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Subsurface Survey Methods.

subsurface

Our team offers several electromagnetic instrumentation solutions to ensure the most cost effect approach to data collection is performed. Our sensor platforms range from handheld to vehicle mounted. They are as follows:

The Geophex GEM-2 offers features that meet or exceed those offered by manufacturers of similar geophysical sensors. It offers a range of programmable frequencies from 300Hz up to 96kHz, enabling selection of optimum frequencies for target detection, target identification or vertical profiling. It can operate with a single frequency or multiple, combined frequencies. The 1.8m long sensor weighs just over 4kg, making it relatively easy to perform manual surveys in rough terrain or on difficult surfaces. Sensor power is provided by a long-life rechargeable battery that can be field-swapped for near-uninterrupted surveying. The electronics console includes both a wireless Bluetooth connection to the standard handheld data logger and provision for a RS-232 serial connection to a standard laptop computer. The console also includes provision for connecting a standard serial output GPS receiver directly to the GEM. The included WinGEM2 software provides for instrument configuration, acquisition of inphase and quadrature data at each selected frequency, acquisition and interpolation of GPS positioning data (or dead-reckoning positioning data if GPS is not available) and real-time display of EM data in a variety of graphical or tabular formats. Data are simultaneously displayed and stored on SD card for subsequent retrieval and further analysis. The WinGEM2 software also allows acquisition of both active and passive frequency spectra, a feature that can be very helpful in selection of appropriate frequencies to both avoid local noise sources and optimize response.

GEM-2 Technical Specifications

Programmable Operation
Bandwidth : 300 Hz to 96 kHz
Frequency Domain : Single frequency or Multiple Frequencies
Sampling Rate : Selectable 30 Hz or 25 Hz
Ski : Fiberglass foam 2 kg, L 183 cm, W 12.5 cm
Coil Configuration : Coplanar
Maximum TX Moment : 3 Amp m2 at 330 Hz
Rechargeable Battery: 12 VDC
Output : Inphase and Quadrature in PPM at each frequency, Apparent conductivity and Magnetic susceptibility, Powerline amplitude

Realtime GPS input for navigation
Remote Operation Options
User Interface and Data Logger : Socket PDA
Communication : Bluetooth with the PDA and Computer or RS232

 
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GEM-2 Survey Data Examples

Forgotten Landfill : While historical photographs showed burial activities at this site, there were no records showing the real extent of the buried waste. GEM-2 was used to map all burial trenches and to prioritize locations for remediation.

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Brine Contamination at an Oil Field : The U.S. Geological Survey (USGS) conducted a GEM-2 survey to characterize shallow brine contamination at an oil field in Oklahoma. Data were collected at five frequencies from 330 Hz to 47,010 Hz . The figures below show the apparent conductivity map at 14 kHz, which define the brine concentration in 3D.

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A conductivity-depth section defines the brine concentration at depths of 2–3 m, which was later confirmed by drilling.

Environmental Site Characterization : A GEM-2 survey was performed for the site characterization of a former industrial plant in Binghamton, NY. The objective was to identify and delineate subsurface features associated with past site operation. The data, shown above, located buried foundations, utility pipes, fill areas, concrete pads, contaminants, and other buried objects associated with the former plant.

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The GEM-3 Array can be towed or mounted in front of a vehicle. This new Electromagnetic Induction (EMI) array is built on a flat one-inch thick panel and, thus, light in weight and easy for packaging and shipping. The standard GEM-3 has seven receiver channels, provides a 2.4 meter swath, and produces high spatial density at a high survey speed. The system may be used for detecting unexploded ordnance (UXO), finding buried utilities, and mapping shallow geology. With proper packaging, the array can also be used for underwater investigations. It may also be used for detecting and characterizing landmines and IEDs in hostile environments under suitable concepts of operation.

The system provides 2-D data display that shows ground images in real time, including flagging potential targets on the screen. The unit comes with a GPS unit at decimeter accuracy. The system can merge data from other sensors and devices on the same platform using standard serial communication lines such as Ethernet, RS232 and RS485. The GEM-3 Array consists of a large transmitter coil along its perimeter, a bucking transmitter and seven equally-spaced receiver coils along its center line. The array has the following advantages:

  • Maximum possible transmitter moment for a given array size
  • Broadband operation, typically 10 frequencies between 90Hz-96kHz
  • Seven equally-spaced receiver channels operating simultaneously
  • High-speed survey; high data density
  • Easy shipping owing to its flat construction
Geophexhummer
The GEM-5 array is a vehicle-mounted or vehicle-towed gradiometer sensor for rapid ground coverage. It has a transmitter in the middle straddled by two identical receiver coils below and above. The difference between the two receiver coils constitutes the signal. The gradiometer sensor has the following advantages:

  • Ability to work as an array in which all sensors can operate simultaneously
  • Immunity to noise induced by sensor motion
  • Immunity to ambient environmental EM noise, and
  • Increased signal-to-noise ratio (SNR) owing to the noise immunities

Technical Specifications

Programmable Operation
Swath : 210cm (7 ft)
Dimensions (variable) : 30cm x 50m x 210cm
Coil Configuration : Coaxial
Transmitter Current : 20 amp RMS max
Power Supply : 12 VDC
Frequency Range : 90 Hz to 96 kHz
Number of Frequencies : 10 – 12
Sampling Rate : 30 Hz
Output : Inphase and quad in PPM, Apparent Conductivity, Magnetic Susceptibility, Powerline Amplitude

Realtime RPS input for navigation
Remote operation options : Vehicle Towed

Data Examples:

An example Q-sum data obtained by painting the screen over the Geophex UXO Test Site, a 10 m x 10 m area, indicated by the white square boundaries. The ground truth shown on the right shows a total of 20 seeded targets of various metal pipes and one magnetic rock (red circle to the left). The seven parallel lines indicate the ground track of the GEM- 5 array having seven receiver channels; the swath is about 2-m wide.

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Example data obtained along streets in Raleigh, NC, superimposed upon an aerial photograph of the area. The survey was undertaken to detect all utility lines (water, sewer, power line, telephone and fiber optic cables) buried under the road. Faint black lines indicate the ground track of the array.

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