Changes

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 (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 [[depth::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|{{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.