Velocity Measurements in Near‐surface Formations

Abstract

Procedures are described for measuring various seismic velocities at shallow depths in the earth, and examples of the resulting logs are presented. Velocities in Austin chalk and Eagle Ford shale show that these formations are not isotropic, and velocities in loose sand are seen to increase smoothly with depth except for an abrupt increase in compressional speed at the water table. Elastic constants for chalk and shale are computed. A discussion is given of the literature dealing with a packing of spheres as a model for loose sand, and an approximate theory is presented which includes tangential forces between spheres.

Identification of subsonicP‐waves

“…White and Sengbush's surface P-and SV-wave velocity determinations are annotated wp and ws along the vertical axis of the plot. White and Sengbush (1953) reported good agreement between the downhole P-wave velocities and those of the crosshole experiments, leading one to conclude that the material was fairly isotropic. Although it was not stated, we may assume that these were moist sands (not dry).…”

“…These data were acquired from the surface to a depth of 24 m (White and Sengbush, 1953). The findings for loose sand have been plotted with circles.…”

“…The findings for loose sand have been plotted with circles. A mass density of 1700 kg/m 3 was used to convert Khair and Ibrahim, 1999), downhole and crosshole velocities (White and Sengbush, 1953), and field-determined velocities (Baker et al, 1999;. Dashed curves are for power law curve fitting (see Table 1).…”

“…Further, earlier crosshole and downhole studies by White and Sengbush (1953) conducted in loose sand reported significantly faster velocities than the more recent seismic testing. Motivated by these apparent discrepancies, I tested the hypothesis that P-waves commonly travel at subsonic velocities in near-surface soils, using an experiment scaled to a previously published case history (Baker et al, 1999).…”

“…The effect is most developed for water saturations in the range of 5% to 20% of void volume (Wu et al, 1984). The S-wave velocities presented by White and Sengbush (1953) may well represent this range of partial water saturation, since truly dry soil is rare (descending recharge or capillary fringe being a common source of soil moisture). White and Sengbush (1953) interpret their velocities in terms of the uniformly packed spherical models that originated with the works of Hertz (1881), Mindlin (1949Mindlin ( , 1954, Mindlin and Deresiewicz (1953), and Gassmann (1951aGassmann ( , b, 1953.…”