Terrain Corrections

Like Bouguer Slab Corrections, when computing Terrain Corrections we need to assume an average density for the rocks exposed by the surrounding topography. Usually, the same density is used for the Bouguer and the Terrain Corrections. Thus far, it appears as though applying Terrain Corrections may be no more difficult than applying the Bouguer Slab Corrections. Unfortunately, this is not the case.

To compute the gravitational attraction produced by the topography, we need to estimate the mass of the surrounding terrain and the distance of this mass from the observation point (recall, gravitational acceleration is proportional to mass over the distance between the observation point and the mass in question squared). The specifics of this computation will vary for each observation point in the survey because the distances to the various topographic features varies as the location of the gravity station moves. As you are probably beginning to realize, in addition to an estimate of the average density of the rocks within the survey area, to perform this correction we will need a knowledge of the locations of the gravity stations and the shape of the topography surrounding the survey area.

Estimating the distribution of topography surrounding each gravity station is not a trivial task. One could imagine plotting the location of each gravity station on a topographic map, estimating the variation in topographic relief about the station location at various distances, computing the gravitational acceleration due to the topography at these various distances, and applying the resulting correction to the observed gravitational acceleration. A systematic methodology for performing this task was formalized by Hammer* in 1939. Using Hammer's methodology by hand is tedious and time consuming. If the elevations surrounding the survey area are available in computer readable format, computer implementations of Hammer's method are available and can greatly reduce the time required to compute and implement these corrections.

Although digital topography databases are widely available, they are commonly not sampled finely enough for computing what are referred to as the near-zone Terrain Corrections in areas of extreme topographic relief or where high-resolution (less than 0.5 mgals) gravity observations are required. Near-zone corrections are terrain corrections generated by topography located very close (closer than 558 ft) to the station. If the topography close to the station is irregular in nature, an accurate terrain correction may require expensive and time-consuming topographic surveying. For example, elevation variations of as little as two feet located less than 55 ft from the observing station can produce Terrain Corrections as large as 0.04 mgals.

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