On the Search – New innovations in geophysical mining exploration

Receiver with network cables

Mining exploration can be costly and time consuming.  In recent years, financial resources and investments into mining have become more difficult to obtain.  In times of lower investor support of exploration it is more important than ever to increase productivity and lower costs.  Detailed characterization of a mining prospect is expensive and time consuming.  Anything to increase efficiency of site characterization and prospection will be a boon to the industry.  In response, Geometrics, Inc. of San Jose, California recently developed a unique product for geophysical mining exploration that enhances data quality and resolution, as well as controlling costs.

The Geode EM3D is an innovative extension of the geophysical exploration technique known as Controlled Source Audio Magnetotellurics (CSAMT).  This system was specifically designed with mining exploration in mind, but the technology is expected to find a place in shallow oil and gas, as well as groundwater exploration.  The distributed 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.  The electric and magnetic fields generated by those currents are measured simultaneously.  From the measurement of the currents flowing in the ground one can produce a map of the underground geologic structure and use that to better interpret the most likely spot for productive drilling and excavating.

Each six-channel receiver acts as a node on the network for synchronization and data communication. The largest system consists of 40 receiver nodes for 240 simultaneous electric and magnetic field measurements. 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.

In the past a traditional survey with only one station measured at a time, the transmitter needs to be run multiple times. In previous CSAMT surveys, to get data from 50 sites (50 stations) the transmitter needs 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 the new Geode EM3D.  All the stations are set up at one time then 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.

Geode EM3D Acquisition Unit

A notable feature of the new system is the ability to scale the equipment to the geophysical requirements of the survey, the exploration budget available, and the complexity of the geologic environment. For example, a system could consist of a single six-channel receiver, two magnetic coils, a ruggedized laptop controller, and a transmitter for a basic six-channel instrument or it could consist of up to 40 six-channel receivers, 20 to 40 magnetic sensors, a geophysical instrument controller, GPS, and transmitter for a large 240-channel network of CSAMT receivers.

A large network of measurement stations allows improved efficiency in laying out the survey to match the requirements of the application.  For example, in simple layered environments many electric channels could use the results of 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 basically the same signal it is more likely to be actual signal and not instrument or localized noise.

The Geode EM3D offers a series of features for easy setup, acquisition of good data, and for making quality control a priority.  For instance, the network allows for automatic and simultaneous measurement and calculation of contact resistance for every electric dipole.  Ground contact resistance measurements mean more accurate calibration of the acquired signal.  Better quality control and calibration of the measured signal means better results.  To that end the Geode EM3D measures total AC and DC noise in the field for better calibration and quality control.

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 assist the exploration team decide where to sample with other methods and ultimately where to dig, and with continuing innovations, mining concerns can continue to compete even in difficult economic times.

Geometrics is a division of OYO Corporation and headquartered in San Jose, CA. The company is a world-leading designer and manufacturer of land, marine and airborne geophysical hardware, sensors and software, covering seismic, magnetic, and electromagnetic technologies. Founded in 1969, Geometrics began operations developing innovative magnetometers and now has representatives worldwide in over 50 countries. With over 40 years of field experience and extensive knowledge, Geometrics’ staff of engineers and technicians has worked with universities, research institutions, government agencies and exploration companies to provide solutions to all kinds of geophysical and geotechnical exploration needs. For more information visit http://www.geometrics.com.

 Article By Doug Groom, Director of GeoElectrical Products, Geometrics, Inc.

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