The GEM-3 broadband electromagnetic induction (EMI) sensor has demonstrated its potential for discriminating between unexploded ordnance (UXO) and clutter. It has been operating mainly as a frequency-domain (FD) sensor, but the sensor and the accompanying electronics essentially operate in time-domain (TD). Combined TD-FD data from a single EMI sensor would provide challenging and exciting opportunities for new data interpretation schemes in support of UXO detection and discrimination.

The objective of this project is to develop and evaulate a new operating software for the GEM-3 that will incorporate both the frequency- and time-domain modes.

Various GEM-3 sensor configurations include the hand-held 40 cm (left) and 64 cm (center), and cart-mounted 96 cm versions. The 64 cm coil was used for the dual-mode data presented in this project.

The GEM-3 has two sensing channels, a receiver (RX) coil at the center and a small reference coil between two transmitter (TX) coils. The RX coil produces the target signal, while the reference coil monitors the TX primary field. The unit time of operation, called “base period”, is commonly 1/30 second and designed to minimize powerline noise. In a 50 hertz (Hz)-power country, the base period is typically 1/25 second. Both coil outputs are digitized at an Analog-to-Digital Converter (ADC) rate of 96 kilohertz in a 24-bit resolution. The digitized time series has a length of 3,200 over a base period of 1/30 second (i.e., 96,000/30). In its simplest form, the GEM-3 may work in FD for one base period and in TD for another base period. In this way, the sensor will collect both the FD and TD data at a sampling rate of 15 Hz, which should be sufficient for most UXO surveys. In the FD mode, the RX and reference channel time series are subjected to sine and cosine convolutions to obtain the inphase and quadrature responses at each transmitted frequency. A complex ratio of the two responses is expressed in a pulse per minute (ppm) unit for each frequency, the raw data output of the sensor. The current operating software (Windows-based WinGEM) automatically converts the ppm data into apparent conductivity at each frequency and apparent susceptibility at the lowest frequency employed.

Application of the TD-FD sensor is expected to improve the UXO detection and discrimination capabilities of the GEM-3 EMI sensor, particularly for deep targets.