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[[file:borehole-gravity_fig1.png|thumb|300px|{{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.]]

[[file:borehole-gravity_fig1.png|thumb|300px|{{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 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 [[reef]]s.

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, D. A. and M. A. Ander, 1999, [http://archives.datapages.com/data/specpubs/beaumont/ch15/ch15.htm Applying borehole gravity methods], in Exploring Oil and Gas Traps: [http://store.aapg.org/detail.aspx?id=545 AAPG Treatise of Petroleum Geology 3], 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, D. A. and M. A. Ander, 1999, [http://archives.datapages.com/data/specpubs/beaumont/ch15/ch15.htm Applying borehole gravity methods], in Exploring Oil and Gas Traps: [http://store.aapg.org/detail.aspx?id=545 AAPG Treatise of Petroleum Geology 3], 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.

==Applications==

==Applications==

The spectrum of BHGM applications is defined on one extreme by density logging and on the other by remote sensing of structure. The first extreme sometimes focuses strictly on formation and reservoir evaluation questions, while the other extends to basic exploration. Figure 1 is an example of both applications. In fact, the purpose of the survey was to detect carbonate porosity in a reef environment that was missed by the other logs. For this objective, the useful radius of investigation of the measurement is on the order of [[length::50 ft]]. The sharp negative density anomaly observed between 6330 and [[depth::6370 ft]] suggests porosity obscured by near-borehole effects or poor volume sampling. However, the broad departure of the BHGM and gamma-gamma logs over the depth range of the logged section is typical of a structural effect, in this case the edge of the reef complex which is within a few hundred feet.For more information about this log and for more examples of applications, see [[Borehole gravity applications: examples]].

The spectrum of BHGM applications is defined on one extreme by density logging and on the other by remote sensing of structure. The first extreme sometimes focuses strictly on formation and reservoir evaluation questions, while the other extends to basic exploration. Figure 1 is an example of both applications. In fact, the purpose of the survey was to detect carbonate porosity in a [[reef]] environment that was missed by the other logs. For this objective, the useful radius of investigation of the measurement is on the order of [[length::50 ft]]. The sharp negative density anomaly observed between 6330 and [[depth::6370 ft]] suggests porosity obscured by near-borehole effects or poor volume sampling. However, the broad departure of the BHGM and gamma-gamma logs over the depth range of the logged section is typical of a structural effect, in this case the edge of the reef complex which is within a few hundred feet.For more information about this log and for more examples of applications, see [[Borehole gravity applications: examples]].

===Density logging===

===Density logging===

[[Category:Geophysical methods]]

[[Category:Geophysical methods]]