Patent ID: 9611736
Date: 2017-04-04
CPC Classifications: E21B,G01V

Claim:
1. An electromagnetic survey apparatus for measuring electrical resistivity properties of subsurface formations of interest, comprising: a transmitter providing electric power for an electric current within the subsurface forming an electromagnetic field; a borehole electrode positioned within a borehole at a depth of interest to inject electric current into the subsurface at the depth of interest; a wellhead electrode at the earth surface adjacent the borehole to inject and receive electric current at the earth surface; a counter electrode at the earth surface and at a spaced position at the earth surface from the borehole to receive electric current at the earth surface; a switching mechanism selectively electrically connecting the transmitter to the borehole electrode and the wellhead electrode for electrical current to flow through subsurface features between the borehole electrode and the wellhead electrode and form an electromagnetic field regarding the subsurface features of interest of the earth between the borehole depth of interest and the wellhead; the switching mechanism selectively electrically connecting the transmitter to the wellhead electrode and the counter electrode for electrical current to flow through the subsurface earth between the wellhead electrode and the counter electrode and form an electromagnetic field regarding near surface electrical resistivity anomalies of the earth between the wellhead and the spaced position at the earth surface from the borehole; an array of electromagnetic sensors at the earth surface disposed to sense the electromagnetic field between the electrodes electrically connected to the transmitter as input electromagnetic field data for processing; a data processor forming a measure of resistivity of the subsurface formation of interest from the sensed electromagnetic field data regarding the subsurface features of interest between the borehole depth of interest and the wellhead; the data processor further forming a measure of resistivity of near surface electrical resistivity anomalies from the sensed electromagnetic field data regarding near surface electrical resistivity anomalies between the wellhead and the spaced position at the earth surface from the borehole; and the data processor further removing effects of near surface electrical resistivity anomalies between the wellhead and the spaced position at the earth surface from the measure of resistivity of the subsurface formations of interest for assessing electrical properties of the subsurface formations of interest.