The most common horizontal loop EM (HLEM) system is the Apex MaxMin. In an HLEM survey, there are two coils or loops. One of these loops is the transmitter, which generates an alternating EM field (the primary field) in the ground beneath. If there is a conductor in the ground, then small circulating currents will flow in the conductor, and give rise to their own EM field (the secondary field) at the surface.
The other loop is a receiver. In the MaxMin the receiver loop is wound around two magnetic cores, which form the ‘horns’ on the receiver. Normally the primary field is much stronger than the secondary field. In order to detect the secondary field, a small part of the primary field is sent from the transmitter via cable to the receiver, and is used to cancel the primary field at the receiver, leaving only the secondary field to be detected. The receiver measures two quantities, the in-phase component and the quadrature component of the secondary field, expressed as a percentage of the primary field at the receiver. There is a phase shift or time delay in generating the secondary field of the conductor. The part of the secondary field that is not delayed is the in-phase component, and the part that is delayed is the quadrature component. Anomalies from good conductors have large in-phase and small quadrature components, while weaker conductors have low in-phase and high quadrature components.
The MaxMin offers a range of frequencies which depends on the model. Typical frequencies increase from 110 hertz (Hz) by factrs of two to 3550 Hz or even higher. In a mineral exploration survey, both lowest and highest frequencies are read, but not all intermediate frequencies are necessarily used.
Interpretation of HLEM data depends on the assumption that the transmitter and receiver coils are coplanar (that is, in the same plane) at a constant separation. In areas of topography, maintaining this assumption is not trivial. The MaxMin system allows the operator to measure the slope to the next station before leaving the current one. If this measurement is done at each station, the computer in the MaxMin receiver will integrate the slope readings to determine the elevation difference between the Tx and Rx coils.