating radar (GPR) tomography has been widely used and has the potential to improve the obtained subsurface models due to its high spatial resolution compared to other methods. Recent advances in full-waveform inversion of crosshole GPR data show that higher resolution images can be obtained compared to conventional ray-based GPR inversion because it can exploit all information present in the observed data. Since the first application of full-waveform inversion on synthetic and experimental GPR data, the algorithm has been significantly improved by extending the scalar to a vectorial approach, and changing the stepped permittivity and conductivity update into a simultaneous update. Here, we introduce new normalized gradients that do not depend on the number of sources and receivers which enable a comparison of the gradients and step lengths for different crosshole survey layouts. An experimental data set acquired at the Boise Hydrogeophysics Research Site is inverted using different source–receiver setups and the obtained permittivity and conductivity images, remaining gradients and final misfits are compared for the different versions of the full-waveform inversion. Moreover, different versions of the full-waveform inversion are applied to obtain an overview of all improvements. Most improvements result in a reducing final misfit between the measured and synthetic data and a reducing remaining gradient at the final iteration. Regions with relatively high remaining gradient amplitudes indicate less reliable inversion results. Comparison of the final full-waveform inversion results with Neutron–Neutron porosity log data and capacitive resistivity log data show considerably higher spatial frequencies for the logging data compared to the full-waveform inversion results. To enable a better comparison, we estimated a simple wavenumber filter and the full-waveform inversion results show an improved fit with the logging data. This work shows the potential of full-waveform inversion as an advanced method that can provide high resolution images to improve hydrological models.     

Spectral bandwidth enhancement of GPR profiling data using multiple-frequency compositing

The amplitude spectrum of ground penetrating radar (GPR) reflection data acquired with a particular antenna set is normally concentrated over a spectral bandwidth of a single octave, limiting the resolving power of the GPR wavelet. Where variously-sized GPR targets are located at numerous depths in the ground, it is often necessary to acquire several profiles of GPR data using antennas of different nominal frequencies. The most complete understanding of the subsurface is obtained when those frequency-limited radargrams are jointly interpreted, since each frequency yields a particular response to subsurface reflectivity. The application of deconvolution to GPR data could improve image quality, but is often hindered by limited spectral bandwidth.We present multiple-frequency compositing as a means of combining data from several frequency-limited datasets and improving the spectral bandwidth of the GPR profile. A multiple-frequency composite is built by summing together a number of spatially-coincident radargrams, each acquired with antennae of different centre frequency. The goal of the compositing process is therefore to produce a composite radargram with balanced contributions from frequency-limited radargrams and obtain a composite wavelet that has properties approximating a delta function (i.e. short in duration and having a broad, uniform spectral bandwidth).A synthetic investigation of the compositing process was performed using Berlage wavelets as proxies for GPR source pulses. This investigation suggests that a balanced, broad bandwidth, effective source pulse is obtained by a compositing process that equalises the spectral maxima of frequency-limited wavelets prior to summation into the composite. The compositing of real GPR data was examined using a set of 225, 450 and 900 MHz GPR common offset profiles acquired at a site on the Waterloo Moraine in Ontario, Canada. The most successful compositing strategy involved derivation of scaling factors from a time-variant least squares analysis of the amplitude spectra of each frequency-limited dataset. Contributions to the composite from each nominal acquisition frequency are clear, and the trace averaged amplitude spectrum of the corresponding composite is broadened uniformly over a bandwidth approaching two-octaves. Improvements to wavelet resolution are clear when a composite radargram is treated with a spiking deconvolution algorithm. Such improvement suggests that multiple-frequency compositing is a useful imaging tool, and a promising foundation for improving deconvolution of GPR data.     

Reconstruction of the Water Table from Self-Potential Data: A Bayesian Approach

Ground water flow associated with pumping and injection tests generates self-potential signals that can be measured at the ground surface and used to estimate the pattern of ground water flow at depth. We propose an inversion of the self-potential signals that accounts for the heterogeneous nature of the aquifer and a relationship between the electrical resistivity and the streaming current coupling coefficient. We recast the inversion of the self-potential data into a Bayesian framework. Synthetic tests are performed showing the advantage in using self-potential signals in addition to in situ measurements of the potentiometric levels to reconstruct the shape of the water table. This methodology is applied to a new data set from a series of coordinated hydraulic tomography, self-potential, and electrical resistivity tomography experiments performed at the Boise Hydrogeophysical Research Site, Idaho. In particular, we examine one of the dipole hydraulic tests and its reciprocal to show the sensitivity of the self-potential signals to variations of the potentiometric levels under steady-state conditions. However, because of the high pumping rate, the response was also influenced by the Reynolds number , especially near the pumping well for a given test. Ground water flow in the inertial laminar flow regime is responsible for nonlinearity that is not yet accounted for in self-potential tomography. Numerical modeling addresses the sensitivity of the self-potential response to this problem.     

探地雷达在多年冻土工程地质勘察中的应用效果研究

由于融土和冻土之间存在明显的电性差异,使探地雷达成为研究多年冻土的有效手段之一.本文结合工程实例,对探地雷达在青藏高原多年冻土工程地质勘察中的应用效果进行了现场试验研究.分析了探地雷达探测多年冻土的物理前提条件,总结了多年冻土主要地质要素的雷达图像特征和探地雷达在多年冻土工程地质勘察中的实际应用效果.研究表明融土与冻土的雷达图像特征存在明显差异:融土雷达反射波为低频强宽振幅的稀疏波,波形较杂乱;冻土反射波为高频低振幅细密波,波形较为规则;探地雷达可较为准确地划分地层、识别多年冻土上限、确定多年冻土分布范围,但尚不能有效确定多年冻土的含冰量;另外,探地雷达对细颗粒土的探测效果要明显好于粗颗粒土.文章还指出了现场探测和图像解译分析中需要注意的事项和下一步需要研究解决的问题,建议在开展现场探测工作之前先在有钻探资料或天然地质剖面处进行对比试验,搞清测区内主要地层的物性参数,掌握有效波和干扰波的分布规律,从而提高雷达探测结果的准确性和可靠性.     

Examining the information content of time-lapse crosshole GPR data collected under different infiltration conditions to estimate unsaturated soil hydraulic properties

Time-lapse geophysical data acquired during transient hydrological experiments are being increasingly employed to estimate subsurface hydraulic properties at the field scale. In particular, crosshole ground-penetrating radar (GPR) data, collected while water infiltrates into the subsurface either by natural or artificial means, have been demonstrated in a number of studies to contain valuable information concerning the hydraulic properties of the unsaturated zone. Previous work in this domain has considered a variety of infiltration conditions and different amounts of time-lapse GPR data in the estimation procedure. However, the particular benefits and drawbacks of these different strategies as well as the impact of a variety of key and common assumptions remain unclear. Using a Bayesian Markov-chain-Monte-Carlo stochastic inversion methodology, we examine in this paper the information content of time-lapse zero-offset-profile (ZOP) GPR traveltime data, collected under three different infiltration conditions, for the estimation of van Genuchten–Mualem (VGM) parameters in a layered subsurface medium. Specifically, we systematically analyze synthetic and field GPR data acquired under natural loading and two rates of forced infiltration, and we consider the value of incorporating different amounts of time-lapse measurements into the estimation procedure. Our results confirm that, for all infiltration scenarios considered, the ZOP GPR traveltime data contain important information about subsurface hydraulic properties as a function of depth, with forced infiltration offering the greatest potential for VGM parameter refinement because of the higher stressing of the hydrological system. Considering greater amounts of time-lapse data in the inversion procedure is also found to help refine VGM parameter estimates. Quite importantly, however, inconsistencies observed in the field results point to the strong possibility that posterior uncertainties are being influenced by model structural errors, which in turn underlines the fundamental importance of a systematic analysis of such errors in future related studies.     

A Potential-Based Inversion of Unconfined Steady-State Hydraulic Tomography

The importance of estimating spatially variable aquifer parameters such as transmissivity is widely recognized for studies in resource evaluation and contaminant transport. A useful approach for mapping such parameters is inverse modeling of data from series of pumping tests, that is, via hydraulic tomography. This inversion of field hydraulic tomographic data requires development of numerical forward models that can accurately represent test conditions while maintaining computational efficiency. One issue this presents is specification of boundary and initial conditions, whose location, type, and value may be poorly constrained. To circumvent this issue when modeling unconfined steady-state pumping tests, we present a strategy that analyzes field data using a potential difference method and that uses dipole pumping tests as the aquifer stimulation. By using our potential difference approach, which is similar to modeling drawdown in confined settings, we remove the need for specifying poorly known boundary condition values and natural source/sink terms within the problem domain. Dipole pumping tests are complementary to this strategy in that they can be more realistically modeled than single-well tests due to their conservative nature, quick achievement of steady state, and the insensitivity of near-field response to far-field boundary conditions. After developing the mathematical theory, our approach is first validated through a synthetic example. We then apply our method to the inversion of data from a field campaign at the Boise Hydrogeophysical Research Site. Results from inversion of nine pumping tests show expected geologic features, and uncertainty bounds indicate that hydraulic conductivity is well constrained within the central site area.     

Mapping agricultural fields with GPR and EMI to identify offsite movement of agrochemicals

Offsite movement of waterborne agrochemicals is increasingly targeted as a non-point source of water quality degradation. Our research has indicated that subsurface water movement is variable and site-specific, and that a small soil volume frequently conducts a large volume of flow. This concentrated flow is usually caused by soil morphology, and it often results in water moving rapidly offsite from certain areas of fields; little or no lateral subsurface flow may occur in other areas. Identifying these subsurface regions is difficult using conventional soil survey and vadose zone sampling techniques. In this study, traditional surveying is combined with electromagnetic induction (EMI) and ground-penetrating radar (GPR) mapping to identify areas with high potential for subsurface offsite movement of agrochemicals, optimizing these identification techniques, and expanding the mapping procedures to make them useful at the field-scale for agricultural production practices. Conclusions from this research are: (1) EMI mapping provides rapid identification of areas of soil with a high electrical conductivity and presumably high potential for offsite movement of subsurface water, (2) GPR mapping of areas identified by EMI mapping provides a means to identify features that are known to conduct concentrated lateral flow of water, and (3) combining the capabilities of EMI and GPR instrumentation makes possible the surveys of large areas that would otherwise be impossible or unfeasible to characterize.     

In situ 2-D and 3-D measurements of radiation patterns of half-wave dipole GPR antennas

Dipole antennas for ground-penetrating radar (GPR) radiate and receive electromagnetic waves with a strong directional dependence. Thus, experiments to measure in situ antenna radiation as functions of direction and polarization are of practical interest. Three field experiments were performed. One experiment was over a layered fluvial/eolian sequence; the other two used controlled targets (buried pipes and a metal ball). The radiation patterns were sampled by incrementally varying the antenna orientations and separations while recording reflections from the known targets. The results show qualitative, but systematic, correspondence with approximate theoretical far-field radiation patterns. Slow variations of amplitude with antenna azimuth and dip indicate that antenna orientations within 20° in the standard TE and TM acquisition geometries are adequate for most field applications, but not if detailed amplitude analysis is to be performed. Variations in antenna orientation or height (particularly for heights less than one-quarter wavelength) above the free surface introduce corresponding biases or uncertainties into recorded amplitudes. The variance within any suite of measurements is, in part, a consequence of differences in ground impedance at each antenna location. The theoretically predicted sensitivity to antenna height is mediated by surface roughness at high frequencies. It is necessary to include, or compensate for, the antenna radiation pattern in analysis of field data amplitudes, in experiment design, and in selecting appropriate antennas for specific applications.     

Reduction of reflections from above surface objects in GPR data

During a ground-penetrating radar (GPR) survey, special attention must be paid to objects located above the earth's surface. Due to the low-loss character of electromagnetic propagation in air and high velocity, above-surface reflections or diffractions can overwhelm subsurface events, making the interpretation a difficult task. The relative sensitivity of reflections and diffractions originating from above-surface objects is a function of the antenna radiation characteristics, the lateral and vertical dimensions of the objects and their position with respect to the antennas. The largest amplitude reflections and diffractions are expected when the polarization of the electric field is parallel to the long-axis of the object. Near the surface in the E-plane, the electric field is vertically polarized and has a larger amplitude than the horizontally polarized electric field in the H-plane. Numerical modeling of reflections from three above surface objects (a vertical plane and elongated horizontal and vertical objects) demonstrate that the largest amplitude difference occurs when an elongated vertical object is present in the E- or H-plane. The calculated reflection from the elongated vertical object present in the E-plane was 21 times larger than when it was present in the H-plane. In 60-m long field data sets, reflections from interfering trees present in the E-plane were at several positions >15 times larger and on average 6 times larger than when the trees were present in the H-plane. These large amplitude differences indicate that appropriate orientation of the antennas can be used to minimize the effects of above-surface reflections and diffractions.     

相控阵探地雷达数据傅氏有限差分偏移研究

偏移归位是探地雷达资料处理中的一种重要方法。本文利用傅里叶有限差分法对相控阵探地雷达样机采集的实测数据进行偏移归位。异常点绕射波形得到有效的归位,异常体空间位置与实际测量的结果基本一致并且处理速度快,不仅表明了该方法的有效性,证明相控阵探地雷达技术具有一定研究价值,也为仪器的进一步完善提供了思路。     

Characterization of a coastal aquifer basin using gravity and resistivity methods: a case study from Aqaba in Jordan

The southern basin of Aqaba forms coastal aquifer and comprises an area of about 90 km². Alluviums and Pleistocene deposits fill the basin bounded by Precambrian Granite Basement to the east and north. Hydrogeophysical investigations were carried out to investigate its groundwater potentials and characteristics. Direct modeling on Bouger and residual gravity revealed some local subsurface faults that form subbasins and sub-grabens in the body of the basin, each one of them is bounded by two to three faults. The spatial distribution of groundwater was found to be affected by the presence of the sub-basins and grabens. Geoelectrical layers have been vertically digitized and put in a four variable space-lattice, 2D slices of the different depths and 3D visualizations have been produced. The extents of seawater intrusion and zones of water qualities were delineated. The geoelectric inferred some local subsurface faults that were found in spatial correlation with those inferred from the gravity. A good combination was made between gravity and resistivity methods to confirm the 3D distribution of groundwater in the basin.     

Application of the Brewster angle to quantify the dielectric properties of ground ice formations

Previous studies of ground ice using moveout type ground-penetrating radar (GPR) surveys indicate that the dielectric permittivity can constrain the type of ground ice present in the subsurface. Due to the high-loss nature of the active layer over permafrost targets, however, the signal strength of GPR signals is often insufficient to resolve the basal boundary required for determining the dielectric permittivity of an underlying unit. We apply a non-conventional antenna orientation and post-processing method to determine the dielectric permittivity of the unit underlying the lowest resolvable boundary. We conduct moveout surveys using a 450 MHz GPR with collinear parallel oriented antennas on two adjacent ground ice formations in the region of Thomas Lee Inlet, Devon Island, Nunavut. We exploit the Brewster angle to calculate the approximate dielectric permittivity of ground ice formations below the active layer. The results agree within 1 dielectric unit with on-ice permittivity measurements made during a complementary study of the site.     

3D FD resistivity modelling and sensitivity analyses applied to a highly resistive phonolitic body

A highly resistive phonolitic body near Sainerholz / Westerwald in Germany has been investigated for geological mapping using vertical electrical soundings in a Schlumberger configuration. Because of its explicitly three-dimensional shape, conventional 1D and 2D interpretation techniques are not applicable. Therefore, a new 3D finite-difference forward modelling algorithm has been applied to acquire information about its subsurface structure and to explain the observed data. This investigation focuses on two exposed soundings: one located near the centre of the body and the other close to its rim. For the interpretation, data from electromagnetic measurements on the lateral extension of the body are additionally taken into account as well as geological a priori information. A possible 3D conductivity model is presented and evidence for its validity is discussed using model studies and sensitivity analyses. The latter are carried out using a newly developed 3D FD sensitivity modelling code with which the total subsurface response can be decomposed. This permits the determination of the resolution of model parameters, indicating the contribution of different parts of the model to the overall response. The results emphasize the feasibility of 3D forward modelling in practice.     

机载探地雷达数值模拟及逆时偏移成像

机载探地雷达可以用于人类无法到达的危险地区、植被严重覆盖的地下目标体探测,然而由于机载探地雷达的特殊性,影响机载探地雷达探测效果的因素包括天线的极化方向、天线的飞行高度以及地表粗糙度等.为了研究这些影响因素与探测效果之间的关系,用三维时间域有限差分模拟电磁波的传播过程,以沙漠地区地下空洞掩体的机载探地雷达探测为实例,分别模拟了不同天线极化方向、天线高度及地表粗糙度情况下的机载探地雷达剖面,分析了各因素对机载探地雷达探测地下空洞目标体的影响.天线极化方向与目标体走向垂直更有利于地下目标体探测;天线距离地表越近,可以获得更高分辨率的雷达剖面;沙漠地表起伏越大,雷达剖面中的散射杂波能量越强,浅部地下目标体信号容易被掩盖.为了消除起伏地形造成的散射杂波,提出用逆时偏移成像技术对共炮集机载探地雷达数据进行偏移成像,成像结果优于基尔霍夫偏移成像结果.     

Site effects in 3D basins using 1D and 2D models: an evaluation of the differences based on simulations of the seismic response of Euroseistest

Site effects are one of the most predictable factors of destructive earthquake ground motion but results depend on the type of model chosen. We compare simulations of ground motion for a 3D model of the Mygdonian basin in northern Greece (Euroseistest) using different approximation for this basin. Site effects predicted using simple 1D models at many points inside the basin are compared to site effects predicted using four different 2D cross sections across the basin and with results for a full 3D simulation. Surface topography was neglected but anelastic attenuation was included in the simulations. We show that lateral heterogeneity may increase ground motion amplification by 100 %. Larger amplification is distributed in a wide frequency range, and amplification may occur at frequencies different from the expected resonant frequencies for the soil column. In contrast, on a different cross section, smaller conversion of incident energy into surface waves and larger dispersion leads to similar amplitudes of ground motion for 2D and 1D models. In general, results from 2D simulations are similar to those from a complete 3D model. 2D models may overestimate local surface wave amplitudes, especially when the boundaries of the basin are oblique to the selected cross section. However, the differences between 2D and 3D site effects are small, especially in regard of the difficulties and uncertainties associated to building a reliable 3D model for a large basin.