COMPARISON OF FIELD AND LABORATORY MEASUREMENTS OF SEISMIC VELOCITIES IN SEDIMENTARY ROCK

In the field, a small dynamite blast was found satisfactory as the source of both longitudinal and transverse waves. Motion thus generated was detected by a single three-component seismometer, successively positioned at different horizontal or vertical distances from the shot point. With vacuum-tube amplifiers and a galvanometer-type, recording oscillograph, the three principal components of particle motion were recorded on photographic paper as a function of time. Analysis of these records for wave types and wave paths yielded the field observations of velocity.

After the wave path of the field measurements was established, cores from relevant parts of the section were studied in the laboratory. Samples were machined to the form of a right circular cylinder and their resonant frequency for forced vibrations in logitudinal and torsional modes determined. The effect on these resonant frequencies of the variables: water content, pressure, and temperature as well as measurement frequency and amplitude was determined, so that conditions of laboratory velocity measurements could be corrected to those of the emplaced rock.

In Osage County, Oklahoma, four attempts to identify transverse body waves by shooting vertical profiles with a three-component seismometer on the surface and the shot at varying depths in an abandoned hole were unsuccessful. Evidently, the fraction of detonation energy delivered to body shear waves was exceedingly small in comparison with that delivered to other extraneous motions.

Vertical profile shooting was abandoned in favor of horizontal profiles. In Section 2, T.25N., R.4E., Osage County, Oklahoma, velocities of: V P = 9200 ± 300 ft/sec. V s = 5000 ± 200 ft/sec. were measured in an upper layer of Permian sand and shale, approximately 250 feet thick, and: V P = 14,000 ± 400 ft/sec. V s = 9900 ± 300ft/sec. in a lower, inhomogeneous layer of limestone.

After correcting to the most probable conditions of water content, pressure, and temperature for the rock, in place, 17 core specimens from 8 closely spaced horizons in the lower limestone gave a bimodal distribution of laboratory velocities with the modes: ![Formula][1] 



(1) Transverse elastic body waves of the SV type can be generated by a small dynamite blast in sedimentary rocks if the shot hole is shallow and high confining pressures are avoided.

(2) The velocity of an SV type body wave can be determined with approximately the same accuracy as a longitudinal type body wave. The transmission path is subject to greater uncertainties.

(3) Among the independent variables affecting velocities in a near-surface, Permian limestone, water content is the most important. Pressure, temperature, and measurement direction, amplitude, or frequency have relatively minor effects.

(4) The Neva limestone may be subdivided on the basis of density and laboratory determinations of velocity into two distinct types of alternating microlayers with thickness of the order of 1 foot.

(5) Under the conditions of this work, elastic parameters and Poisson's ratio computed from field measurements differ significantly from values determined by laboratory measurement.

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Field of study: Geology