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Geology topics

D.L. Wright

Publications and source records attributed to D.L. Wright.

15 recordsLinked to original sources
USGS2017

Systems for measuring thickness of temperate and polar ice from the ground or from the air.

Equipment has been designed and tested for ground-based and airborne sounding of temperate glaciers. The transmitter is a free-running pulse generator that uses avalanche-mode transistor breakdown to create high-voltage pulses. The transmit and receive antennas are resistively loaded dipoles; for the airborne system, a twin-lead transmit element and a three-layer coaxial receive element are used on the inboard end of the respective antennas. The sounders are broadband systems; oscilloscopes are used for receivers. The oscilloscope trace is recorded photographically in the ground-based systems. A sampling oscilloscope is used in the airborne system—the sampling process strobes the waveform to audio frequencies so that it can be recorded on magnetic tape. Echoes have been obtained from ice depths of 550 m using the airborne system and about 1 000 m using the ground-based system.

Journal of Glaciology↗
USGS2008

ALLTEM UXO detection and discrimination

ALLTEM is a multi-axis electromagnetic induction system designed for unexploded ordnance (UXO) applications. It uses a continuous triangle-wave excitation and provides good late-time signal-to-noise ratio (SNR) especially for ferrous targets. Multi-axis transmitter (Tx) and receiver (Rx) systems such as ALLTEM provide a richer data set from which to invert for the target parameters required to distinguish between clutter and UXO. Inversions of field data over the Army's UXO Calibration Grid and Blind Test Grid at the Yuma Proving Ground (YPG), Arizona in 2006 produced polarizability moment values for many buried UXO items that were reasonable and generally repeatable for targets of the same type buried at different orientations and depths. In 2007 a test stand was constructed that allows for collection of data with varying spatial data density and accurate automated position control. The behavior of inverted ALLTEM test stand data as a function of spatial data density, sensor SNR, and position error has been investigated. The results indicate that the ALLTEM inversion algorithm is more tolerant of sensor noise and position error than has been reported for single-axis systems. A high confidence level in inversion-derived target parameters is required when a target is declared to be harmless scrap metal that may safely be left in the ground. Unless high confidence can be demonstrated, state regulators will likely require that targets be dug regardless of any "no-dig" classifications produced from inversions, in which case remediation costs would not be decreased.

Conference Paper↗
USGS2008

Numerical modeling of higher order magnetic moments in UXO discrimination

The surface magnetic anomaly observed in unexploded ordnance (UXO) clearance is mainly dipolar, and consequently, the dipole is the only magnetic moment regularly recovered in UXO discrimination. The dipole moment contains information about the intensity of magnetization but lacks information about the shape of the target. In contrast, higher order moments, such as quadrupole and octupole, encode asymmetry properties of the magnetization distribution within the buried targets. In order to improve our understanding of magnetization distribution within UXO and non-UXO objects and to show its potential utility in UXO clearance, we present a numerical modeling study of UXO and related metallic objects. The tool for the modeling is a nonlinear integral equation describing magnetization within isolated compact objects of high susceptibility. A solution for magnetization distribution then allows us to compute the magnetic multipole moments of the object, analyze their relationships, and provide a depiction of the anomaly produced by different moments within the object. Our modeling results show the presence of significant higher order moments for more asymmetric objects, and the fields of these higher order moments are well above the noise level of magnetic gradient data. The contribution from higher order moments may provide a practical tool for improved UXO discrimination. ?? 2008 IEEE.

IEEE Transactions on Geoscience and Remote Sensing↗
USGS2008

Measuring the electrical properties of soil using a calibrated ground-coupled GPR system

Traditional methods for estimating vadose zone soil properties using ground penetrating radar (GPR) include measuring travel time, fitting diffraction hyperbolae, and other methods exploiting geometry. Additional processing techniques for estimating soil properties are possible with properly calibrated GPR systems. Such calibration using ground-coupled antennas must account for the effects of the shallow soil on the antenna's response, because changing soil properties result in a changing antenna response. A prototype GPR system using ground-coupled antennas was calibrated using laboratory measurements and numerical simulations of the GPR components. Two methods for estimating subsurface properties that utilize the calibrated response were developed. First, a new nonlinear inversion algorithm to estimate shallow soil properties under ground-coupled antennas was evaluated. Tests with synthetic data showed that the inversion algorithm is well behaved across the allowed range of soil properties. A preliminary field test gave encouraging results, with estimated soil property uncertainties (????) of ??1.9 and ??4.4 mS/m for the relative dielectric permittivity and the electrical conductivity, respectively. Next, a deconvolution method for estimating the properties of subsurface reflectors with known shapes (e.g., pipes or planar interfaces) was developed. This method uses scattering matrices to account for the response of subsurface reflectors. The deconvolution method was evaluated for use with noisy data using synthetic data. Results indicate that the deconvolution method requires reflected waves with a signal/noise ratio of about 10:1 or greater. When applied to field data with a signal/noise ratio of 2:1, the method was able to estimate the reflection coefficient and relative permittivity, but the large uncertainty in this estimate precluded inversion for conductivity. ?? Soil Science Society of America.

Conference Paper↗
USGS2007

Improving GPR image resolution in lossy ground using dispersive migration

As a compact wave packet travels through a dispersive medium, it becomes dilated and distorted. As a result, ground-penetrating radar (GPR) surveys over conductive and/or lossy soils often result in poor image resolution. A dispersive migration method is presented that combines an inverse dispersion filter with frequency-domain migration. The method requires a fully characterized GPR system including the antenna response, which is a function of the local soil properties for ground-coupled antennas. The GPR system response spectrum is used to stabilize the inverse dispersion filter. Dispersive migration restores attenuated spectral components when the signal-to-noise ratio is adequate. Applying the algorithm to simulated data shows that the improved spatial resolution is significant when data are acquired with a GPR system having 120 dB or more of dynamic range, and when the medium has a loss tangent of 0.3 or more. Results also show that dispersive migration provides no significant advantage over conventional migration when the loss tangent is less than 0.3, or when using a GPR system with a small dynamic range. ?? 2007 IEEE.

Conference Paper↗
USGS2005

Radiation pattern of a borehole radar antenna

The finite-difference time-domain method was used to simulate radar waves that were generated by a transmitting antenna inside a borehole. The simulations were of four different models that included features such as a water-filled borehole and an antenna with resistive loading. For each model, radiation patterns for the far-field region were calculated. The radiation patterns show that the amplitude of the radar wave was strongly affected by its frequency, the water-filled borehole, the resistive loading of the antenna, and the external metal parts of the antenna (e.g., the cable head and the battery pack). For the models with a water-filled borehole, their normalized radiation patterns were practically identical to the normalized radiation pattern of a finite-length electric dipole when the wavelength in the formation was significantly greater than the total length of the radiating elements of the model antenna. The minimum wavelength at which this criterion was satisfied depended upon the features of the antenna, especially its external metal parts.

Geophysics↗
USGS2004

Numerical study of electromagnetic waves generated by a prototype dielectric logging tool

To understand the electromagnetic waves generated by a prototype dielectric logging tool, a numerical study was conducted using both the finite-difference, time-domain method and a frequency-wavenumber method. When the propagation velocity in the borehole was greater than that in the formation (e.g., an air-filled borehole in the unsaturated zone), only a guided wave propagated along the borehole. As the frequency decreased, both the phase and the group velocities of the guided wave asymptotically approached the phase velocity of a plane wave in the formation. The guided wave radiated electromagnetic energy into the formation, causing its amplitude to decrease. When the propagation velocity in the borehole was less than that in the formation (e.g., a water-filled borehole in the saturated zone), both a refracted wave and a guided wave propagated along the borehole. The velocity of the refracted wave equaled the phase velocity of a plane wave in the formation, and the refracted wave preceded the guided wave. As the frequency decreased, both the phase and the group velocities of the guided wave asymptotically approached the phase velocity of a plane wave in the formation. The guided wave did not radiate electromagnetic energy into the formation. To analyze traces recorded by the prototype tool during laboratory tests, they were compared to traces calculated with the finite-difference method. The first parts of both the recorded and the calculated traces were similar, indicating that guided and refracted waves indeed propagated along the prototype tool.

Geophysics↗
USGS2003

Three-dimensional imaging of buried objects in very lossy earth by inversion of VETEM data

The very early time electromagnetic system (VETEM) is an efficient tool for the detection of buried objects in very lossy earth, which allows a deeper penetration depth compared to the ground-penetrating radar. In this paper, the inversion of VETEM data is investigated using three-dimensional (3-D) inverse scattering techniques, where multiple frequencies are applied in the frequency range from 0-5 MHz. For small and moderately sized problems, the Born approximation and/or the Born iterative method have been used with the aid of the singular value decomposition and/or the conjugate gradient method in solving the linearized integral equations. For large-scale problems, a localized 3-D inversion method based on the Born approximation has been proposed for the inversion of VETEM data over a large measurement domain. Ways to process and to calibrate the experimental VETEM data are discussed to capture the real physics of buried objects. Reconstruction examples using synthesized VETEM data and real-world VETEM data are given to test the validity and efficiency of the proposed approach.

IEEE Transactions on Geoscience and Remote Sensing↗
USGS2002

Radiation pattern of a borehole radar antenna

To understand better how a borehole antenna radiates radar waves into a formation, this phenomenon is simulated numerically using the finite-difference, time-domain method. The simulations are of two different antenna models that include features like a driving point fed by a coaxial cable, resistive loading of the antenna, and a water-filled borehole. For each model, traces are calculated in the far-field region, and then, from these traces, radiation patterns are calculated. The radiation patterns show that the amplitude of the radar wave is strongly affected by its frequency, its propagation direction, and the resistive loading of the antenna.

Conference Paper↗
USGS2000

Numerical modeling of an enhanced very early time electromagnetic (VETEM) prototype system

In this paper, two numerical models are presented to simulate an enhanced very early time electromagnetic (VETEM) prototype system, which is used for buried-object detection and environmental problems. Usually, the VETEM system contains a transmitting loop antenna and a receiving loop antenna, which run on a lossy ground to detect buried objects. In the first numerical model, the loop antennas are accurately analyzed using the Method of Moments (MoM) for wire antennas above or buried in lossy ground. Then, Conjugate Gradient (CG) methods, with the use of the fast Fourier transform (FFT) or MoM, are applied to investigate the scattering from buried objects. Reflected and scattered magnetic fields are evaluated at the receiving loop to calculate the output electric current. However, the working frequency for the VETEM system is usually low and, hence, two magnetic dipoles are used to replace the transmitter and receiver in the second numerical model. Comparing these two models, the second one is simple, but only valid for low frequency or small loops, while the first modeling is more general. In this paper, all computations are performed in the frequency domain, and the FFT is used to obtain the time-domain responses. Numerical examples show that simulation results from these two models fit very well when the frequency ranges from 10 kHz to 10 MHz, and both results are close to the measured data.

IEEE Antennas and Propagation Magazine↗