Short-offset transient electromagnetic geophysical surveying
A method for electromagnetic surveying subsurface formations includes inducing an electromagnetic field in the subsurface formations by passing electric current through a transmitter. Response of the subsurface formations to the induced electromagnetic field is detected at a first plurality of spaced apart positions disposed longitudinally within a bipole length of the transmitter. A direct induction response is removed from the detected response.
1. A method for electromagnetic surveying subsurface formations, comprising:
inducing an electromagnetic field in the subsurface formations by passing electric current through a transmitter, wherein the transmitter is deployed above an area of the subsurface to be surveyed and is disposed in a cable towed by a vessel;
detecting response of the subsurface formations to the induced electromagnetic field at a first plurality of spaced apart positions disposed longitudinally entirely within a bipole length of the transmitter using receivers deployed in a plurality of cables laterally spaced apart from the transmitter cable and towed by the vessel, the receivers disposed entirely within the bipole length of the transmitter; and
removing a response component resulting from energy propagating directly from the transmitter to each of the spaced apart positions from the detected response.
2. The method of claim 1 wherein the detecting response comprises measuring voltages impressed across spaced apart pairs of electrodes.
3. The method of claim 1 wherein the transmitter comprises a pair of spaced apart electrodes.
4. The method of claim 1 further comprising detecting response of the subsurface formations at a plurality of second spaced apart positions disposed longitudinally entirely within the bipole length of the transmitter and laterally spaced apart from the first positions.
5. The method of claim 1 wherein the detecting is performed by deploying a line of electromagnetic receivers above an area of the subsurface to be surveyed.
6. The method of claim 5 wherein the line of receivers is disposed in a cable towed by a vessel.
7. The method of claim 1 wherein the current includes at least one switching event.
8. The method of claim 7 wherein the at least one switching event comprises one of switching current on, switching current off, reversing current polarity, a pseudorandom binary sequence and combinations thereof.
9. The method of claim 1 wherein the bipole length of the transmitter is about five times a depth in the subsurface of a target formation.