Geology topics
David L. Wright
Publications and source records attributed to David L. Wright.
Four models used for numerical simulation of a borehole radar antenna
In this report are four different models that represent an antenna used by personnel at the U.S. Geological Survey for crosswell investigations. The four models vary in complexity and concomitantly the accuracy with which they represent the actual antenna. These models are used in numerical simulations of the antenna to determine how it radiates radar waves.
Imaging and modeling new VETEM data
In previously reported work (Wright and others, 2000) we found that the very early time electromagnetic (VETEM) prototype system produced data from which high resolution images of a buried former foundry site at the Denver Federal Center were made. The soil covering the site is about 30 mS/m conductivity, and is thus relatively unfavorable for ground penetrating radar (GPR) imaging. We have surveyed portions of this site again with new electric field dipole antennas and a new receiver designed for these antennas. Comparisons of the images produced using the loop antennas to those produced using the electric field dipole antennas illustrate that for this application the loop antennas produced more useful images. The larger man-made structures can be seen more clearly because they are not masked by dispersion and/or smaller scale variations as with the electric field dipole antennas. The VETEM system now contains an array of antennas with appropriate transmitters and receivers and can be operated as a low frequency time domain GPR or as a high frequency time domain electromagnetic (EM) system with several possible antenna spacings and polarizations. We plan to examine additional configurations. Numerical modeling of the perpendicular loop antenna configuration has been done and depth estimates produced. We conclude that, as with other GPR and time domain EM systems, the best choice of operating parameters depends on the application and the environment, but the inherent flexibility of the VETEM system allows a wide range of options.
Mapping hydraulically permeable fractures using directional borehole radar and hole-to-hole tomography with a saline tracer
Reflection-mode borehole radar and transmission-mode radar tomograms image heterogeneity in the electromagnetic properties of rock. Heterogeneity may be produced by interfaces between different rock types, foliation, and fracturing. In crystalline rock, hydraulic flow is primarily through fracture networks rather than through the rock matrix. Borehole radar methods have been applied to help map flow paths in crystalline rock. Correlation of features identified in borehole radar reflection records and tomograms with hydraulic flow paths is generally uncertain because the records show responses to heterogeneity of all- kinds, not just to hydraulically permeable fractures. Even in lithologically uniform rock, it is often not possible to distinguish fractures of high hydraulic permeabilities from those with low permeabilities. It is possible to “erase” signatures from lithologic interfaces and rock fabric to identify the signatures of hydraulically permeable fractures by using a saline tracer in fractured crystalline rock because the electrical properties of the rock, except for the fractures that are open to infiltration by the brine solution, remain the same after the injection of the brine and may be removed by examining differences. Saline tracer experiments were carried out in 1995, 1996, and 1997 in the FSE well field at the Mirror Lake fractured-rock hydrology research site in Grafton County, New Hampshire. Comparisons of results from directional radar reflection surveys to well-to-well difference attenuation tomography in the same pairs of wells show generally good correspondence between the location of radar reflections and attenuation anomalies. Our results demonstrate the advantage of using a saline tracer for before-and-after difference mapping of hydraulically permeable fractures in lithologically heterogeneous rock and the utility of the coordinated use of directional borehole radar and hole-to-hole radar tomography.
Radar tomograms at Mirror Lake, New Hampshire--3D visualization and a brine tracer experiment
No abstract available.
Microprocessor-based data-acquisition system for a borehole radar
An efficient microprocessor-based system has been implemented that permits real-time acquisition, stacking, and digital recording of data generated by a borehole radar system. Although the system digitizes, stacks, and records independently of a computer, it is interfaced to a desktop computer for program control over system parameters such as sampling interval, number of samples, number of times the data are stacked prior to recording on nine-track tape, and for graphics display of the digitized data. The data can be transferred to the desktop computer during recording, or played back from a tape at a later time. Using the desktop computer, the operator can observe results while recording data and generate hard-copy graphics in the field. Thus, the radar operator can immediately evaluate the quality of data being obtained, modify system parameters, study the radar logs before leaving the field, and re-run borehole logs if necessary. The system has proven to be reliable in the field and has increased productivity both in the field and in the laboratory.
A short-pulse electromagnetic transponder for hole-to-hole use
We have made hole-to-hole observations through nearly 20 m of granite using an electromagnetic transponder (an active reflector) in one borehole and a single-hole short-pulse radar in another. We found that the transponder is inexpensive, operationally simple, and effective in extending the capability of a short-pulse borehole radar system to allow hole-to-hole operation without requiring timing cables. A detector in the transponder senses the arrival of each pulse from the radar (which may be millivolts in amplitude); each pulse detection triggers a kilovolt-amplitude pulse for retransmission. The transponder “echo” may be stronger than that of a passive reflector by a factor of as much as 120 dB. The result is an increase in range capability by a factor which depends on attenuation in the medium and hole-to-hole wavepath geometry. Single-hole reflection-mode echoes are still available at times prior to the transponder pulse arrival. The transponder is helpful in yielding velocity information, because the radar-transponder distance is known and the echo time is observed. Field tests have demonstrated that the transponder is a useful alternative to employing timing cables in some short-pulse hole-to-hole measurement situations.