Apparatus and method for gravity correction in borehole survey systems

摘要

Disclosed is an inertial navigation borehole survey system wherein the signals supplied by accelerometers (40) that are contained within the borehole survey system probe (10) are corrected for gravitational gradients encountered as the probe (10) travels through a borehole (12). The gravity correction is effected in the survey system signal processor (24) and is based on a gravity gradient signal that mathematically corresponds to: ##EQU1## where f represents the specific force due to gravity; f.sub.o represents the specific force of gravity at wellhead (20) of borehole (12); R.sub.o represents the average radius of the earth; . rho.(H) represents the local density of the geological formation penetrated by the borehole as a function of depth H; and &rgr;. sub.ave represents the means density of the earth. In utilizing the gravitational gradient to generate a gravity correction signal, the signal processor (24) effects a summation process that mathematically corresponds to: ##EQU2## where (&Dgr;H).sub.i represents the depth change between the "ith" signal processing cycle and the nextmost antecedent processing cycle, and, the summation range extends from the first signal processing cycle performed during the borehole survey through the final signal processing cycle of the borehole survey operation.