Method for subsurface electromagnetic surveying using two or more simultaneously actuated electromagnetic sources to impart electromagnetic signals into a subsurface formation and thereby determining a formation response to each signal
A method for electromagnetic exploration includes imparting a first electromagnetic signal into subsurface formations and imparting a second electromagnetic signal into the formations substantially contemporaneously with imparting the first electromagnetic signal. The first and second electromagnetic signals are substantially uncorrelated with each other. A combined electromagnetic response of the formations to the first and second imparted electromagnetic signals is detected. A response of the formations to each of the first and the second imparted signals is determined from the detected response.
1. A method for electromagnetic exploration, comprising:
imparting a first electromagnetic signal into subsurface formations;
imparting a second electromagnetic signal into the formations substantially contemporaneously with imparting the first electromagnetic signal, the first and second electromagnetic signals substantially uncorrelated with each other;
detecting a combined electromagnetic response of the formations to the first and second imparted electromagnetic signals; and
determining a response of the formations to each of the first and the second imparted signals from the detected response, wherein determining formation response to the first electromagnetic signal comprises:
(a) deconvolving the detected signal using the first electromagnetic signal;
(b) estimating the formation response to the first electromagnetic signal using the deconvolved detected signal;
(c) convolving the estimated formation response from (b) with the first electromagnetic signal; and
(d) subtracting a result of the convolution of the estimated formation response with the first electromagnetic signal from the detected signal.
2. The method of claim 1 , wherein the first and second electromagnetic signals are substantially uncorrelated pseudo-random binary sequences.
3. The method of claim 1 , wherein the first and second electromagnetic signals have a cross correlation function of substantially zero.
4. The method of claim 1 further comprising detecting a combined response at a Plurality of locations; separated from a location of the imparting and a location of the detecting locations.
5. The method of claim 4 wherein a switching frequency of the first and second imparted signals is inversely related to a square of a distance between a location of imparting the first signal and a location of imparting the second signal and a location of detecting the combined electromagnetic response.
6. The method of claim 1 further comprising imparting a third electromagnetic signal into the subsurface formations, the third imparted signal being substantially uncorrelated with the first and second imparted signals.
7. The method of claim 6 further comprising detecting combined first, second and third electromagnetic signals at a location, and wherein a switching frequency of each of the first, second and third electromagnetic signals is inversely related to a square of the distance between each imparting location and each detecting location.
8. The method of claim 1 wherein the first and second imparted signals have overlapping bandwidths.
9. The method of claim 1 involving measuring a property of the first and the second electromagnetic signals proximate the location of imparting thereof.
10. The method of claim 9 wherein the property is current used to actuate an electromagnetic transmitter.
11. The method of claim 1 wherein the first and second electromagnetic signals are imparted at substantially a same location.
12. The method of claim 1 wherein the estimating the formation response to the first electromagnetic signal using the deconvolved detected signal includes identifying a peak associated with the first electromagnetic signal and using the peak to determine the estimate.
13. The method of claim 1 further comprising (c) deconvolving the result of (d) with the second electromagnetic signal to produce an estimate of formation response to the second electromagnetic signal.
14. The method of claim 13 further comprising:
(f) convolving the estimate of the formation response to the second electromagnetic signal with the second electromagnetic signal;
(g) determining a difference between the result of (f) and the detected signal;
(h) deconvolving the result of (g) with the first electromagnetic signal to produce an improved estimate of the formation response to the first electromagnetic signal; and
(j) using the improved estimate produced in (h) by (1) convolving the estimated formation response from (h) with the first electromagnetic signal, and (2) subtracting a result of the convolution of the estimated formation response from (h) with the first electromagnetic signal from the detected signal.
15. The method of claim 14 further comprising iteratively repeating (c) to (j) until convergence occurs.
16. The method of claim 1 wherein the imparting the first and second electromagnetic signals comprises imparting electric current across an electric bipole.
17. The method of claim 1 wherein the detecting comprises measuring voltage imparted across an electric bipole.