Detection of subsurface resistivity contrasts with application to location of fluids

Abstract

The invention relates to a method of mapping subsurface resistivity contrasts by making multichannel transient electromagnetic (MTEM) measurements on or near the earth's surface using at least one source, receiving means for measuring the system response and at least one receiver for measuring the resultant earth response. All signals from the or each source-receiver pair are processed to recover the corresponding electromagnetic impulse response of the earth and such impulse responses, or any transformation of such impulse responses, are displayed to create a subsurface representation of resistivity contrasts. The invention enables subsurface fluid deposits to be located and identified and the movement of such fluids to be monitored.

1 . A method of mapping subsurface resistivity contrasts comprising making multichannel transient electromagnetic (MTEM) measurements using at least one source, and at least one receiver for measuring the resultant earth response, measuring the system response for the or each source-receiver pair and for each transient; deconvolution of the measured signal for the measured system response from the or each source-receiver pair to recover the corresponding electromagnetic impulse response of the earth, and displaying such impulse responses, or any transformation of such impulse responses, to create a subsurface representation of resistivity contrasts.  
   
   
       2 . A method according to  claim 1 , wherein the impulse response of the earth is obtained from the equation 
         a   k ( x   s   , x   r   , t )= s   k ( x   s   , x   r   , t )* g ( x   s   , x   r   , t )+ n   k ( x   r   , t ) 
     where k indicates the kth measurement in a suite of measurements for a given source-receiver pair, a k (x s ,x r ,t) is the measured transient response for a given source-receiver pair of said MTEM measurements, * denotes convolution, s k (x s ,x r ,t) is the system response, g(x r ,x r ,t) is the impulse response of the earth for a given source-receiver pair, and n k (x r ,t) is uncorrelated electromagnetic noise at the receiver.  
   
   
       3 . A method according to  claim 2 , wherein said source comprises a current in a wire grounded at each end, and the receiver comprises a device for measuring the potential difference between two grounded electrodes.  
   
   
       4 . A method according to  claim 2 , wherein said source comprises a current in a wire grounded at each end, and the receiver comprises a device for measuring the current induced in at least one horizontal loop.  
   
   
       5 . A method according to  claim 2 , wherein said source comprises at least one current loop and the receiver comprises a device for measuring the potential difference between two grounded electrodes.  
   
   
       6 . A method according to  claim 2 , wherein said source comprises at least one current loop and the receiver comprises a device for measuring the current induced in at least one receiver loop.  
   
   
       7 . A method according to  claim 2 , wherein said source comprises a current in a wire grounded at each end and the system response in measured with a device, for example a current meter for measuring the current in the wire.  
   
   
       8 . A method according to  claim 2 , wherein said source comprises at least one current loop and the system response is measured with a device, for example a current meter for measuring the current in the loop.  
   
   
       9 . A method according to  claim 2 , wherein the recording system used to measure the system response has the same characteristics as the system used to record the measurement a k (x s ,x r ,t).  
   
   
       10 . A method according to  claim 2 , wherein the recording system used to measure the system response has different characteristics from the recording system used to record the measurement a k (x s ,x r ,t) and wherein these differences are eliminated using the Fourier transform of the transfer function between the two recording systems.  
   
   
       11 . A method according to  claim 2 , wherein an estimate of the earth impulse response with noise is obtained by deconvolution of the said equation.  
   
   
       12 . A method according to  claim 2 , wherein the transients are stacked and an estimate of the earth impulse response is obtained by deconvolution of the system response.  
   
   
       13 . A method according to  claim 11 , wherein said estimate of the earth impulse response is improved by stacking the estimated impulse responses.  
   
   
       14 . A method according to  claim 2 , wherein the measured system response and corresponding measured transient are synchronised.  
   
   
       15 . A method according to  claim 2 , wherein any different time origin between the measured system response and corresponding measured transient is measured and compensated for.  
   
   
       16 . A method according to  claim 1 , wherein the MTEM measurements are made on the earth's surface.  
   
   
       17 . A method according to  claim 1 , wherein the MTEM measurements are made at or near a sea floor of the earth's surface.  
   
   
       18 . A method according to  claim 1 , wherein the system response is approximately a step and an approximation to the deconvolution is made by differentiation.  
   
   
       19 . Apparatus for mapping subsurface resistivity contrasts comprising a multichannel transient electromagnetic (MTEM) measuring device comprising at least one source, and at least one receiver for measuring the resultant earth response, a system response measuring device for measuring the system response for the or each source-receiver pair and for each transient and a processor for deconvolution of the measured signal for the measured system response from the or each source-receiver pair to recover the corresponding electromagnetic impulse response of the earth, and a display for displaying such impulse responses, or any transformation of such impulse responses, to create a subsurface representation of resistivity contrasts.