GEM‐2A: A programmable broadband helicopter‐towed electromagnetic sensor

We present a new helicopter-towed broadband electromagnetic sensor, GEM-2A, for mineral prospecting and geologic mapping. The sensor uses one set of transmitter and receiver coils for a multifrequency operation. For a given survey, the user initially specifies a set of operational frequencies in the current bandwidth of 90 Hz to 48 kHz. The transmitter coil then emanates a current waveform that contains all specified frequencies. The duration of this current waveform, called the base period, is typically 1/30 s (a submultiple of local powerline frequency) resulting in an overall data rate at 30 Hz. Receiver channels digitize the secondary field into a time series over a base period, which is then subjected to discrete sine and cosine transforms or convolutions at each transmitted frequency to produce the raw in-phase and quadrature data. Additional convolutions may be included for passively monitoring environmental noise, including powerline emissions. The entire operation, including the system upload/download and realtime monitoring and communication, is done in Microsoft Windows. The fact that the sensor contains only a single set of coils for the broadband operation provides several unique advantages, some of which include (1) corelatable and coherent drift characteristics among frequencies, (2) spectral integrity among frequencies that may be useful for anomaly classification and, possibly, mineral discrimination, (3) tolerance to sferics, (4) tolerance to powerline noise, and (5) light tow body with minimal cockpit hardware suitable for a small helicopter. This paper presents the sensor construction, operation, and data examples.