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==Basic concepts==

==Basic concepts==

The borehole gravity meter (BHGM) can be described simply as a deep-investigating density logging tool. Applications range beyond this simple description to include detection of oil- and gas-filled [[porosity]] and detection and definition of remote structures, such as salt domes, faults, and reefs.

The borehole gravity meter (BHGM) can be described simply as a deep-investigating density logging tool. Applications range beyond this simple description to include detection of oil- and gas-filled [[porosity]] and detection and definition of remote structures, such as salt domes, faults, and reefs.

One of the great advantages of the BHGM is that it is the only logging method that can directly measure density at a significant distance away from a well. It is also the only logging method that can reliably obtain density through well casing <ref name=Chapin, David A. and Mark A. Ander>Chapin, David A. and Mark A. Ander, 1999,Applying borehole gravity methods, in Treatise of Petroleum Geology, p. 15-15,</ref>.  It is practically unaffected by near-borehole influences, which are the scourge of nuclear tools: casing, poor cement bonding, rugosity, washouts, and fluid invasion. Another advantage is the fundamental simplicity of the relationships among gravity, mass, rock volume, and density. Complex geology can be easily modeled so that the response of a range of hypothetical models can be studied and understood before undertaking a survey.

One of the great advantages of the BHGM is that it is the only logging method that can directly measure density at a significant distance away from a well. It is also the only logging method that can reliably obtain density through well casing <ref name=Chapin, David A. and Mark A. Ander>Chapin, David A. and Mark A. Ander, 1999,Applying borehole gravity methods, in Treatise of Petroleum Geology, p. 15-15,</ref>.  It is practically unaffected by near-borehole influences, which are the scourge of nuclear tools: casing, poor cement bonding, rugosity, washouts, and fluid invasion. Another advantage is the fundamental simplicity of the relationships among gravity, mass, rock volume, and density. Complex geology can be easily modeled so that the response of a range of hypothetical models can be studied and understood before undertaking a survey.

What is actually measured is referred to as ''BHGM apparent density'', which is a simple function of the measured vertical gradient of gravity. To obtain an apparent density measurement, gravity is measured at two depths. The accuracy of the computed density depends on the accuracy of both measured differences: gravity and depth. Operationally, BHGM surveys resemble vertical seismic profiling (VSP) surveys. The BHGM is stopped at each planned survey level, and a 5- to 10-min reading is taken. The blocky appearance of the log reflects the station interval (Figure 1).

What is actually measured is referred to as ''BHGM apparent density'', which is a simple function of the measured vertical gradient of gravity. To obtain an apparent density measurement, gravity is measured at two depths. The accuracy of the computed density depends on the accuracy of both measured differences: gravity and depth. Operationally, BHGM surveys resemble vertical seismic profiling (VSP) surveys. The BHGM is stopped at each planned survey level, and a 5- to 10-min reading is taken. The blocky appearance of the log reflects the station interval ([[:file:borehole-gravity_fig1.png|Figure 1]]).

 

[[file:borehole-gravity_fig1.png|thumb|{{figure number|1}}An example of a BHGM log. The sharp difference In density between 6330 and 6370 ft is caused by porosity not detected by the gamma-gamma density log. The broader difference anomaly observed over the length of the logged interval is explained by the structural influence of the reef complex.]]

The log is not continuous. BHGM measurements are taken at discrete depths usually at intervals of 10 to [[length::50 ft]], depending on the resolution required. While the BHGM has remarkable resolution in the measurement of density over intervals of [[length::10 ft]] or more (less than 0.01 g/cm³), surveys requiring closer vertical resolution must sacrifice density resolution.

The log is not continuous. BHGM measurements are taken at discrete depths usually at intervals of 10 to [[length::50 ft]], depending on the resolution required. While the BHGM has remarkable resolution in the measurement of density over intervals of [[length::10 ft]] or more (less than 0.01 g/cm³), surveys requiring closer vertical resolution must sacrifice density resolution.