Geology Reference
In-Depth Information
Resistivity Logs
The resistivity log is analogous to electrical resistivity measurements made
at the Earth's surface. It measures the electrical resistance of material between
electrodes placed on the sonde in the borehole. This log is especially sensitive
to the electrical properties of fluids contained in underground formations. The
resistivity log cannot generally be made through casing, although research is
now being done to develop this capability. There are several types of resistivity
logs including direct-current resistivity logs and electromagnetic induction logs.
Gamma Logs
Gamma logs measure natural gamma radiation, and are particularly useful
for finding shales that have a high gamma output because clay collects
radionuclides. They can be used in cased holes, so they are also run on casing
collar logs to tie the exact location of casing to geologic rock units. This is
essential if we are to perforate the casing at exactly the right spot to test the oil
or gas (or fresh water) zones. Newer techniques include gamma-ray spectral
logging to look at clays, based on ratios of gamma rays of known energy from
uranium, thorium, and potassium.
Spontaneous Potential Logs
The spontaneous potential logs measure voltage between formations by
attaching one voltmeter electrode on the logging sonde and the other at the
Earth's surface (see above). It does not work if the drill is using salt-based mud.
Sonic Logs
Sonic logs are essentially a borehole seismic refraction survey. Sonic logs
use a 20-kilohertz transducer and two sensors. The method makes use of the
Wyllie equation which assumes that transit time is a function of the mineralogy,
the percentage of pore space, and the P -wave velocity of the fluid within the
pores. The equation works surprisingly well, but it is sometimes treacherous to
extrapolate the velocity measured by sonic tools to
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