The Geode EM3D (GEM3D) is an innovative extension of the geophysical exploration techniques known as Controlled Source Audio Magnetotellurics (CSAMT) and audio magnetotellurics (AMT). The distributed GEM3D system is a network of synchronized receivers providing high-data density surveys to characterize mining prospects. The CSAMT technique uses a transmitter to generate currents in the ground that are measured by the CSAMT receivers at the survey site. AMT uses naturally-occurring currents flowing in the ground. The electric and magnetic fields generated by those currents are measured simultaneously. From the measurement one can produce a map of the underground geologic structure and use that to better interpret the most likely spot for drilling and excavating. Each six-channel receiver acts as a node on the network for synchronization and data communication. For CSAMT, a powerful geophysical transmitter provides the source of the measured signals. A typical CSAMT transmitter sequence takes from 30 to 60 minutes depending on the requirements of the survey. A traditional CSAMT survey only measured one station at a time, so the transmitter needed to be run multiple times. To get data from 50 sites (50 stations) the transmitter would have to be run 50 times. That means the survey would require from 25 to 50 hours of transmitter time, not including the time for setting up and moving the gear 50 times. The survey time is greatly reduced using a network of stations all acquiring data at the same time. After all stations are set up only one transmitter sequence is required. That means all 50 stations could be measured with a single one-hour transmitter run instead of requiring up to 50 hours to do the same acquisition one station at a time.
A system could consist of a single six-channel receiver, two magnetic coils, a ruggedized laptop controller, and a transmitter; or it could consist of up to 40 six-channel receivers, 40 magnetic sensors, a geophysical instrument controller, GPS, and transmitter for a large 240-channel network of CSAMT receivers.
In simple layered environments many electric channels could use a single set of magnetic field measurements to calculate ground resistivity from up to 20 electric field measurements. In complex geology with lateral variations in structure, the survey may best be served by having a set of magnetic coils for every 10 electric field measurements. In other words, a distribution of measurement stations allows the user to “tune” the survey parameters to the targets of interest and the environment of the prospect. Another advantage of having multiple, simultaneous magnetic field measurement is the ability to do “local referencing” to help eliminate noise in the data processing. With two parallel coils making magnetic field measurements one can be used as a reference for the other. If both coils are seeing the same frequencies it is more likely to be actual signal and not instrument or localized noise.
In short, the new instrument offers an opportunity to expand the capabilities of exploration teams for enhanced geophysical characterization of prospective mine sites before a single borehole or excavation is started. Geophysical techniques are intended to enhance geologic and geochemical exploration, not replace them. The goal is for geophysics to provide a rapid, accurate method to help the exploration team decide where to sample with other methods and ultimately where to drill or dig. With continuing innovations like the Geode EM3D, mining enterprises can continue to compete even in difficult economic times.
By Doug Groom, director of GeoElectrical Products, Geometrics, Inc.