Of all the many geophysical exploration techniques, the electromagnetic (EM) method provides significant advantages for shallow geophysical exploration. The GEM-2 is a handheld, digital, multi-frequency broadband electromagnetic sensor weighing only 3.6 kg and is ready to be deployed by drones and UAVs. It operates in a frequency range of about 30 Hz to 93 kHz and can transmit an arbitrary waveform containing multiple frequencies. The unit is capable of transmitting and receiving any digitally-synthesized waveform by means of the pulse-width modulation technique.

The built-in operating software allows a surveyor to cover about one acre per hour at line spacings of 2 metres. Along a survey line, the data rate is about 5 per metre, resulting in about 20,000 data points per acre per hour. Such portability, survey speed, and high data density are important requirements for geophysical surveys at environmental sites.

Depth of exploration for a given earth medium is determined by the operating frequency. Therefore, measuring the earth response at multiple frequencies is equivalent to measuring the earth response from multiple depths. Hence, such data can be used to image a 3D distribution of subsurface objects. Results from several environmental sites indicate that multi-frequency data from the GEM-2 is far superior to data from single-frequency sensors in characterizing buried, metallic and non-metallic targets.

The GEM-2 offers significant advantages over geometrical single-frequency sensors by providing depth-dependent information, superior target discrimination, and better performance in challenging soil conditions. While single-frequency sensors are generally limited to mapping horizontal surface conductivity, multi-frequency systems can be used to invert data to model vertical electrical conductivity changes.

For depth-dependent information (Vertical Sounding) a single-frequency sensor using different coil separations, also known as geometrical sounding, only provides an “apparent conductivity” value that is a weighted average of all material within its sensitive volume. It cannot distinguish whether a high reading comes from shallow or deep soil. However, because electromagnetic penetration depth is frequency-dependent (low frequencies penetrate deeper, high frequencies are shallower), multi-frequency sensors like the GEM-2 can step through different frequencies allowing the user to invert this data to produce a vertical profile of electrical conductivity to determine if a target is shallow or deeply buried.

Different types of metals (ferrous, non-ferrous) respond best to different frequencies. Multi-frequency detectors like the GEM-2 provide superior discrimination because they analyze how an object responds across a range of frequencies to identify its composition, significantly reducing false positives from trash. They are also capable of detecting small, valuable targets located next to large pieces of iron trash—which single-frequency sensors often miss.

Ultimately, the GEM-2 is the “Swiss army knife” for geophysical applications and can be used in a variety of ways for effective mapping of permafrost boundaries, landfills, contaminant plumes, archaeology, forensics, precision agriculture, geology and geotechnical environments, as it provides a quick and efficient way to model the thickness of layers rather than just their presence.

Think the GEM-2 might be a fit for your needs? Whether you’re looking to acquire a system or prefer the flexibility of renting, we’d be pleased to assist. Contact our team to learn more.