Changes

===Density===

===Density===

The density tool measures the apparent density of the formation using a radioactive source that bombards the formation with high energy gamma rays and then measures the number of lower energy gamma rays returning to the detectors. The detectors and source are mounted in a pad that is forced against the borehole wall. The measurement attempts to correct automatically for the effects of mudcake and minor hole rugosity. The measurement is sensitive to significant borehole wall rugosity and pad standoff, which cause the tool to read too low of a density. A typical presentation of the density (as well as several other parameters) is shown in the log in Figure 2.

The density tool measures the apparent density of the formation using a radioactive source that bombards the formation with high energy gamma rays and then measures the number of lower energy gamma rays returning to the detectors. The detectors and source are mounted in a pad that is forced against the borehole wall. The measurement attempts to correct automatically for the effects of mudcake and minor hole rugosity. The measurement is sensitive to significant borehole wall rugosity and pad standoff, which cause the tool to read too low of a density. A typical presentation of the density (as well as several other parameters) is shown in the log in [[:Image:basic-open-hole-tools_fig2.png|Figure 2]].

===Compensated neutron===

===Compensated neutron===

Compensated neutron devices measure the hydrogen index of the formation using a radioactive neutron source that bombards the formation with fast-moving neutrons. Neutrons collide with atoms of the formation, transferring their energy through these collisions. The most efficient transfer of energy occurs with hydrogen atoms because the mass of hydrogen is approximately the same as the mass of a neutron. Two detectors count the number of deenergized (thermal) neutrons returning from the formation. The ratio of the detector count rates is primarily related to the hydrogen index or the apparent water-filled porosity.

Compensated neutron devices measure the hydrogen index of the formation using a radioactive neutron source that bombards the formation with fast-moving neutrons. Neutrons collide with atoms of the formation, transferring their energy through these collisions. The most efficient transfer of energy occurs with hydrogen atoms because the mass of hydrogen is approximately the same as the mass of a neutron. Two detectors count the number of deenergized (thermal) neutrons returning from the formation. The ratio of the detector count rates is primarily related to the hydrogen index or the apparent water-filled porosity.

The source and detectors are mounted in a mandrel that, ideally, is pressed against the borehole to minimize the influence of the high apparent porosity of the borehole. This measurement is very sensitive to tool standoff, hole size, temperature, and salinity. Environmental corrections are highly recommended before attempting to interpret results. Gas has a very low hydrogen index compared to water, which causes the tool to report abnormally low porosities in gas-bearing formations. When used in conjunction with density measurements, gas-bearing intervals are often easy to identify. A typical presentation of a compensated neutron measurement is shown in the log in Figure 2.

The source and detectors are mounted in a mandrel that, ideally, is pressed against the borehole to minimize the influence of the high apparent porosity of the borehole. This measurement is very sensitive to tool standoff, hole size, temperature, and salinity. Environmental corrections are highly recommended before attempting to interpret results. Gas has a very low hydrogen index compared to water, which causes the tool to report abnormally low porosities in gas-bearing formations. When used in conjunction with density measurements, gas-bearing intervals are often easy to identify. A typical presentation of a compensated neutron measurement is shown in the log in [[:Image:basic-open-hole-tools_fig2.png|Figure 2]].

===Sonic===

===Sonic===

Sonic devices measure the velocity of various acoustic waves, most notably compressional, shear, and Stoneley waves. The velocity of the waves is a function of the elastic properties and the density of the formation. Logs normally present the inverse of velocity, called the ''interval transit time'' or ''delta t'' (Δ''t''). A number of empirical relationships have been developed to relate compressional velocity to porosity (which are explained in [[Standard Interpretation]]).

Sonic devices measure the velocity of various acoustic waves, most notably compressional, shear, and Stoneley waves. The velocity of the waves is a function of the elastic properties and the density of the formation. Logs normally present the inverse of velocity, called the ''interval transit time'' or ''delta t'' (Δ''t''). A number of empirical relationships have been developed to relate compressional velocity to porosity (which are explained in [[Standard Interpretation]]).

Two versions of the compressional sonic device are available: the compensated sonic and the full waveform sonic (FWS). The full waveform sonic contains an array of receivers that are used to determine both compressional and shear velocities. Sonics are available in a variety of transmitter-to-receiver spacings from 3 to [[length::12 ft]] or more. The longer spacings are capable of investigating deeper into the formation. Both the conventional sonic and the full waveform sonic devices are used to measure compressional velocity. A typical presentation of compressional sonic measurements is shown in the log in Figure 1.

Two versions of the compressional sonic device are available: the compensated sonic and the full waveform sonic (FWS). The full waveform sonic contains an array of receivers that are used to determine both compressional and shear velocities. Sonics are available in a variety of transmitter-to-receiver spacings from 3 to [[length::12 ft]] or more. The longer spacings are capable of investigating deeper into the formation. Both the conventional sonic and the full waveform sonic devices are used to measure compressional velocity. A typical presentation of compressional sonic measurements is shown in the log in [[:Image:basic-open-hole-tools_fig1.png|Figure 1]].

Shear velocities are used to determine mechanical properties of the formations and to determine Poisson's ratio for use in interpreting seismic data. Shear velocities can be determined from the FWS (monopole), the dipole sonic, or the quadrupole sonic. The monopole sonic is not able to measure shear velocities when the shear velocity of the formation is slower than the compressional velocity of the mud. Mud interval transit times are typically in the 190 μsec/ft range. When this condition is not met, no shear energy is refracted toward the receivers, making shear velocity measurements impossible. The dipole overcomes this limitation by directly exciting shear flexural energy in the formation regardless of the mud velocities.

Shear velocities are used to determine mechanical properties of the formations and to determine Poisson's ratio for use in interpreting seismic data. Shear velocities can be determined from the FWS (monopole), the dipole sonic, or the quadrupole sonic. The monopole sonic is not able to measure shear velocities when the shear velocity of the formation is slower than the compressional velocity of the mud. Mud interval transit times are typically in the 190 μsec/ft range. When this condition is not met, no shear energy is refracted toward the receivers, making shear velocity measurements impossible. The dipole overcomes this limitation by directly exciting shear flexural energy in the formation regardless of the mud velocities.