Electromagnetic Surveying

A computer readable media comprising instructions when executed by a processor to perform the following:

• obtaining a set of electromagnetic field data obtained using at least one electromagnetic receiver and at least one electromagnetic source for a range of source-receiver separations;
• identifying a subset of the electromagnetic field data which comprises data obtained for source-receiver separations greater than a threshold offset;
• processing the subset of the electromagnetic field data for source-receiver separations greater than the threshold offset in accordance with a first technique to obtain information on the background strata configuration;
• and
• processing electromagnetic field data obtained for source-receiver separations less than the threshold offset in accordance with a second technique to obtain information on any subterranean resistive or conductive body within the background strata configuration.

A computer system comprising instructions when executed by a processor to perform the following:

• obtaining a set of electromagnetic field data obtained using at least one electromagnetic receiver and at least one electromagnetic source for a range of source-receiver separations;
• identifying a subset of the electromagnetic field data which comprises data obtained for source-receiver separations greater than a threshold offset;
• processing the subset of the electromagnetic field data for source-receiver separations greater than the threshold offset in accordance with a first technique to obtain information on the background strata configuration;
• and
• processing electromagnetic field data obtained for source-receiver separations less than the threshold offset in accordance with a second technique to obtain information on any subterranean resistive or conductive body within the background strata configuration.

A method of planning an electromagnetic survey of an area that is thought or known to contain a subterranean resistive or conductive body within a background strata configuration, comprising:

• creating a model of the area to be surveyed including a rock formation comprising the background strata configuration containing a postulated resistive or conductive body, and a body of water above the rock formation;
• setting values for water depth, depth of the postulated resistive or conductive body, and resistivity structure of the rock formation;
• performing a simulation of an electromagnetic survey in the model of the survey area by calculating horizontal electric and magnetic field data obtained by at least one simulated electromagnetic receiver detecting signals from at least one simulated electromagnetic source for a range of source-receiver separations;
• identifying a subset of the electromagnetic field data which comprises data obtained for source-receiver separations greater than a threshold offset;
• processing the subset of the electromagnetic field data for source-receiver separations greater than the threshold offset in accordance with a first technique to obtain information on the background strata configuration;
• and
• processing electromagnetic field data obtained for source-receiver separations less than the threshold offset in accordance with a second technique to obtain information on the postulated resistive or conductive body.

A computer readable media comprising instructions when executed by a processor to perform the following:

• creating a model of the area to be surveyed including a rock formation comprising the background strata configuration containing a postulated resistive or conductive body, and a body of water above the rock formation;
• setting values for water depth, depth of the postulated resistive or conductive body, and resistivity structure of the rock formation;
• performing a simulation of an electromagnetic survey in the model of the survey area by calculating horizontal electric and magnetic field data obtained by at least one simulated electromagnetic receiver detecting signals from at least one simulated electromagnetic source for a range of source-receiver separations;
• identifying a subset of the electromagnetic field data which comprises data obtained for source-receiver separations greater than a threshold offset;
• processing the subset of the electromagnetic field data for source-receiver separations greater than the threshold offset in accordance with a first technique to obtain information on the background strata configuration;
• and
• processing electromagnetic field data obtained for source-receiver separations less than the threshold offset in accordance with a second technique to obtain information on the postulated resistive or conductive body.

A computer system comprising instructions when executed by a processor to perform the following:

• creating a model of the area to be surveyed including a rock formation comprising the background strata configuration containing a postulated resistive or conductive body, and a body of water above the rock formation;
• setting values for water depth, depth of the postulated resistive or conductive body, and resistivity structure of the rock formation;
• performing a simulation of an electromagnetic survey in the model of the survey area by calculating horizontal electric and magnetic field data obtained by at least one simulated electromagnetic receiver detecting signals from at least one simulated electromagnetic source for a range of source-receiver separations;
• identifying a subset of the electromagnetic field data which comprises data obtained for source-receiver separations greater than a threshold offset;
• processing the subset of the electromagnetic field data for source-receiver separations greater than the threshold offset in accordance with a first technique to obtain information on the background strata configuration;
• and
• processing electromagnetic field data obtained for source-receiver separations less than the threshold offset in accordance with a second technique to obtain information on the postulated resistive or conductive body.

A method of performing an underwater electromagnetic survey of an area that is thought or known to contain a subterranean resistive or conductive body within a background strata configuration, comprising:

• providing at least one electromagnetic source and at least one electromagnetic receiver for transmission and detection of electromagnetic signals;
• obtaining a set of electromagnetic field data from the at least one electromagnetic receiver and at least one electromagnetic source for a range of source-receiver separations;
• identifying a subset of the electromagnetic field data which comprises data obtained for source-receiver separations greater than a threshold offset;
• processing the subset of the electromagnetic field data for source-receiver separations greater than the threshold offset in accordance with a first technique to obtain information on the background strata configuration;
• and
• processing electromagnetic field data obtained for source-receiver separations less than the threshold offset in accordance with a second technique to obtain information on any subterranean resistive or conductive body within the background strata configuration.

A method for obtaining hydrocarbon from an area that contains a subterranean hydrocarbon reservoir, comprising:

• providing at least one electromagnetic source and at least one electromagnetic receiver for transmission and detection of electromagnetic signals;
• obtaining a set of electromagnetic field data from the at least one electromagnetic receiver and at least one electromagnetic source for a range of source-receiver separations;
• identifying a subset of the electromagnetic field data which comprises data obtained for source-receiver separations greater than a threshold offset;
• processing the subset of the electromagnetic field data for source-receiver separations greater than the threshold offset in accordance with a first technique to obtain information on the background strata configuration;
• processing electromagnetic field data obtained for source-receiver separations less than the threshold offset in accordance with a second technique to obtain information on any hydrocarbon reservoir within the background strata configuration;
• identifying the hydrocarbon reservoir from the results of processing electromagnetic field data in accordance with the second technique;
• penetrating the identified hydrocarbon reservoir with a hydrocarbon-producing well;
• extracting the hydrocarbon from the reservoir using the well.

A volume of hydrocarbon obtained from an area that contains a subterranean hydrocarbon reservoir, the volume of hydrocarbon obtained by:

• providing at least one electromagnetic source and at least one electromagnetic receiver for transmission and detection of electromagnetic signals;
• obtaining a set of electromagnetic field data from the at least one electromagnetic receiver and at least one electromagnetic source for a range of source-receiver separations;
• identifying a subset of the electromagnetic field data which comprises data obtained for source-receiver separations greater than a threshold offset;
• processing the subset of the electromagnetic field data for source-receiver separations greater than the threshold offset in accordance with a first technique to obtain information on the background strata configuration;
• processing electromagnetic field data obtained for source-receiver separations less than the threshold offset in accordance with a second technique to obtain information on any hydrocarbon reservoir within the background strata configuration;
• identifying the hydrocarbon reservoir from the results of processing electromagnetic field data in accordance with the second technique;
• penetrating the identified hydrocarbon reservoir with a hydrocarbon-producing well;
• and
• extracting the hydrocarbon from the reservoir using the well to obtain the volume of hydrocarbon.