Thomas E. Owen

Modeling resistivity anomalies from localized voids under irregular terrain

In applying earth resistivity methods to the problem of locating and delineating subsurface structures, surface elevation variations along the surveyed terrain introduce distortions in the soundings. The analysis presented here is aimed at characterizing such terrain variations in the detection of relatively small subsurface targets such as caves, sinks, and tunnels in otherwise homogeneous earth materials. The analytical approach involves, first, the development of a suitable earth resistivity model for localized three‐dimensional subsurface anomalies in a homogeneous flat half‐space. Next, in order to apply the half‐space resistivity model to irregular terrain, a Schwarz‐Christoffel transformation is utilized to map the terrain surface variations into an equivalent flat half‐space. The technique is illustrated by calculating the resistivity response of three tunnels located below a hill with 40-m valleys on either side.

Acoustic amplitude-doppler target ranging system

An acoustic target sensor and ranging system automatically detects military targets and provides a munition firing signal at the appropriate target position and time. An amplitude-doppler circuit is utilized to predict the time of closest approach to a munition by a target by measuring the time interval between zero crossings of the second and third derivatives of the received acoustic wave amplitude function. The circuit produces a firing signal by logically ANDing the closest point of approach signal it develops with a signal that indicates when target range is within specified limits. The circuit is realized by means of conventional electronic zero crossing detectors, an up-down counter, dividers, sample and hold devices and voltage comparators. The acoustic target signal is obtained from an omni-directional microphone the output of which is amplified and rectified.

Model studies of electrical leak detection surveys in geomembrane‐lined impoundments

Pole‐dipole array electrical potential distributions are calculated for a geomembrane‐lined liquid impoundment having single or multiple leaks. A three‐dimensional numerical model is employed to represent a small circular leak in the highly resistive plastic liner. The liquid waste material, the liner, and the soil under the impoundment are simulated by infinite horizontal layers, with approximate corrections for the finite size of the impoundment. Parametric curves for a single leak show that with optimum selection of electrode spacing and positioning and other field survey parameters, leaks can be detected effectively. To identify and resolve the presence of a cluster of leaks, the potential measurements must be made close to the liner and the detector dipole spacing must be smaller than the separation of the leaks. The results also indicate that the survey speed may be increased when portable leak detection equipment employing a vertical dipole detector is used.

Acoustic amplitude-threshold target ranging system

Munition actuation signals are generated by an acoustic target ranging system that predicts the time and distance of closest approach of a moving target to a munition. The predictive capability of the system is based on the rate of change of the sound level emanating from the target compared to its absolute magnitude. The system generates munition standby, arm and fire signals by differencing the received acoustic amplitude function signal and its first derivative and subsequently comparing the difference signal with various reference level signals. The reference level signals are a function of munition effective range and target conditions. Mechanization of the system is realized by simple circuitry consisting of an acoustic signal detector and processor, a differentiating operational amplifier, a programmable gain amplifier, a reference level signal source, an adder and a comparator circuit.

Asymmetrical lateral-force seismic source transducer

A spark discharge acoustic pulse transducer for borehole operation provides an insulating ceramic barrier which contains a small cross-section aperture separating two bodies of electrolyte through which the two zones of liquid make contact. An electrical arc discharge is caused to occur through the aperture to produce a transient pressure pulse and associated acoustic pulse which is coupled to the geologic formation at the borehole wall. The specific shape and geometry of the aperture channel in the insulating barrier being exponentially shaped provides a means for controlling the waveform and frequency spectrum of the acoustic pulse. The transducer has an elastic outer housing and an elastic inner housing with hydraulic fluid between the inner and outer housings. The pressure of the hydraulic fluid may be increased to expand the outer housing against a borehold prior to a spark discharge. The transducer is also provided with a system for venting gas produced as a result of the spark discharge. The transducer is designed to produce asymmetrical forces on the borehole wall in the azimuthal plane about the discharge chamber axis.

Wax‐embedded borehole seismic detector for high‐resolution measurements

High resolution seismic measurements are now being extended well above the conventional frequency limit of about 150200 Hz to the kilohertz frequency range in applications such as interwcll seismic imaging, shallow reflectionsurveys, and reverse vertical seismic profiling. High-resolution measurements demand high-quality data; meaning widcband frequency response, the highest practical signal-to-noise ratio, precision response uncluttered by artifacts related to the source or detector plants, and more complele information content afforded by both compressional and shear wave measurements Artifact-free seismic measurements at frequencies up to about 2,003 Hz are needed if a projected highresolution limit on the order of one meter in spatial dimensions is to be realized in reservoir structure delineation, interwell sonic logging, and shallow reverse VSP applications. For this reason, we exammed the detector rcqmrement from an objective view as well as from a transitional view, secking, first, a practical methodology that could meet the various demands of high-resolution measurements. The result was a prototype reference detector capable of demonstrating state-of-the-art wide bandwidth response and data quality up to the high frequency limits of interest.

Hole‐to‐hole resistivity signatures of cylindrical cavities: Dipole‐dipole electrode array

A numerical procedure for predicting cavity signatures for a dipole-dipole array configuration in hole-to-hole resistivity measurements has been developed. This electrode geometry is implemented from the general solution for a point source of current near an air-filled cylindrical cavity or an air-filled cylinder surrounded by a concentric conductive or resistive halo region embedded in a homogeneous conducting host medium to simulate hole-to-hole resistivity measurements. Cavity signatures obtained for several vertical offset distances between the source and detector dipoles as well as multiply spaced dipole-dipole responses suggest that a processing technique may be devised to identify directly the position of the cavity inhomogeneity with respect to the boreholes. The results also show that the presence of a concentric halo region more conductive than the host medium influences the overall signature by either reducing or enhancing the effect of the air-filled cavity depending upon the halo size and conductivity contrast. In comparison, a halo region more resistive than the host medium always influences the composite signature by enhancing the effect of the air-filled cavity.

A cylindrical bender acoustic transducer

An acoustic transducer having hollow cylinder geometry has been developed for generation of compressional waves in fluid or solid media or of shear waves in solid media using the bender concept of a two‐layered piezoelectric ceramic transducer element. The device can be constructed with either two active piezoelectric layers or with one active piezoelectric layer and one passive layer. The latter configuration allows construction of a cylindrical transducer with a relatively thick metal exterior shell for electrical shielding. The active ceramic can be either monolithic or a mosaic of small elements. Mechanical details of several transducers that have been constructed will be presented with their operational parameters. Low resonance frequency, small size, rugged construction, excellent electrical shielding, and electrically switchable modes of operation are a few of the positive attributes of the transducer design. [Work supported by U.S. Department of the Interior, Bureau of Mines.]