Electromagnetic surveying for resistive or conductive bodies

A computer program product bearing machine readable instructions for implementing a method of analyzing results from an electromagnetic survey of an area that is thought or known to contain a subterranean resistive or conductive body, comprising:

• providing horizontal electric or magnetic field data obtained by at least one receiver from at least one horizontal electric dipole transmitter;
• determining a horizontal gradient in a first component of the electric or magnetic field data along a first direction;
• determining a horizontal gradient in a second component of the electric or magnetic field data along a second direction;
• and
• combining the horizontal gradients along the first and second directions to generate combined response data.

A computer apparatus loaded with machine readable instructions for implementing the method of analyzing results from an electromagnetic survey of an area that is thought or known to contain a subterranean resistive or conductive body, comprising:

• providing horizontal electric or magnetic field data obtained by at least one receiver from at least one horizontal electric dipole transmitter;
• determining a horizontal gradient in a first component of the electric or magnetic field data along a first direction;
• determining a horizontal gradient in a second component of the electric or magnetic field data along a second direction;
• and
• combining the horizontal gradients along the first and second directions to generate combined response data.

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

• creating a model of the area to be surveyed including a rock formation 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;
• and
• performing a simulation of an electromagnetic survey in the model of the survey area by calculating horizontal electric or magnetic field data obtained by at least one simulated receiver detecting signals from at least one simulated horizontal electric dipole transmitter;
• determining a horizontal gradient in a first component of the electric or magnetic field data along a first direction;
• determining a horizontal gradient in a second component of the electric or magnetic field data along a second direction;
• and
• combining the horizontal gradients along the first and second directions to generate combined response data.

A computer program product bearing machine readable instructions for implementing a method of planning an electromagnetic survey of an area that is thought or known to contain a subterranean resistive or conductive body, comprising:

• creating a model of the area to be surveyed including a rock formation 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;
• and
• performing a simulation of an electromagnetic survey in the model of the survey area by calculating horizontal electric or magnetic field data obtained by at least one simulated receiver detecting signals from at least one simulated horizontal electric dipole transmitter;
• determining a horizontal gradient in a first component of the electric or magnetic field data along a first direction;
• determining a horizontal gradient in a second component of the electric or magnetic field data along a second direction;
• and
• combining the horizontal gradients along the first and second directions to generate combined response data.

A computer apparatus loaded with machine readable instructions for implementing a method of planning an electromagnetic survey of an area that is thought or known to contain a subterranean resistive or conductive body, comprising:

• creating a model of the area to be surveyed including a rock formation 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;
• and
• performing a simulation of an electromagnetic survey in the model of the survey area by calculating horizontal electric or magnetic field data obtained by at least one simulated receiver detecting signals from at least one simulated horizontal electric dipole transmitter;
• determining a horizontal gradient in a first component of the electric or magnetic field data along a first direction;
• determining a horizontal gradient in a second component of the electric or magnetic field data along a second direction;
• and
• combining the horizontal gradients along the first and second directions to generate combined response data.

An electromagnetic survey method applied to a survey area that is thought or known to contain a subterranean resistive or conductive body, comprising:

• providing at least one transmitter for transmission of electromagnetic signals;
• providing at least one receiver for detection of electromagnetic signals comprising two pairs of electric or magnetic dipole detectors, a first pair of which are separated along a first direction and a second pair of which are separated along a second direction, the first and second directions being horizontal when the receiver is in normal use;
• and
• obtaining electromagnetic field data by detection at a plurality of different locations over the survey area.

An electromagnetic survey method applied to a survey area that is thought or known to contain a subterranean resistive or conductive body, comprising:

• providing at least one source comprising two pairs of electric or magnetic dipole transmitters a first pair of which are separated along a first direction and a second pair of which are separated along a second direction, the first and second directions being horizontal when the source is in normal use
• providing at least one receiver for respective transmission and detection of electromagnetic signals;
• and
• obtaining electromagnetic field data by detection at a plurality of different locations over the survey area.