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KIRTLAND AIR FORCE BASE ALBUQUERQUE, NEW MEXICO ...

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APPENDIX B<br />

SOP B1.2<br />

Borehole Geophysical Logging<br />

Geophysical techniques have proven to be valuable tools in determining lithology, porosity and moisture<br />

condition of various stratigraphic units. This information is useful in the decision making process for<br />

choosing monitoring well completion intervals and characterizing hydrogeology. Logging suites are<br />

determined by the information required. The following is a brief description of various borehole<br />

geophysical instruments and measurements.<br />

Temperature Measurement and Fluid Resistivity Measurement<br />

A temperature log is the first measurement performed in a borehole. This probe continuously records<br />

borehole fluid temperature immediately surrounding a thermal sensor as the probe is lowered downhole.<br />

The logging equipment is calibrated to present data in degrees Celsius (or other appropriate temperature<br />

scale). Temperature data can provide information concerning fluid flow in or out of the borehole, which<br />

is reflected by changes in the thermal gradient.<br />

Fluid resistivity logs are run in conjunction with temperature logs and record borehole fluid resistivity<br />

directly as the borehole fluid passes between electrodes. Changes in fluid resistivity can indicate in-flow<br />

or out-flow of water as well as water quality. In highly resistive environments some of the electrical<br />

current leaks into the formation and the tool thus behaves somewhat like a micro-resistivity probe.<br />

Localized portions of the log appear to respond as fractures.<br />

Natural Gamma Measurement<br />

The natural gamma measurement detects natural gamma radiation occurring in lithology and is recorded<br />

in counts per second. This log, which exhibits relative changes in natural (or man-made) radiation, is<br />

used for general lithologic identification and stratigraphic correlation. Natural occurring radiation comes<br />

from three principal areas: potassium 40 which occurs with all potassium minerals, uranium-238, and<br />

thorium-232 which is associated primarily with biotite. The typical radius of investigation for the natural<br />

gamma log is approximately 10 to 12 inches from the borehole wall.<br />

Electric Measurements<br />

Several types of electrical measurements can be performed in the borehole. Electrical measurements can<br />

only be run below fluid level since fluid is required to complete the electrical circuit between the probe<br />

and the borehole wall. If no fluid is available, the induction measurements can be performed with<br />

different instrumentation (e.g., Geonics EM-39).<br />

The Single Point Resistance log measures direct resistance between an electrode on the downhole probe<br />

and a surface reference point electrode. The relatively shallow radius of investigation (approximately 5<br />

to 6 inches) enables precise recording of formation contacts and fracture zones. The log response,<br />

measured in ohms, is strongly affected by lithology, resistance of the drilling fluid, quality of the surface<br />

contact, and the rugosity (e.g. irregularity) of the borehole. It is therefore only qualitative in nature.<br />

The Spontaneous Potential log basically utilizes the same electrical configuration as the Single Point<br />

Resistance and is used to measure variations in natural electrical potential. This potential, measured in<br />

millivolts (mvs), results from one of three principal causes:<br />

Electrochemical potential from a contrast between the resistivities of the borehole fluid and<br />

the formation pore water,<br />

Kirtland AFB<br />

SOPs for Field Investigations B-29 April 2004

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