Gravity Notes

Applications of the Gravity Method

Gravimeters-use a zero length spring

A zero length spring is a spring which when all turns are touching has a given tension still present which is equal to the tension required to extend the spring a distance equal to its original length,i.e. tension is proportional to actual length of the spring

This condition is met by pre-stressing the spring in winding so an initial force is required to separate the coils.

Gravity Corrections

The difference in gravity between two stations is in part due to other factors in addition to the attraction of unknown anomalous masses, i.e.:

  1. variations in latitude
  2. elevation
  3. topography

The process of correcting for these factors is known as gravity reduction and most often we reduce our data as if taken on the geoidal surface, this is the most convenient equipotential surface.

The difference between observed gravity (gobs) and theoretical gravity (gth) at any point on the Earth's surface after reducing the gravity readings to the geoidal surface (i.e., making the Free Air, Bouguer slab, and terrain corrections) is known as the Bouguer gravity anomaly or Bouguer gravity and results due to lateral variations in density in the subsurface.

Corrections to Gravimeter Reading

1.) Correct for drift in terms of dial units

2.) Calculate difference between stations and base ( Rdg)

3.) Convert difference to units of gravity ( g) by multiplying Rdg values by the gravimeter scale constant

4.) Calculate gobs at the station by adding g to observed gravity at base

5.) Calculate the Free Air (CFA) correction (below reference surface h is negative, above h is positive) and add to gobs of the station

6.) Calculate Bouguer slab (CBS) correction and subtract from gobs + CFA of the station.

7.) Calculate terrain (CTC) correction if necessary and add to gobs + CFA - CBS

8.) Calculate gth for the station and subtract from gobs + CFA - CBS + CTC. This is the complete Bouguer anomaly.

9.) The simple Bouguer anomaly is gobs + CFA - CBS - gth.

10.) The Free Air anomaly is gobs + CFA - gth.

Density determinations

1. Assume =2.67 Mg/m3 or 2.67 g/cm3.

2. Assign density based on literature search.

3. Measurements on hand samples, cuttings, and core.

4. Gamma-Gamma density logs.

5. Seismic velocities.

6. Borehole gravity:

7. Nettleton’s method of gravity (density) profiling.

8. Accounting for porosity: = P fluid + (1-P) dry; where P = fractional porosity