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

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