Apparatus and method for electromagnetic geophysical exploration

Claim 1

An electromagnetic geophysical exploration system including means for generating a varying current to a primary radiator for providing a primary magnetic field for coupling with the earth, the frequency of said varying current being a function of time, apparatus for installation on a moving craft, said apparatus including means for detecting and measuring the intensity of a time-varying secondary magnetic field responsive to said varying primary magnetic field, an optically pumped, optically monitored nuclear resonance magnetometer, said magnetometer being coupled to said craft such that, in use, the positional relationship between said primary magnetic field generating means and said magnetometer is known at all times, variable magnetic field generating means including Helmholtz coil means to provide an accurately controllable magnetic vector in a confined space interior of said Helmholtz coil means, said controllable magnetic vector being substantially adjustable in any direction within said confined space, said magnetometer being positioned within said confined space, means coupled with said Helmholtz coils and responsive to said current in said radiator for producing a cancelling magnetic field vector within said confined space, which cancelling vector is equal and opposite to said primary magnetic field, said means for producing said cancelling magnetic field vector includes a plurality of variable circuit elements, each said variable circuit element having a variable voltage output which is in phase with said current in said radiator, and means for measuring said time-varying secondary magnetic field, said measuring means including an FM detector being connected to said magnetometer output.

Claim 7

A method of measuring secondary magnetic fields due to geophysical structures in the presence of a primary magnetic field created by current in a primary radiator comprising: causing the frequency of said current to vary as a function of time; locating an optically pumped, optically monitored nuclear resonance magnetometer within a confined space between the windings of a Helmholtz coil positionally fixed in a craft with respect to the position of said primary radiator positioning said craft in a location where said secondary fields are non-existent; generating in said Helmholtz coil a magnetic field equal and opposite to a component of said primary field in said confined space by providing a driving signal to said coil, which driving signal is derived from a signal having an in-phase relation to said current in said primary radiator such that substantially all response of said magnetometer to said primary magnetic field component is avoided; FM detecting the output of said magnetometer while operating said craft in regions having secondary magnetic fields.

Claim 8

A method of measuring secondary magnetic fields due to geophysical structures in the presence of a primary magnetic field created by current in a primary radiator comprising: causing the frequency of said current to vary as a function of time; locating an optically pumped, optically monitored nuclear resonance magnetometer within a confined space between the windings of a Helmholtz coil positionally fixed in a craft with respect to the position of said primary radiator positioning said craft in a location where said secondary fields are non-existent; generating in said Helmholtz coil a magnetic field equal and opposite to a component of said primary field in said confined space by providing a driving signal to said coil, which driving signal is derived from a signal having an in-phase relation to said current in said primary radiator such that substantially all response of said magnetometer to said primary magnetic field component is avoided; FM detecting the output of said magnetometer while operating said craft in regions having secondary magnetic fields.