measure the number and energy level of detected gamma rays and uses the data to calculate
the concentrations of each component.
Gamma gamma logs record the intensity of gamma radiation at the tool detectors
resulting from the backscattering and attenuation of gamma radiation emitted by the tool
source. The primary use of the gamma gamma tool is for the identification of lithology and
for the measurement of the bulk density of the formation. The modern gamma gamma log
records the bulk density of the measured formation using a compensating, skid mounted,
borehole sidewall device that contains a gamma ray source and two detectors. The instrument
skid is pressed against the borehole sidewall by a spring activated arm with sufficient force to
cut through soft mudcakes. The density sonde measures the formation's ability to attenuate
gamma rays emitted from the tool's radioactive source by measuring the number of scattered
gamma rays reaching the detectors. The number of scatterings is related to the number of
electrons in the formation, therefore, the response of the tool is determined by the electron
density of the formation. The electron density is related to the true bulk density of the
formation. The bulk density information is used to provide a measure of the formation
density and to calculate the formation porosity. More advanced density tools, in addition to
providing measurement of formation density, record low energy gamma rays in the domain of
photoelectric absorption. By comparing the number of gamma rays detected in each domain,
these density tools can determine a photoelectric absorption cross section index, Pe. The Pe
value is primarily a function of the formation mineralogy and is used to estimate the in situ
mineralogic composition of the formation. The depth of penetration of the density tool is
approximately 4 feet with vertical resolution ranging from 1.5 to 3 feet, depending on the
logging speed. Instruments of lesser quality may obtain penetration depths of only 6 inches.
Neutron logging is one of several methods used to derive porosity values for
subsurface formations. The neutron log response is a function of the hydrogen content of the
borehole environment and is used for the measurement of moisture content above the water
table and of total porosity below the water table. A modern, compensated neutron tool uses
an americium beryllium radioactive source (3 16 curies) to generate high energy neutrons that
interact with the formation. The sonde is a dual spaced device with two sets of thermal
neutron detectors, near and far. The tool compensation resulting from the dual detector
arrangement reduces the effects of borehole conditions by using the ratio of two counting
rates similarly affected by the environment. As the neutrons are attenuated or rebounded
from the formation, the tool detects and counts neutrons in the thermal energy regime. The
ratio of the counting rates from the two detectors is processed by the surface equipment to
produce a linearly scaled measure of the neutron porosity index.
The response of the neutron tool is affected by formation elements having high
thermal neutron capture cross sections (elements having higher probabilities of capturing
thermal neutrons) that act to moderate (attenuate) neutrons in the formation. Hydrogen,
boron, and chlorine are particularly effective. Reduced counting rates as a result of neutron
attenuation by an element result in poorer counting statistics and unrealistically higher
November 1992
4 32






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