ELF EM Space Domain Gradiometry


Trinity Research Labs have developed and tested in the field a novel ELF (Extremely Low Frequency) EM Gradiometer (based on ELFISTM proprietary technology) which is readily available for commercial use. It can be deployed from a small helicopter or towed behind a 4WD vehicle of medium size in reasonably flat terrain.

Major Applications

  • Geophysical reconnaissance, such as the search for nickel bearing geological structures
  • The search for underground water reserves
  • The detection of shipwrecks and shallow water conducting artifacts ( cannons, etc… )
  • UXO ( Unexploded Ordnance ) detection

The instrument's mobile transmitter-receiver section consists of two concentric transmitter coils mounted together on a solid non-conducting and non-magnetic frame, with two pairs of induction coils connected in a gradiometric configuration and separated spatially by a 0.5 m baseline. The latter coils are wound on high magnetic permeability cores with about 1 kHz operational corner frequency. The receiver coils are firmly mounted inside the cylindrical space, occupied by the transmitter loops, in such a way that reduces firstly, to a minimum, the effect of any possible mechanical motion of the receiver coils relative to the transmitter coils, and, secondly, reduces to a minimum the direct transmitter-receiver EM coupling. The system enables the detection of five AC gradients, namely - Bxz, Byz, Bxx, Byy and Bzz dependent on the orientation of the transmitter-receiver section with respect to a coordinate reference frame.

Also, the ELF EM Gradiometer is the first commercial surveying system that is immune to the primary EM field, which, normally, is strongly coupled to a receiver in the standard EM surveying systems. At the heart of the gradiometric surveying system is ELFISTM -  the first of its kind, a phase only sensitive interferometer-on-board (or phase bridge) that combines the advantages of microwave and RF Mahn-Zehnder type of interferometers for ultra-sensitive phase measurements, with the use of active compensating components, such as digitally controlled phase shifters, operating in the ELF frequency range. ELFISTM operates in a closed loop mode. Both the digital amplitude control and digital phase control facilities, embedded into the DSP architecture, are integral parts of two feedback loops. The first one locks the interferometer amplitude imbalance to a near zero value. The second one locks the reference signal phase to the phase of the secondary EM field, coming through the measurement arm of the interferometer. The reference arm and the measurement arm of ELFISTM are both balanced in such a way that the frequency fluctuations in the carrier, which result in the direct frequency-to-phase-noise conversion, cancel each other at the output of the interferometer. The real time data recorded at the end of the signal processing chain are direct phase variations between the primary and the secondary EM fields.