A light-weight portable velocity probe has been built to permit rapid ultrasonic measurements on core, outcrops, rock samples, and other materials. This tool can be used to estimate porosity, examine anisotropy and inhomogeneity, correlate among units, assist calibration of down-hole logs, and estimate rock mechanical properties. Probe velocity logs made from cores show good correlation to lithology and in-situ sonic logs. Fresh, mechanically open fractures can be distinguished from welded or partially sealed fractures. However, probe measurements are made at zero effective pressure and incomplete liquid saturation so probe velocities are systematically lower than values at depth. Measurements are also sensitive to weathering and surface roughness. Thus probe measurements are primarily used to examine relative velocity changes at different locations, among different lithologies, or for porosity and texture variations within a single lithology. The probe has proved useful when rapid yet detailed characterizations are desired.
An ultrasonic velocity probe has been built to assist in geologic characterization of rock. This equipment is small, light weight, and completely portable. This tool provides rapid and extensive velocity measurements for applications requiring detailed descriptive data. Jordan et. al. (1991) and Provo et. al. (1977) have demonstrated the usefulness of gamma ray surveys of outcrops in stratigraphic correlation, heterogeneity studies, and log interpretation. In addition to these types of studies, ultrasonic measurements can be used to examine rock properties such as porosity, anisotropy, diagenetic grade, fracture potential, and cementation. Mechanically open fractures can be distinguished from those partially sealed or welded. Seismic description of stratigraphic sequences is an area of great potential use. Velocity measurements may be one of the few ways to correlate lithologies such as carbonates, for which other methods are inadequate. Thus far, the probe has been useful in outcrop, core, and laboratory studies.
Probe measurements are made on the sample surface and the depth of penetration is only a few millimeters. Weathering, surface roughness, and rough treatment can substantially alter velocities. In addition, measurements are made at zero effective pressure and usually the rock is not saturated with liquids so values are systematically lower than those at depth. Thus, probe measurements are primarily for comparison of one location to another, of one lithology to another, or of property changes within a single lithology. Velocities usually can not be applied alone and a detailed geological examination of the measured locations greatly improves interpretation.