Geophysical electromagnetic (“EM”) prospecting techniques can be effective in determining the electrical conductivity of soils, rocks, and other bodies at and under the earth's surface.
Geophysical EM prospecting can be carried out using surface based equipment and airborne equipment. Airborne methods in which equipment is transported by aircraft such as helicopter, airplane or airship may be useful for large area surveys. For airborne electromagnetic (“AEM”) systems, survey data may be acquired while an airplane or helicopter flies at a nearly constant speed along nearly-parallel and close to equally-spaced lines at an approximately constant height above ground. In some applications, geophysical EM prospecting of a seabed may be carried out using equipment located under the surface of a body of water.
Some geophysical surveying methods are active in that the equipment is used to transmit a signal to a targeted area, and then measure a response to the transmitted signal. Other geophysical surveying methods are passive in that signals produced from a target area are measured without first transmitting a signal to the target area.
An example of a passive geophysical EM prospecting method is Audio Frequency Magnetic (“AFMAG”) surveying in which the EM fields resulting from naturally occurring primary signal sources such as lightning discharges are measured. These EM fields propagate around the earth as plane waves guided by the ionosphere and earth's surface. Lightning activity occurring remote from the measurement point can produce signals with a nearly flat spectral density at frequencies between, for example, 8 Hz and 500 Hz, varying with geographical location, time of the day, seasons and weather conditions. An example of a passive AFMAG geophysical EM prospecting method is shown in U.S. Pat. No. 6,876,202.
Examples of active geophysical EM prospecting methods include methods in which a transmitter is used to illuminate a target area with a primary field and a receiver is used to measure the secondary field generated by the target area. Such systems are often frequency domain or time domain systems. In at least some frequency-domain electromagnetic (“FDEM”) systems, a transmitter coil continuously transmits an electromagnetic signal at fixed multiple frequencies, while the receiver coil measures the secondary field signals continuously over time.
In at least some time-domain electromagnetic (“TDEM”) systems, a pulse of current is applied to a transmitter coil during an on-period and switched off during the off-period, typically at a repetition rate equal to an odd multiple of half of the local power line frequency. A response signal is measured at a receiver as a function of time. The response signal amplitude decay during the off-period, combined with modeling of the conductivity and geometry of geological bodies in the ground, can be utilized to yield the conductivity contour maps. An example of a TDEM system is shown in U.S. Pat. No. 7,157,91.