- How is GMR different from other geophysical equipment?
- What is meant by the “dead-time” and why is a short dead-time important?
- What is the advantage of having a high-power transmit capability?
- Why is it important to have a low receiver input noise?
- What does wide-band detection mean and what advantage does it offer?
- What are the advantages of GMR’s multi-channel architecture?
- To what extent is safety considered in the GMR design?
GMR is the world’s most advanced surface NMR system, boasting a unique combination of features that enable more efficient, accurate, and informative groundwater investigations. Unlike other geophysical techniques which measure physical properties that may be related to groundwater (e.g. TEM and MT measure electrical conductivity), the GMR system measures the NMR signal emitted directly by hydrogen in groundwater. The system features four fully functional and synchronized transmit and receive channels (expandable to eight or more channels)**, high-power transmit capabilities, a very short dead-time, and ultra-low-noise wide-band detection‡ electronics. These features combined with advanced acquisition, processing, and inversion software deliver unparalleled data quality and capabilities for groundwater characterization.
A short-dead time serves to extend the range of environments in which surface NMR measurements can be effectively used. The surface NMR measurement is acquired by transmitting a pulse of current through one or more surface loops and then recording the NMR signal from groundwater on these same loops. The dead-time represents the period of time between the end of the transmit pulse and the first accurate recording of the NMR signal. During the dead-time, some portion of the NMR signal from groundwater will decay and, thus, will not be accurately recorded. Short NMR signals tend to be associated with water in magnetic geology, water in fine-grained sediments, or water in the unsaturated zone. In order to accurately detect these important signals, a short dead-time is required. Through fast switching and wide-band receive electronics, GMR system offers a uniquely short instrument dead-time of less than 4ms and a total post processed dead time of 4ms + 1/BW (BW is the user-selected filter bandwidth). The exceptionally short dead-time of the GMR system enables reliable detection of groundwater under conditions in which other surface NMR instruments have failed and vastly expands the range of environments and applications in which this method can be utilized.
High-power pulses enable detection of deeper groundwater as well as better detection of groundwater with short NMR signals. In the surface NMR measurement, large current pulses are passed through the surface loop, transmitting a magnetic field which radiates to depth and excites an NMR response from groundwater. The depth of investigation or the amplitude of the NMR response from a given depth varies with the total pulse moment (given by the product of the transmit pulse current times the pulse duration). Therefore, a higher-current, long transmit pulse enables detection of groundwater at greater depths. In order to excite water with a short NMR decay time (water in magnetic geology or fine sediments), shorter pulses must be used. In this case, using a higher transmit current makes it possible to achieve equivalent pulse moments using shorter duration pulses to detect shorter NMR signals. The maximum transmit power specification for the GMR is 600A, 4000V, and 25 A*s. The GMR’s efficient power conversion architecture also enables transmission through thinner gauge wires. This feature can be especially advantageous and convenient when laying out and reeling up large diameter loops.
A lower receiver input noise makes it possible to detect smaller NMR signals. The NMR signal from groundwater that is detected on a surface coil is very weak — sometimes as small just a few nanovolts. In order to detect these very small signals, the level of noise influencing the measurement must be kept as small as possible. Noise can come from environmental EM noise sources but also from internal receiver input noise that is present in any type of detection electronics. It is particularly important to keep this receiver input noise small so that the measurement is only limited by environmental EM noise sources, which can be separately reduced through adaptive noise cancellation. The receiver input noise on the GMR system is less than 0.5nV/rt(Hz). This means that for a typical measurement with 10 stacks a bandwidth of 200 Hz, the total instrument noise level will be less than (0.5)*sqrt(200)/sqrt(10) = 2nV. Keeping the total noise level low is particularly important for measurements where water content is low or for high-resolution surveys with small loops.
The GMR system uses wide-band detection, which means the system accurately records a wide range of frequencies. This is different from a system that is electronically tuned to be sensitive to a narrow range of frequencies. Tuned detection can corrupt the recorded signal in several unwanted ways, including non-linear errors in amplitude, phase, and the shape of the signal at early-times. Wide-band detection, on the other hand, robustly preserves this information, which is critical for high-quality groundwater investigations. Robust measurement of amplitudes is required to determine water content distribution, while phase information is critical for two-dimension imaging and for incorporating the effect of subsurface conductivity. By providing more accurate recordings of the signal at early-times, wide band detection also serves to shorten the effective dead-time, enabling detection of shorter NMR signals. The use of untuned wide-band electronics also prevents mutual coupling between surface coils, an effect which can cause unwanted artifacts in 2D or 3D imaging.
The multi-channel design GMR system enables adaptive noise cancellation for vastly improved data quality as well as more efficient 2D or 3D groundwater imaging**. Four independent channels (expandable to eight or more) allow for simultaneous detection on multiple loops. Because of the unique system architecture, all four channels of the GMR system provide fully functional and equivalently calibrated transmit and receive capabilities. Additional channels may be connected to reference loops, to characterize the local environmental noise. Reference noise recordings are used in Vista Clara’s adaptive noise cancellation processing to dramatically improve signal quality in noisy environments. Spatially distributed loops can also be used to efficiently conduct 2D or even 3D imaging surveys. Because all channels of the GMR are identically calibrated, any combination of transmit and receive loops can be used without having to worry about variable characteristics or scaling between channels.
Safety is the foremost consideration in GMR system and is incorporated through several unique design features. Surface NMR measurements require the use of very high power electrical energy, stored as charge on capacitors and discharged as current through surface loops. In order to protect users from this powerful electrical energy, the GMR design includes a number of critical safety features not found on other instruments. First, a robust safety lock design automatically depowers the system if any element is improperly connected or if a hazardous state is detected. For example, if the tuning unit lid is opened, the system immediately depowers and rapidly dissipates stored charge to a safe level. High-visibility analog voltage meters on the front panel of the DC/DC converter and bright warning lights provide constant indication of the voltage state during operation following shutdown. As a further safeguard, all wiring connections on the GMR system use environmentally-rated no-touch contacts so that the operator never directly contacts any conductive elements of the high-voltage circuitry. Safety should always be the first priority in field work and has been thoughtfully integrated into the GMR system at every level of design.