Electromagnetic surveying

Abstract

A submersible electromagnetic (EM) field generator for seafloor electromagnetic surveying comprising an AC to DC converter operable to generate a DC output from an AC input and a switching module operable to generate a waveform driving signal by selectively switching the DC output. The EM field generator also comprises an antenna that is operable to generate an EM field in response to the waveform driving signal. This design approach of using a switched DC source allows square or rectangular wave EM signals to be generated which have sharp transition characteristics and which are substantially independent of the AC input characteristics.

1 . A submersible electromagnetic (EM) field generator for seafloor electromagnetic surveying, comprising: 
 an AC to DC converter operable to generate first and second DC outputs from an AC input;    a switching module operable to generate a waveform driving signal by selectively switching the DC output by means of a plurality of switches;    surge protection circuitry comprising a set of transient suppression circuits connected in parallel across the first and second DC outputs, each transient suppression circuit providing suppression tuned to a particular frequency or frequency range, thereby to provide protection for the switches against induced back EMF generated when the switches are operated; and    an antenna operable to generate an EM field in response to the waveform driving signal.    
   
   
       2 . The submersible electromagnetic field generator according to  claim 1 , wherein the AC to DC converter is operable to generate the DC output from a three-phase AC input.  
   
   
       3 . The submersible electromagnetic field generator according to  claim 1 , wherein the switches are semiconductor switches.  
   
   
       4 . The submersible electromagnetic field generator according to  claim 3 , wherein each semiconductor switch has a transient voltage suppressor connected between its source and gate.  
   
   
       5 . The submersible electromagnetic field generator according to  claim 3 , wherein the semiconductor switches are MOSFETs or IGBTs.  
   
   
       6 . The submersible electromagnetic field generator according to  claim 1 , wherein the surge protection circuitry comprises one or more diodes.  
   
   
       7 . The submersible electromagnetic field generator according to  claim 6 , wherein each of the switches is connected in parallel with a respective one of the diodes.  
   
   
       8 . The submersible electromagnetic field generator according to  claim 1 , wherein the transient suppression circuits have capacitors, diodes and/or resistors with values selected according to their respective particular frequency or frequency range.  
   
   
       9 . The submersible electromagnetic field generator according to  claim 1 , wherein the AC to DC converter is further operable to generate at least one additional DC output so that there is a plurality of DC outputs, and the switching module is further operable to generate the waveform driving signal by selectively switching the plurality of DC outputs.  
   
   
       10 . The submersible electromagnetic field generator according to  claim 1 , further comprising a controller that is operable to control selective switching of the switching module so as to generate the waveform driving signal.  
   
   
       11 . The submersible electromagnetic field generator according to  claim 10 , wherein the controller is operable to receive an external synchronisation signal and, in response thereto, to control the operation of the switching module so as to start generation of the waveform driving signal.  
   
   
       12 . The submersible electromagnetic field generator according to  claim 10 , wherein the controller is operable to receive an external command signal and to control the switching module to generate one of a plurality of predetermined waveform driving signals in dependence upon the external command signal.  
   
   
       13 . The submersible electromagnetic field generator according to  claim 1 , further operable to receive an external clock signal.  
   
   
       14 . The submersible electromagnetic field generator according to  claim 13 , wherein the switching module is operable to generate the waveform driving signal in synchronisation with the external clock signal.  
   
   
       15 . The submersible electromagnetic field generator according to  claim 1 , further comprising a data gathering module.  
   
   
       16 . The submersible electromagnetic field generator according to  claim 1 , further operable to receive an external clock signal, and further comprising a data gathering module, and wherein the data gathering module is operable to receive the external clock signal and to time-stamp data gathered by the data gathering module with a time-stamp generated from the external clock signal.  
   
   
       17 . The submersible electromagnetic field generator according to  claim 15 , wherein the data gathering module is further operable to transmit data gathered by the data gathering module from a submersible vehicle to topside.  
   
   
       18 . An electromagnetic (EM) field generation system for seafloor electromagnetic surveying, comprising: 
 a submersible electromagnetic field generator, comprising: an AC to DC converter operable to generate first and second DC outputs from an AC input; a switching module operable to generate a waveform driving signal by selectively switching the DC output by means of a plurality of switches; surge protection circuitry comprising a set of transient suppression circuits connected in parallel across the first and second DC outputs, each transient suppression circuit providing suppression tuned to a particular frequency or frequency range, thereby to provide protection for the switches against induced back EMF generated when the switches are operated; an antenna operable to generate an EM field in response to the waveform driving signal; and a controller that is operable to control selective switching of the switching module so as to generate the waveform driving signal;    a power source operably coupled to the AC to DC converter of the electromagnetic field generator; and    a control module operably coupled to the controller of the submersible electromagnetic field generator, wherein the control module is operable to control the waveform driving signal that drives the antenna.    
   
   
       19 . The electromagnetic field generation system according to  claim 18 , wherein the control module is operably coupled to a global positioning satellite (GPS) receiver, and is further operable to generate a clock signal in synchronisation with a GPS transmitter signal.  
   
   
       20 . The electromagnetic field generation system according to  claim 19 , wherein the control module is operable to supply the controller with the clock signal.  
   
   
       21 . The electromagnetic field generation system according to  claim 19 , wherein the control module is operable to supply a data gathering module with the clock signal.  
   
   
       22 . The electromagnetic field generation system according to  claim 21 , wherein the data gathering module is operable to time-stamp gathered data with a time-stamp generated from the clock signal.  
   
   
       23 . The electromagnetic field generation system according to  claim 13 , wherein the control module is operably coupled to the controller of the submersible electromagnetic field generator using a fibre optic cable.  
   
   
       24 . A method of generating an electromagnetic (EM) field for seafloor electromagnetic surveying, comprising: 
 providing an AC signal;    generating a DC signal on first and second DC outputs from the AC signal;    generating a waveform driving signal by switching the DC signal using a plurality of switches protected by surge protection circuitry, wherein the surge protection circuitry comprises a set of transient suppression circuits connected in parallel across the first and second DC outputs, each transient suppression circuit providing suppression tuned to a particular frequency or frequency range, thereby protecting the switches against induced back EMF generated when the switches are operated; and    driving an electric dipole with the waveform driving signal to generate the EM field.    
   
   
       25 . The method of  claim 24 , further comprising generating at least one additional DC signal so that there is a plurality of DC signals, and generating the waveform driving signal by selectively switching the plurality of DC signals.  
   
   
       26 . The method of  claim 24 , wherein the switches are semiconductor switches.  
   
   
       27 . The method of  claim 26 , wherein each semiconductor switch has a transient voltage suppressor connected between its source and gate.  
   
   
       28 . The method of  claim 26 , wherein the semiconductor switches are MOSFETs or IGBTs.  
   
   
       29 . The method of  claim 24 , wherein the surge protection circuitry comprises one or more diodes.  
   
   
       30 . The method of  claim 29 , wherein each of the switches is connected in parallel with a respective one of the diodes.  
   
   
       31 . The method of  claim 24 , wherein the transient suppression circuits have capacitors, diodes and/or resistors with values selected of their respective particular frequency or frequency range.  
   
   
       32 . The method of  claim 24 , comprising synchronising generation of the waveform driving signal produced at a submersible electromagnetic field generator with a clock signal provided from topside.  
   
   
       33 . A method for obtaining hydrocarbon from an area that contains a subterranean hydrocarbon reservoir, comprising: 
 providing survey data from an electromagnetic survey of the area performed using a submersible electromagnetic (EM) field generator comprising: an AC to DC converter operable to generate first and second DC outputs from an AC input; a switching module operable to generate a waveform driving signal by selectively switching the DC output by means of a plurality of switches; surge protection circuitry comprising a set of transient suppression circuits connected in parallel across the first and second DC outputs, each transient suppression circuit providing suppression tuned to a particular frequency or frequency range, thereby to provide protection for the switches against induced back EMF generated when the switches are operated; and an antenna operable to generate an EM field in response to the waveform driving signal;    analyzing the survey data to identify the subterranean hydrocarbon reservoir;    penetrating the subterranean hydrocarbon reservoir with a hydrocarbon-producing well; and    extracting hydrocarbon from the subterranean hydrocarbon reservoir using the hydrocarbon-producing well.    
   
   
       34 . A results data set representing an area that is thought or is known to contain a subterranean hydrocarbon reservoir, the results data set obtained by: 
 performing an electromagnetic survey of the area using a submersible electromagnetic (EM) field generator comprising: an AC to DC converter operable to generate first and second DC outputs from an AC input; a switching module operable to generate a waveform driving signal by selectively switching the DC output by means of a plurality of switches; surge protection circuitry comprising a set of transient suppression circuits connected in parallel across the first and second DC outputs, each transient suppression circuit providing suppression tuned to a particular frequency or frequency range, thereby to provide protection for the switches against induced back EMF generated when the switches are operated; and an antenna operable to generate an EM field in response to the waveform driving signal; and    generating the results data set based on data obtained during the survey.    
   
   
       36 . A computer readable storage medium having a results data set according to claim  35  recorded thereon.