Mapping the range front with gravity - are the corrections up to it?

The currently used gravity data reduction procedures were formulated in the petroleum industry and have not changed since the 1930’s. Simplifications needed to facilitate data reduction by hand have remained, in spite of the cheap and powerful computer processing that is now in common use. Corrections such as the Free Air Correction still use a linear relationship with height, even though the full latitude and height dependent formula is simple and available. More significantly, the Simple Bouguer Correction is still being used routinely in spite of the significant errors it introduces by assuming an infinite “Bouguer slab” and that the density of all rocks in the survey area can be approximated by one value. The result is that most gravity data is over or under corrected.

The Complete Bouguer Correction, incorporating a full terrain correction, is rarely applied, and only then when the topography is extreme. Yet it is this correction which promises to deliver an accuracy in data processing and presentation commensurate with the accuracies currently being achieved in data collection in the field with DGPS and modem gravimeters. The availability of high resolution Digital Terrain Models and faster terrain correction software certainly allows these corrections to be applied routinely. Even more rarely used is a variable density model. Obtaining such a model is the next major hurdle to be overcome.

Density models may be classified into three types: assumed, statistical, and inferred. Assumed densities are typically based on measurements of hand specimen samples and extended to the entire survey area. Statistical density models are based on minimising the correlation between topography and gravity whilst inferred densities are derived from a transformation of the actual gravity data.

The Hamersley Basin of Western Australia is a hostile environment for the gravity method. Precipitous cliffs of high density banded iron formations abut low lying plains of unconsolidated sediments. Exploration for iron is focussed on this topographically challenging range front. The strict application of the Complete Bouguer Correction coupled with a variable density model allows the gravity method to be used as a direct detector of iron deposits in spite of the obvious problems.