U.S. Geological Survey, Water Resources of Pennsylvania

BOREHOLE GEOPHYSICAL METHODS

Geophysical Logs
The standard suite of borehole geophysical logs consists of caliper, natural-gamma, single-point-resistance, fluid-resistivity, and fluid-temperature logs.

Caliper Log
Caliper logs provide a continuous record of average borehole diameter, which is related to fractures, lithology, and drilling technique. Caliper logs are used to identify fractures and water-bearing openings. Correlation of caliper logs with fluid-resistivity and fluid-temperature logs is used to identify water-producing and water-receiving fractures or zones. Caliper logs also are used to locate smooth sections of the borehole for setting straddle packers.

 Natural-Gamma Log
Natural-gamma logs, also called gamma-ray logs, record the natural-gamma radiation emitted from rocks penetrated by the borehole. Gamma radiation can be measured through casing, but the gamma response is dampened. Uranium-238, thorium-232, and the progeny of their decay series and potassium-40 are the most common emitters of natural-gamma radiation.

Single-Point-Resistance Log
Single-point-resistance logs record the electrical resistance between the borehole and an electrical ground at land surface. In general, resistance increases with grain size and decreases with borehole diameter, density of water-bearing fractures, and increasing dissolved-solids concentration of borehole fluid. A fluid-filled borehole is required for single-point-resistance logs, and they are run only for the saturated part of the formation below the casing. Single-point-resistance logs sometimes help to identify the location of water-bearing zones because a fluid-filled fracture is less resistive than solid rock.

Fluid-Temperature Log
Fluid-temperature logs provide a continuous record of the temperature of the fluid in the borehole. Fluid-temperature logs are used to identify water-producing and water-receiving zones and to determine intervals of vertical borehole flow. Water-producing and water-receiving zones usually are identified by sharp changes in temperature, and intervals of vertical borehole flow are identified by little or no temperature gradient.

Fluid-Resistivity Log
Fluid-resistivity logs measure the electrical resistance of fluid in the borehole. Resistivity is the reciprocal of fluid conductivity, and fluid-resistivity logs reflect changes in the dissolved-solids concentration of the borehole fluid. Fluid-resistivity logs are used to identify water-producing and water-receiving zones and to determine intervals of vertical borehole flow. Water-producing and water-receiving zones usually are identified by sharp changes in resistivity.

Acoustic Borehole Televiewer Log
The acoustic borehole televiewer log is a magnetically oriented, 360 degree, photograph-like image of the acoustic reflectivity of the borehole wall. The acoustic televiewer is an ultrasonic imaging tool operating at a frequency of about 1 megahertz that scans the borehole wall with an acoustic beam generated by a rapidly pulsed piezoelectric source rotating at about three revolutions per second as the tool is moved up the borehole. Digital images from the televiewer are recorded by the computer collecting logging data. A smooth and hard borehole wall produces a uniform pattern of reflectivity. The intersection of a fracture with the borehole wall scatters the acoustic waves, producing a dark, linear feature on the image. Because the image is magnetically oriented, the dip and strike of the fracture can be determined.

Borehole Deviation Log
Borehole deviation logs, also called dipmeter logs, record the deviation of a borehole from true vertical. Deviation of boreholes from the vertical is common, and deviation logs are used to calculate true vertical depth of features of interest and to correct the strike and dip of fractures or bedding obtained from acoustic televiewer logs.

 Heatpulse Flowmeter
The direction and rate of borehole-fluid movement are measured with a high-resolution heatpulse flowmeter. The heatpulse flowmeter operates by diverting nearly all flow to the center of the tool where a heating grid slightly heats a thin zone of water. If vertical borehole flow is occurring, the water moves up or down the borehole to one of two sensitive thermistors (heat sensors). When a peak temperature is recorded by one of the thermistors, a measurement of direction and rate is calculated by the computer collecting the logging data. The range of flow measurement is about 0.01-1.5 gallons per minute in a 2- to 8-inch diameter borehole. Heatpulse-flowmeter measurements may be influenced by poor seal integrity between the borehole and the flowmeter or contributions of water from storage within the borehole during pumping. If the seal between the borehole and the heatpulse flowmeter is not complete, some water can bypass the flowmeter, resulting in flow measurements that are less than the actual rate. The quantity of water bypassing the tool is a function of borehole size and shape and degree of fracturing. Although the heatpulse flowmeter is a calibrated tool, the data primarily are used as a relative indicator of fluid-producing zones.

Aquifer-Isolation (Packer) Tests
Aquifer-isolation tests, commonly known as a packer tests, are conducted to test discrete fractures or fracture zones. Because ground water in fractured rock moves through discrete fractures or fracture zones, the hydraulic characteristics and chemical quality of water in each fracture or fracture zone can differ. These differences are characterized by isolating discrete fractures or fracture zones using a straddle-packer assembly to determine depth-discrete specific capacity values or hydraulic conductivity, obtain depth-discrete water samples, and to determine the effect of pumping an individual fracture or fracture zone on water levels in nearby wells. The packer assembly consists of two inflatable rubber bladders (packers) about 4 feet long set on 2-inch-diameter lift pipe with a pump set between the packers. The distance from the center of the upper packer to the center of the lower packer varies from borehole to borehole and sometimes within a borehole. On the basis of borehole geophysical logs and heat-pulse flowmeter measurements, intervals are selected for aquifer-isolation tests. The packer assembly is lowered to the selected depth in the borehole, and the packers were inflated against the borehole wall, isolating the selected interval. Exact depths to set packers are based on the location of smooth sections of borehole wall determined from the caliper logs.

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