Advanced sonic log tools are capable of providing high resolution borehole caliper

measurements and full borehole imagery. Borehole televiewer surveys are capable of taking

high resolution acoustic pictures of the walls of fluid filled boreholes. The televiewer allows

identification of fractures and fracture orientation, deformation, pitting, vugs, bedding planes,

lithology changes, and well casing and screen integrity. Limitations of sonic logging are

related both to signal attenuation as a result of borehole environmental or tool factors, and to

variability in formation properties affecting the elastic wave transmission and attenuation.

The previously discussed "matrix effect" is applicable to the interpretation of sonic data.

Physical Methods

Physical methods of subsurface investigation include caliper, temperature, borehole

deviation, and downhole video surveying. Caliper surveys are commonly run in combination

with the other tools and are used to apply borehole corrections to the measured log data.

Caliper surveys also are valuable in delineating enlarged borehole zones that may be

indicative of subsurface fracturing, karstification/solution channels, or water bearing zones.

High resolution, multi arm caliper devices can provide valuable information regarding the

borehole geometry and directional aspects of borehole enlargement. Caliper devices range in

resolution from single arm tools measuring the borehole diameter in a single direction, to

multi arm tools measuring the hole diameter in several simultaneous directions.

A temperature log is obtained by lowering a temperature sonde into a fluid filled

borehole at a constant rate. The probe is constructed so that borehole fluid flows by a

temperature sensor on the probe. Temperature is recorded as a function of depth. A

temperature log can provide information on the temperature variation with depth and can

provide a measure of the thermal gradient. The log is commonly run in open hole

environments although cased hole applications are common, particularly for locating cement

grout behind a casing or for confirming fluid flow in perforated intervals. Temperature

anomalies in open boreholes may be indicators of permeable zones reflecting the movement

of cooler, unequilibrated water into a warmer, equilibrated borehole environment.

Borehole geometry and deviation are determined from high resolution microresistivity

measurements obtained using dipmeter or gyroscopic tools. The dipmeter tool uses four dual

electrodes to record eight microconductivity curves and a triaxial accelerometer and three

magnetometers to provide detailed information on borehole microresistivity, tool deviation,

and azimuth. Caliper measurements are obtained at 90 degree intervals for input to borehole

geometry and volume calculations. Borehole video surveys are a valuable means of visually

assessing downhole conditions in stable, open holes and in cased wells. Completion of a

successful video survey is contingent on the clarity of the fluid filling the borehole or well.

November 1992

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