BOREHOLE ELECTRIC FIELD SURVEY WITH IMPROVED DISCRIMINATION OF SUBSURFACE FEATURES

摘要

Errors produced in a borehole to surface electromagnetic (BSEM) survey by near surface electrical anomalies on the estimates of formation properties are reduced. The effects of variations in subsurface electrical resistivity near the surface are separated from electrical resistivity changes at locations in the formations of interest far from the measurement region. A survey system includes one or more electrodes to inject electrical current at formation depth within a borehole, one or more counter electrodes which collect such current on the surface of the earth, and one or more electrodes to inject current on the surface. A transmitter is selectively connectable to different sets of the electrodes to cause a current to flow between the selected electrode sets. The resultant fields from the current flow are sensed and processed. The effects of near surface anomalies are detectable in the data and removable from the survey data of interest regarding the formations. The survey data more accurately indicates formation features or properties of interest.

主权项

1. A electromagnetic survey apparatus for measuring electrical properties of subsurface earth features, comprising:
a transmitter providing electric power for an electric current within the subsurface forming an electromagnetic field; a borehole electrode positioned within a well borehole at a depth of interest; a wellhead electrode at the at the earth surface adjacent the borehole; a counter electrode at the earth surface and spaced from the borehole; a switching mechanism selectively electrically connecting the transmitter to selected pairs of the current electrodes for electrical current to flow between the connected electrodes and form an electromagnetic field; an array of electromagnetic sensors at the earth surface disposed to sense the electromagnetic field between the connected electrodes.