Dwain K. Butler

Microgravimetric and gravity gradient techniques for detection of subsurface cavities

.ABSTRACT Microgravimetric and gravity gradient surveying techniques are applicable to the detection and delineation of shallow subsurface cavities and tunnels. Two case histories of the use of these techniques to site investigations in karst regions are presented. In the first case history, the delineation of a shallow (_ 10 m deep), airfilled cavity system by a microgravimetric survey is demonstrated. Also, application of familiar ring and center point techniques produces derivative maps which demonstrate (1) the use of second derivative techniques to produce a “residual” gravity map, and (2) the ability of first derivative techniques to resolve closely spaced or complex subsurface features. In the second case history, a deeper (-30 m deep), water-filled cavity system is adequately detected by a microgravity survey. Results of an interval (tower) vertical gradient survey along a profile line are presented in the second case history; this vertical gradient survey successfully detected shallow (< 6 m) anomalous features such as limestone pinnacles and clay pockets, but the data are too “noisy” to permit detection of the vertical gradient anomaly caused by the cavity system. Interval horizontal gradients were determined along the same profile line at the second site, and a vertical gradient profile is determined from the horizontal gradient profile by a Hilbert transform technique. The measured horizontal gradient profile and the computed vertical gradient profile compare quite well with corresponding profiles calculated for a two-dimensional model of the cavity system.

Microgravity investigations of foundation conditions

A microgravity investigation was conducted in the upstream and downstream switchyards of the Wilson Dam powerplant, Florence, Alabama. The objective of the survey was the detection in the switchyard foundations of subsurface cavities or other anomalous conditions that could threaten the stability of the switchyard structures. The survey consisted of 288 gravity stations in the downstream switchyard and 347 stations in the upstream switchyard. Significant anomalous areas in the switchyards were selected on the basis of residual gravity anomaly maps. These results were prioritized and used to guide an exploratory drilling program to investigate the cause of the anomalies. Highest-priority boring location recommendations were in negative gravity anomaly areas, since negative anomalies could be caused by actual cavities or low-density zones that might represent incipient cavity formation. Remaining boring locations were in positive anomaly areas for verification purposes. The results of the borings confirm the presence of cavities and soft zones indicative of cavity formation.

Analytical Modeling of Magnetic and Gravity Signatures of Unexploded Ordnance

The dominant cost and time driver in unexploded ordnance (UXO) cleanup is the necessity of digging (excavating) “false alarm” geophysical anomalies. As many as 75% of all the anomalies investigated are false alarms, e.g., scrap metal, ordnance debris, cans, wire, etc. The effort to develop capability to discriminate UXO anomalies from false alarm anomalies is a driver for the development of geophysical model-based signature modeling. The signature modeling is important for forward signature prediction and ultimately for geophysical inversion of buried object parameters from measured data. Researchers are actively developing forward and inverse modeling capability for time-domain and frequency-domain electromagnetic induction, ground penetrating radar, magnetic, and gravity anomaly signatures for realistic UXO shapes. This paper documents progress in developing forward gravity and total magnetic field analytical solutions. Gravity and magnetic methods are passive and share similarity in mathematical formal...

Crosshole seismic testing; procedures and pitfalls

Crosshole seismic testing is a valuable technique for determining seismic velocity profiles for critical structure siting investigations. Improper field procedures, however, can result in the acceptance of apparent velocities as true velocities, leading to unconservative design values. Common pitfalls in crosshole seismic testing are (1) the use of source‐receiver borehole spacings too large to enable the determination of true formation velocities, (2) the use of in‐hole station spacings too large to allow resolution of thin velocity layers of interest, (3) the use of incremental traveltimes between widely spaced receiver boreholes, and (4) the naive assumption that boreholes are vertical and hence parallel. A procedure is presented whereby field programs can be more rationally planned.

Generalized gravity gradient analysis for 2-D inversion

Gravity gradient profiles across subsurface structures that are approximately 2-D contain diagnostic information regarding depth, size, and structure (geometry). Gradient space plots, i.e., plots of horizontal gradient versus vertical gradient, present the complete magnitude and phase information in the gradient profiles simultaneously. Considerable previous work demonstrates the possibility for complete structural interpretation of a truncated plate model from the gradient space plot. The qualitative and quantitative diagnostic information contained in gradient space plots is general, however. Examination of the characteristics of gradient space plots reveals that 2-D structures are readily classified as extended or localized. For example, the truncated plate model is an extended model, while the faulted plate model is a localized model. Comparison of measured or calculated gradient space plots to a model gradient space plot catalog allows a rapid, qualitative determination of structure or geometry. “Cor...

Comprehensive geophysics investigation of an existing dam foundation; engineering geophysics research and development

Part 1 of this paper (TLE, August 1989) reviews recent geotechnical investigations conducted at Beaver Dam, Arkansas. The problem addressed was anomalous seepage beneath Dike 1, adjacent to the main embankment dam. The paper presents a summary of the site geology, seepage history, and foundation grouting programs. Figure 17 is a plan map showing the north and south bounding fault zones of a graben structure beneath Dike 1. The foundation is a down‐faulted block of severely weathered limestone/dolomite of the Boone formation. Figure 4 of part 1 shows a simplified geologic cross‐section through Dike 1 showing the graben structure. Overall objectives of the geotechnical investigations were to assess the anomalous seepage and plan remedial measures to eliminate or significantly abate the seepage. The engineering geophysics investigations discussed in part 1 were designed to detect, map, and monitor anomalous seepage paths and delineate geologic structure beneath Dike 1.

Comprehensive geophysical investigation of an existing dam foundation

Engineering geophysics has long played a role in geotechnical site investigations, although the significance and acceptance of the results have varied considerably. There has been a tremendous surge in acceptance and applications of engineering geophysics within the past 10 years. This improved status has resulted from a number of factors, not the least of which are improved instrumentation and microcomputers, better trained personnel, and the recognition of classes of geotechnical problems for which engineering geophysics is not only ideally suited but in many cases the only viable option. One such class of problems is the investigation of existing structures and their foundations, such as earth and rockfill dams. This class of problems has emerged in the United States, for example, because of an aging and decaying infrastructure; the key words in this effort are evaluation, repair, maintenance, and rehabilitation. Remediation efforts are directed to increasing the useful life of structures and insuring ...

Report on a workshop on electromagnetic induction methods for UXO detection and discrimination

To decrease cost and increase reliability and efficiency of unexploded ordnance (UXO) environmental remediation, government research and development programs and industry efforts seek to establish a practice of UXO location using modern digital survey systems, sound geophysical survey procedures, and postprocessing of data for enhanced detection and discrimination. The new practice will replace most traditional “mag and flag” type UXO location surveys that typically use analog instruments, where all detected targets (anomalies) must be investigated (i.e., excavated). Several factors, both technical and nontechnical, drive and justify the change in practice: (1) digital data allow maintenance of a permanent record of the locations of targets and sensor signatures over targets; (2) digital data will typically have higher dynamic range (hence higher signal-to-noise ratios) than the analog data that rely on audio or visual output; (3) digital data allow and facilitate postprocessing to enhance signatures for ...

Interval gravity-gradient determination concepts

Considerable attention has been directed recently to applications of gravity gradients, e.g., Hammer and Anzoleaga (1975), Stanley and Green (1976), Fajklewicz (1976), Butler (1979), Hammer (1979), Ager and Liard (1982), and Butler et al. (1982). Gravity‐gradient interpretive procedures are developed from properties of true or differential gradients, while gradients are determined in an interval or finite‐difference sense from field gravity data. The relations of the interval gravity gradients to the true or differential gravity gradients are examined in this paper. Figure 1 illustrates the concepts of finite‐difference procedures for gravity‐gradient determinations. In Figure 1a, a tower structure is illustrated schematically for determining vertical gradients. Gravity measurements are made at two or more elevations on the tower, and various finite‐difference or interval values of vertical gradient can be determined. For measurements at three elevations on the tower, for example, three interval gradient ...

Selected short stories on novel applications of near‐surface geophysics

Applications of near‐surface geophysics can be grouped under engineering and geotechnical, environmental, ground water, archaeological, forensic or law enforcement, military, and miscellaneous. However, the distinction is often arbitrary. For example, cavity or tunnel detection might be classified as engineering/geotechnical, archaeological, military, or forensic/law enforcement although the geophysical methods and approach are likely the same in each case. Near‐surface geophysics, for most applications, has three distinguishing characteristics not shared with regional geophysics or oil and gas exploration: (1) very high resolution surveys; (2) public health and safety concerns; (3) “near real‐time” validation or “ground truthing” of the interpretations.