GMR ----Overview----Features----Specifications----Case Studies----FAQ
Surface NMR ( Magnetic Resonance Sounding and Tomography )

 

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	GMR featured in Envrionmental Engineering Geophysical Society (EEGS) publication FastTIMES

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Overview

GMR represents the state-of-the-art in surface NMR and magnetic resonance sounding (MRS) technology. The GMR system combines powerful multi-channel instrumentation with advanced software for precise and economical groundwater investigations. GMR technology enables direct and non-invasive detection of groundwater and characterization of aquifer properties without drilling expensive wells. Download a GMR product brochure.

GMR Measurements Provide

• Non-invasive direct detection of groundwater
• Spatial resolution of hydrogeologic parameters in 1D, 2D, or even 3D
• Quantitative determination of water content and porosity
• Estimation of mobile versus bound water content
• Estimation of relative or calibrated hydraulic conductivity and transmissivity
• Investigation to depths up to 150 meters

GMR Applications in Hydrology Include

• Groundwater exploration and well site selection
• Determination of water storage and specific yield
• "Virtual pump testing": estimation of relative or calibrated hydraulic conductivity and transmissivity
• Detection and imaging of preferential flow zones, including water-filled fractures, karst conduits, and paleochannels
• Parameterization of groundwater models at local to basin scales
• Integration of logging and surface-based NMR measurements

How GMR Works

GMR detects groundwater using the same physics as a medical MRI scanner, but GMR uses the Earth's magnetic field in place of the much stronger magnetic field generated inside an MRI magnet. In the geomagnetic field, hydrogen nuclei in groundwater will emit a measurable NMR signal when they are energized at a specific resonant frequency. This NMR signal provides information about the abundance of water and also the size of the pore spaces in which the water resides.

In a GMR survey, the NMR response of groundwater is probed using large wire loops (up to 200 meters on a side), which are laid out on the surface. Short current pulses are routed through one or more of the wire loops to energize the hydrogen nuclei in groundwater at their resonant frequency (1-3 kHz, depending on location). The surface NMR method is also commonly known as magnetic resonance sounding (MRS).

Example GMR coil layout, with large wire loops arranged across a groundwater investigation site.

Following the transmit pulse and a short delay, known as the dead-time, the surface loop(s) switch to receive mode and record the NMR signal generated by the energized groundwater.  Repeated measurements achieve varied depths of investigation by increasing or decreasing the amount of energy through the surface coils. Advanced signal processing techniques are used to cancel environmental and cultural noise. An advanced, high-resolution linear inversion is implemented to isolate the NMR signals arising from groundwater at different depth locations. Using these isolated NMR signals, the spatial distribution of water content can be directly and quantitatively derived from the signal amplitude. Estimates of key hydrogeologic properties can also be derived, based on the decay time spectrum of the signal.

Top government agencies and research institutes around the world are currently using GMR technology to advance groundwater science.

	Leibniz Institute for Applied Geophysics		Australian Commonwealth Scientific and Industrial Research Organization		Stanford University Geophysics		Rutgers University Earth & Enviornmental Sciences

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GMR Features

Multi-channel Design
At the core of the GMR's design are Vista Clara's patented multi-channel instrumentation, acquisition, and processing techniques**. The GMR system provides the most complete and robust multi-channel capabilities: the ability to transmit and receive on any surface coil or combination of surface coils. The patented mutli-channel approach provides key advantages for rapid surface NMR surveys, imaging in 2D or 3D, and noise cancellation, which is a necessity for measurements near power lines or other noise sources.

 

Advanced Noise Cancellation
GMR instrumentation and software includes powerful noise cancellation that leverages our proprietary multi-channel data acquisition design **. This critical advantage allows a GMR user to produce high quality surface NMR data and inversion results in locations where a single-channel instrument cannot produce useful data.

Graph of a real surface NMR signal collected using a 91m loop in central Nebraska in the time (top) and frequency (bottom) domain. Blue curves show the signal before noise cancellation. Red curves show the signal after GMR noise cancellation.

 

2D/3D Groundwater Imaging (Magnetic Resonance Tomography):
Vista Clara pioneered the development and use of 2D/3D surface NMR imaging using multiple surface coils. GMR's multi-channel data acquisition and software make 2D imaging fast and precise. Direct 2D NMR imaging is especially useful for detecting and imaging structures such as karst conduits, paleochannels, and other highly localized hydrological features. GMR's patented high-impedance ultralow-noise receivers‡ are designed to prevent mutual coupling, which is essential for accurate 2D or 3D surface NMR imaging.**

2D GMR image of a limestone aquifer near San Antonio Texas. A probable water-filled cave or fracture system is shown at the right of the image. The investigation also identified a limestone formation with lower permeability delineated at the left edge of this image.

Sophisticated Hydrological Analysis Software:
GMR includes the most advanced commercial software for 1D magnetic resonance soundings, inversions, and hydraulic parameter estimation. The GMR 1D inversion software package includes:

• Our patented low-SNR permeability indicator**, as well as other empirical formulas for estimating relative and calibrated hydraulic conductivity.
• Our proprietary all-linear 1D inversion code with 3D source modeling
• Multi-exponential analysis software, to estimate the distribution of water content versus T2* decay time. This feature is required to estimate the pore size distribution and to distinguish bound versus mobile water content.
• Estimation of T1 and T2 decay times from double-pulse and spin-echo measurements.

GMR advanced 1D inversion of data from Nebraska, USA, with low-SNR permeability indicator shown at far left.

Data from the same site processed via multi-exponential analysis illuminating the complex nature of the aquifer. Signals with short decay times(towards left of image) indicate water in small pores of low permeability silt and clay. Signals with long decay times (towards right of image) indicate water in productive aquifers, in this case sands, gravels and sandstones.

Short Instrument Dead-time:
Water bound in silts and clays or water in iron-rich formation (e.g., basalts, magnetite-rich sands) typically exhibits very short decay times (<20 ms). The uniquely short dead-time of the GMR instrument (4 ms preprocessed) makes it possible to detect and characterize these very short signals, which otherwise could not be observed. GMR thus provides detection of groundwater in the widest range of environments, enabling direct imaging of aquicludes and confining units as well as more accurate estimation of hydrogeologic properties.

Double-Pulse Measurements:
The GMR hardware and acquisition software employs sophisticated measurements in which two energizing pulse are transmitted in rapid succession through the surface coils. These double-pulse measurements allow T1/recovery and T2/spin-echo measurements which can yield more robust estimates of hydrogeologic properties and are required for surveys in environments with magnetic geology. The use of phase cycling supresses measurement artifacts leading to improved data quality.

NMR signals from groundwater in an aquifer with magnetic mineralogy. The FID signal (in black) from a single pulse measurement decays with a very short T2* rate (<50ms). Spin echo signals (color) measured at varied echo time delays exhibit a much longer decay time T2 (>200ms) revealing that the aquifer actually has high permeability. Phase cycling in the double-pulse sequence used to measure spin echoes supresses artifacts and improves data quality.

 

 

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GMR Specifications

GMR 540V DC/DC Converter

Performance Specs:

• Max. DC bus voltage: +/- 270V DC
• Capacitance: 0.054 F
• Charge current: +/- 1 A
• Power supply: two 12V batteries

Safety Features:

• Auto-shut off
• Analog voltmeters on panel.

Options:

• Two DC/DC converters can be used in parallel.

GMR 4000V Transmitter

Performance Specs:

• Max. transmit voltage: 4000V AC
• Max. transmit current: 600A, 25 A*s (software limited)
• Instrument dead time: 4ms
• Post-processed dead time: 4ms +1/(filter bandwidth)
• Four transmit/receive channels
• Receiver input noise: 500 pV/rt(Hz)
• 24-bit wideband simultaneous sampling.
• Pulse sequences for FID (single-pulse),T1 (90-90 pulse),and T2 (spin echo) acquisition with phase-cycling.

Safety Features:

• Auto-shut down in overcurrent state.

Options:

• Expandable to 8 or 12 channels

GMR 16uF/32uF Tuning Unit

Performance Specs:

• Max. transmit voltage: 4000V AC
• Max. transmit current: 600A.

Safety Features:

• Transmitter will not operate unless tuning cover is securely closed.
• Rapid discharge (time constant ~ 1s).

Options:

• Available with 16uF or 32uF capacitance.
• 32uF recommended for low earth latitudes.
• 16uF and 32uF expansion modules available.

Software, Accessories and Options

Standard Accessories and Software:

• High voltage environmental-rated connectors with carrying case
• Panasonic Toughbook field computer
• GMR data acquisition software
• GMR noise cancellation software
• GMR 1D inversion software

Optional Software:

• GMR 2D inversion software

Surface Wire Options:

• #8 AWG, 4kV polyurethane insulated
• #10 AWG, 4kV polyurethane insulated

Optional Accessories:

• Custom ATA shipping cases
• 4-channel Transmitter Expansion Module
• 16uF Tuning Expansion Module
• 32uF Tuning Expansion Module

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