Xiaoxian Zeng

GPR characterization of buried tanks and pipes

Ray-based numerical simulations of monostatic and bistatic GPR responses for several tank and pipe configurations reveal the potential for noninvasive diagnostic evaluations. Examples include discrimination of the material from which a tank is constructed, its size, contents, fluid levels, and shape changes. Ambiguities occur when different configurations give similar responses, and evaluations become less reliable as noise increases. Simulations are able to reproduce the salient features of field GPR data recorded over a metal pipe, and over plastic pipes filled with air, fresh water, and salt water.

Ground penetrating radar imaging of a collapsed paleocave system in the Ellenburger dolomite, central Texas

Abstract Ground penetrating radar (GPR) can image near-surface features with high (submeter) resolution. A field feasibility test over a collapsed paleocave system in the Lower Ordovician Ellenburger dolomites in central Texas shows the ability to image both large (10s of m) and small (submeter) scale features. Two-dimensional GPR profiles were recorded along the top of quarry walls to image similar features displayed in the walls. GPR data collected over the transition between the dipping, stratified host rock and the brecciated cave fill are interpreted in terms of the curved contact of a cave wall and roof, pebble to boulder (1–50 cm) sized chaotic breccia, and tension fractures paralleling the cave wall. GPR data collected over a zone of megablocks that resulted from massive cave roof collapse outline blocks that range in size from a few meters to more than 6 m. The scales of features in the GPR data lines correspond well with those noted in the adjacent quarry faces. The different carbonate facies produce readily distinguishable GPR facies. This demonstrates potential feasibility for detailed study of carbonate facies and features from GPR data and suggests 3-D surveys as a desirable next step. Potential applications include 3-D characterization of analogs of collapsed paleocave hydrocarbon reservoirs.

Forensic GPR: finite-difference simulations of responses from buried human remains

Abstract Time domain 2.5-D finite-difference simulations of ground-penetrating radar (GPR) responses from models of buried human remains suggest the potential of GPR for detailed non-destructive forensic site investigation. Extraction of information beyond simple detection of cadavers in forensic investigations should be possible with current GPR technology. GPR responses are simulated for various body cross-sections with different depths of burial, soil types, soil moisture contents, survey frequencies and antenna separations. Biological tissues have high electrical conductivity so diagnostic features for the imaging of human bodies are restricted to the soil/skin interface and shallow tissue interfaces. A low amplitude reflection shadow zone occurs beneath a body because of high GPR attenuation within the body. Resolution of diagnostic features of a human target requires a survey frequency of 900 MHz or greater and an increment between recording stations of 10 cm or less. Depth migration focuses field GPR data into an image that reveals accurate information on the number, dimensions, locations and orientations of body elements. The main limitation on image quality is attenuation in the surrounding soil and within the body. 3-D imaging is also feasible.

Comparison of ray and Fourier methods for modeling monostatic ground-penetrating radar profiles

With increasing emphasis on shallow, high‐resolution geophysical techniques for environmental and engineering applications, it has become important to implement and evaluate tools for quantitative interpretation of ground‐penetrating radar (GPR) data. Both ray and Fourier algorithms are viable for numerical simulation of 2-D monostatic GPR data, but they have different characteristics. The ray algorithm uses geometrically complicated layers, where within each the dielectric permittivity and attenuation are constant. The algorithm produces accurate amplitudes for reflection but does not include wave effects such as diffractions from layer truncations. The Fourier algorithm uses a gridded parameterization in which reflections are constructed by superposition of diffractions in a background of constant dielectric permittivity and constant attenuation. This technique includes all wave effects, but it does not contain the antenna directivities. Both algorithms are able to simulate the main features in two repr...

Estimation of the spatial distribution of fluid permeability from surface and tomographic GPR data and core, with a 2‐D example from the Ferron Sandstone, Utah

Reservoir analogs provide detailed information that is applicable to fluid transport simulations but that cannot be obtained directly from reservoirs because of inaccessibility. The Ferron Sandstone of east‐central Utah is an analog for fluviodeltaic reservoirs; its excellent outcrop exposures are ideal for detailed study. Ground‐penetrating radar (GPR) data were collected in and between two cored boreholes and are used to build a 2‐D fluid permeability model in four steps. First, an anisotropic GPR propagation velocity model is obtained from traveltime tomography between two boreholes and between each borehole and the earths surface. Second, the geometry of the sedimentological features is imaged by prestack Kirchhoff depth migration of constant‐offset GPR data acquired along a line between the two holes at the earths surface. Third, a background permeability is assigned to each layer by interpolating the geometrical average of the measured permeabilities in each sedimentological element. Finally, the ...

Synthesis of amplitude-versus-offset variations in ground-penetrating radar data

To evaluate the importance of amplitude‐versus‐offset information in the interpretation of ground‐penetrating radar (GPR) data, GPR reflections are synthesized as a function of antenna separation using a 2.5-D finite‐difference solution of Maxwell’s equations. The conductivity, the complex dielectric permittivity, and the complex magnetic permeability are varied systematically in nine suites of horizontally layered models. The source used is a horizontal transverse‐electric dipole situated at the air‐earth interface. Cole‐Cole relaxation mechanisms define the frequency dependence of the media. Reflection magnitudes and their variations with antenna separation differ substantially, depending on the contrast in electromagnetic properties that caused the reflection. The spectral character of the dielectric and magnetic relaxations produces only second‐order variations in reflection coefficients compared with those associated with contrasts in permittivity, conductivity, and permeability, so they may not be s...

Two methods for determining geophone orientations from VSP data

Vertical seismic profile (VSP) data are usually acquired with three-component geophones of unknown azimuthal orientation. The geophone orientation must be estimated from the recorded data as a prerequisite to processing such as P- and S-wave separation, calculation of wave-incident directions, and 3D migration. We compare and combine two methods for estimating azimuthal orientation by least-squares fitting over a large number of shots. Combining the two methods can be done in an automated manner, which provides more accurate estimates of the geophone orientations than previous methods. In the polarization-plane method, we calculate the polarization plane of the first P-wave arrival. Then we subtract the source azimuth to determine the geophone orientation, independently for each geophone, with an angular uncertainty of π , and with no accumulated errors. In the relative-angle method, we obtain relative angles between adjacent geophone pairs using trace crosscorrelations, and operate on all coherent signal...

Characterization of a coalesced, collapsed paleocave reservoir analog using GPR and well‐core data

The three‐dimensional architecture, spatial complexity, and pore‐type distribution are mapped in a near‐surface analog of a coalesced, collapsed paleocave system in the Lower Ordovician Ellenburger Group near the city of Marble Falls in central Texas. The surface area of the site has dimensions of about 350 × 1000 m. The data collected include about 12 km of 50‐MHz ground‐penetrating radar (GPR) data arranged in a grid of orthogonal lines, 29 cores of about 15‐m length, and detailed facies maps of an adjacent quarry face. Electrical property measurements along with detailed core descriptions were the basis of integrated interpretation of the GPR data. Three main GPR facies are defined on the basis of degree of brecciation in the corresponding cores: undisturbed host rock, disturbed host rock, and paleocave breccia. This GPR facies division defined the major paleocave trends and the distribution of porosity types, which correlate with reservoir quality. Highly brecciated zones are separated by disturbed an...

Three-dimensional facies architecture and three-dimensional calcite concretion distributions in a tide-influenced delta front, Wall Creek Member, Frontier Formation, Wyoming

Ground-penetrating radar (GPR) has been used to image the three-dimensional (3-D) internal structure (and, thus, the 3-D facies architecture) of a top-truncated delta front in the topmost parasequence in the Wall Creek Sandstone Member of the Frontier Formation in Wyoming and to estimate the distribution of low-permeability concretions throughout the 3-D GPR volume. The interpretation of the 3-D GPR data is based both on correlations with outcrop and on calibration with core data from holes within the survey grid. Two main radar facies (RF) are identified. Radar facies 1 corresponds to tide-influenced mouth bars formed by a unidirectional flow during delta progradation or bidirectional flow during tides, whereas RF2 is correlated with laterally migrating channels developed on previous bar deposits. The delta-front foreset beds dip in the same direction as the dominant paleocurrent indicators. The GPR interpretation is consistent with the outcrop interpretation that, following a regressive period, bars and channels were developed at the Raptor Ridge site before subsequent transgressive ravinement. The individual 3-D deltaic facies architectures were reconstructed from the 3-D GPR volume and indicate that the depositional units are larger than the survey grid. Cluster analysis of the GPR attributes (instantaneous amplitudes and wave numbers) calibrated with the cores and the outcrop was used to predict the distribution of near-zero permeability concretions throughout the 3-D GPR volume; clusters of predictive attributes were defined and applied separately in the bars and channels. The predicted concretions in the bars and the channels are 14.7 and 10.2% by volume, respectively, which is consistent with those observed in the cores (14.7 and 10.5%, respectively), and their shape and thickness are also generally in consonance with those in the outcrop and cores. The estimated concretions are distributed in an aggregate pattern with irregularly shaped branches within the 3-D GPR volume, indicating that the cementation does not follow a traditional center-to-margin pattern. The concretions and 3-D geological solid model provide cemented flow baffles and a 3-D structural framework for 3-D reservoir modeling, respectively.

A ground-penetrating radar survey of a delta-front reservoir analog in the Wall Creek Member, Frontier Formation, Wyoming

Ground-penetrating radar (GPR) measurements, in conjunction with outcrop sedimentology, were carried out at Murphy Creek reservoir in the Upper Cretaceous Turonian Wall Creek Member of the Frontier Formation in Wyoming. The objectives were to apply GPR to map geometrical details of a top-truncated lowstand delta front and to estimate the volumes of the prograding bar deposits of the delta lobe. Eleven GPR profiles totaling about 4400 m (14,435 ft) were acquired using 50-MHz antennas on a coarsely spaced, two-dimensional grid of lines lying parallel and perpendicular to the average depositional dip. Ground-penetrating radar reflections were detected from within the outcrop to a depth of about 10–15 m (33–49 ft). Four southerly dipping major surfaces identified in the GPR data are correlated with the boundaries of progradational delta-front facies, stacked as distal mouth-bar deposits, in the outcrop. The major boundaries correspond to lithological changes between relatively clean sandstones that are interpreted to have been deposited during floods with high sediment supply, alternating with bioturbated sandstones and mudstones deposited during interflood periods with correspondingly low sedimentation rates. These two lithological units, which also correspond to the two main GPR facies, repeat at least three times with no change in dominant average sand-grain size. Subsequent erosion by transgressive ravinement caused the significantly truncated lowstand delta long after the sandstones were deposited. The bar assemblage volume at successive stages of growth is estimated using measurements from the outcrop and the GPR data. The migrating bars have an estimated average half-length of 650 m (2132 ft); a lower bound on the average volume of the bar is 3.9 106 m3 (1.37 108 ft3). As the volume of the bars increases, the bar deposits appear to have a landward as well as a basinward component of accretion.