Figure 1. A gravity survey in progress.
The measurement of gravity, or gravitational acceleration, is a highly developed science. Present instrumentation has a sensitivity about +/- 0.01mgal or +/- 1 part in 10 billion of the earths field. This sensitivity allows the detection of small density variations within the near surface of the earth. The measurement of these density variations can assist in undertanding the earth’s subsurface.
The common exploration geophysics gravity meters measure the relative difference in gravity. There are very sensitive and portable absolute gravity meters that are becoming commercially available. The typical exploration gravity meter is about the size of a coffee pot and needs careful handling. Four commercial gravity meter providers are: Scintrex Ltd., Sodin Ltd., LaCoste and Romberg, and Worden.
The field procedure for a gravity survey is not much different than the other geophysical methods. The main difference is in the vertical control needed at each station. A good quality gravity survey provides sufficient control on the uncertainty in elevation at each station to permit ‘seeing’ the feature of interest. For instance, if the vertical control is poor then no matter how well the gravity meter performs you will probably never observe the feature of interest. So a good gravity survey also means a good (+/- 0.1m) to excellent (+/- 0.01m) level (elevation) survey for all stations. In some cases an uncertainty of +/- 0.001 m is used where visibility is good.
- Good gravity readings in a proper base station looping procedure
- Good elevation control
A quality gravity survey then requires two things:
These two requirements affect the survey cost as two people are needed to perform the level survey and the internal data checks require time to loop-back along the line to check consistency and for errors. One can probably record about 80 to 100 stations per day including base station tie-ins.
Gravity surveys in rugged topography also require corrections for the local topographic deviations from a planar surface. These corrections then require very good topographic maps, which sometimes do not exist due to forest canopy, remoteness of site, lack of airphoto coverage etc. In many cases nearby topographic effects are quite important and these are often the most difficult to attain due to the forest canopy. The more distant topographic effects, as one can calculate from a 1:50,000 scale map, also require a good representative estimate of the local density structure which is usually unknown but can be assumed, at least to begin, to be the average density of the earths crust of 2670 kg/cubic-metre.
Figure 2. Photograph of a Scintrex Ltd. CG-3M gravity meter set up on a station.
The scale is used to measure the instrument height and the plywood was used to shelter the instrument from the wind. This instrument can measure small density differences in the shallow subsurface that may assist in the characterization of the overburden thickness, or locate voids or areas of lower density in the shallow subsurface. These measurements are very successful in the characterization of metallic mineral deposits prior to drilling.
Figure 3. Tidal Gravity prediction.