Changes

==How the tool measures gravity==

==How the tool measures gravity==

The figure below illustrates the fundamentals of measuring density using a [[borehole gravity]] sensor. Two gravity measurements, ''g''₁ and ''g''₂, are made downhole, separated in depth by Δ''z''. The value ''G'' is the universal gravity constant. Thus, the gravity gradient, Δ''g''/Δ''z'', is related directly to the density of the intervening layer. The result is a direct computation of the bulk density of that layer.

[[:file:applying-gravity-in-petroleum-exploration_fig15-10.png|Figure 1]] illustrates the fundamentals of measuring density using a [[borehole gravity]] sensor. Two gravity measurements, ''g''₁ and ''g''₂, are made downhole, separated in depth by Δ''z''. The value ''G'' is the universal gravity constant. Thus, the gravity gradient, Δ''g''/Δ''z'', is related directly to the density of the intervening layer. The result is a direct computation of the bulk density of that layer.

[[file:applying-gravity-in-petroleum-exploration_fig15-10.png|thumb|{{figure number|15-10}}]]

[[file:applying-gravity-in-petroleum-exploration_fig15-10.png|thumb|{{figure number|1}}]]

 

[[file:applying-gravity-in-petroleum-exploration_fig15-11.png|thumb|{{figure number|15-11}}After .<ref name=ch15r9>McCulloh, T., H., Kandle, J., R., Schoellhamer, J., E., 1968, Application of gravity measurements in wells to problems of reservoir evaluation: Transactions of the 9th Annual SPWLA Logging Symposium. Fundamental work describing the distance of sources seen by the borehole gravity meter.</ref> Copyright: SPWLA.]]

==Depth of investigation==

==Depth of investigation==

The figure below shows how the depth of investigation is tunable by means of varying the separation between two gravity measurements, Δ''z''. The rule of thumb is that 90% of the gravity effect can be imaged at a distance away from the borehole within five times Δ''z''.

[[:file:applying-gravity-in-petroleum-exploration_fig15-11.png|Figure 2]] shows how the depth of investigation is tunable by means of varying the separation between two gravity measurements, Δ''z''. The rule of thumb is that 90% of the gravity effect can be imaged at a distance away from the borehole within five times Δ''z''.

 

==Depth of investigation of various logging methods==

==Depth of investigation of various logging methods==