Geophysical logs provide a detailed measure of physical quantity as a function of depth within a borehole. Changes in lithology, porosity, bulk density, electrical conductivity (resistivity), temperature, and hole diameter can be measured using various logging probes.

PROSONIC has a complete geophysical logging capability with a full suite of probes and in-house equipment. We initially established our logging capabilities to support our full service drilling department. All logs can fit into a 2-inch casing. Our standard logging system includes natural gamma, resistance/spontaneous potential, temperature, and caliper logs. Depth capabilities for this system are about 1,000 feet.

Our logging unit is specifically designed for groundwater applications. It is also PC-based and has menu-driven software for the collection, display, and analysis of digital log data. Drawworks have plastic-coated logging cables for easy decontamination. PROSONIC is capable of collecting multiple geophysical parameters with a single logging run, thereby greatly increasing the efficiency of the logging operation.

What types of logs are available?

Below is a menu for the various categories of logging tools.

Caliper logs record borehole diameter. Changes in borehole diameter are related to well construction, such as casing or drilling-bit size, and to fracturing or caving along the borehole wall. Because borehole diameter commonly affects log response, the caliper log is useful in the analysis of other geophysical logs, including interpretation of flowmeter logs.

Gamma logs record the amount of natural gamma radiation emitted by the rocks surrounding the borehole. The most significant naturally occurring sources of gamma radiation are potassium-40 and daughter products of the uranium- and thorium-decay series. Clay- and shale-bearing rocks commonly emit relatively high gamma radiation because they include weathering products of potassium feldspar and mica and tend to concentrate uranium.

Spontaneous-potential logs record potentials or voltages developed between the borehole fluid and the surrounding rock and fluids. Spontaneous-potential logs can be used in the determination of lithology and water quality. Collection of spontaneous-potential logs is limited to water- or mud-filled open holes.

Normal-resistivity logs record the electrical resistivity of the borehole environment and surrounding rocks and water as measured by variably spaced potential electrodes on the logging probe. Typical spacing for potential electrodes is16 inches for short-normal resistivity and 64 inches for long-normal resistivity. Normal-resistivity logs are affected by bed thickness, borehole diameter, and borehole fluid and can only be collected in water- or mud-filled open holes.

Fluid resistivity logs record the electric resistivity of water in the borehole. Changes in fluid resistivity reflect differences in the dissolved-solids concentration of water. Fluid resistivity logs are useful for delineating water-bearing zones and identifying vertical flow in the borehole.

Temperature logs record the water temperature in the borehole. Temperature logs are useful for delineating water-bearing zones and identifying vertical flow in the borehole between zones differing hydraulic head penetrated by wells. Borehole flow between zones is indicated by temperature gradients that are less than the regional geothermal gradient, which is about 1 degree Fahrenheit per 100 feet of depth.