The present invention relates generally to a method of geophysical exploration and more particularly to a novel method of magnetotelluric exploration. In one embodiment, a series of continuous electric dipole measurements of one component of the earth's electric field are obtained along a zigzag line. Concurrently, at least two components of the earth's magnetic field are obtained in the vicinity of the zigzag line. Each electric dipole measurement of the earth's electric field comprise measures of the potential difference between electrodes spaced along the zigzag line in which the included angle between adjacent pairs of electric dipole measurements is less than 180.degree.. Preferably, the included angle between adjacent electric dipole measurements is between 90.degree. and 150.degree..

An electromagnetic survey method for geophysical exploration, in which the variations in the earth's magnetic field are measured in two, non-parallel directions at one point in the survey area. Simultaneously, the variations in the earth's electrical field parallel to the survey line are measured at a number of points along the survey line. These measured variations are transformed to the frequency domain, and then the horizontal component of the magnetic field orthogonal to the direction of the measured electrical field is calculated. The impedance at each measurement point on the survey line is calculated as a function of frequency, and weighted averages of the impedance for predetermined frequencies using a zero phase length weight function corresponding to a low pass filter applied to the electric field are used to calculate the subsurface conductivity distribution.

A method of magnetotelluric exploration is described, wherein measures of the earth's electric and magnetic fields are obtained at a plurality of sensing locations comprising an areal array of sensing locations. Specifically, first and second components of the earth's electric field are measured at a plurality of sensing locations within an areal array of sensing locations whereby each of the components of the earth's electric field are spatially continuous. Additionally, first and second components of the earth's magnetic field are measured simultaneously at at least one location within the area covered by the areal arrays of sensing locations.

A system and method are disclosed for detecting an event-related signal among noise, where the system includes memory for storing measurement values and may include a processor for generating residual values associated with the measurement values, and for detecting the event-related signal using the residual values.

The magnetotelluric system for seafloor petroleum exploration comprises a first waterproof pressure case containing a processor, AC-coupled magnetic field post-amplifiers and electric field amplifiers (the "logger unit"), a second waterproof pressure case containing an acoustic navigation/release system, four silver-silver chloride (Ag-AgCl) electrodes mounted on booms and at least two magnetic induction coil sensors. These elements are mounted together on a plastic and aluminum frame along with flotation devices and an anchor for deployment to the seafloor. The acoustic navigation/release system serves to locate the system by responding to acoustic pings generated by a ship-board unit and receives a release command which initiates detachment from the anchor so that the buoyant package floats to the surface for recovery. The electrodes used to detect the electric field are configured as grounded dipole antennas. Booms by which the electrodes are mounted onto frame are positioned in an "X" configuration to create two orthogonal dipoles, which are used to measure the complete vector electric field. The magnetic field sensors are multi-turn Mu-metal core coils which detect within the frequency range typically used for land-based MT surveys. The magnetic field coils are encased in waterproof pressure cases and are connected to the logger package by high pressure waterproof cables. The logger unit includes the amplifiers for amplifying the signals received from the various sensors, which signals are then provided to the processor which controls timing, logging, storing and power switching operations. Temporary and mass storage is provided within and/or peripheral to the processor.