odern GPR systems are very easy to operate and much of the complexity of the underlying electromagnetic (EM) signal character is hidden from the user. In fact, GPR signals are electromagnetic fields which are invisible to the human, vector in nature and spread out over space and time. The ability to capture GPR signals in three dimensions around a GPR transmitting antenna offers huge advantages and creates a path to a wide variety of sensing applications that have not, to date, been addressed by GPR technology.
The exploration seismic field, which searches for deposits of oil and natural gas, has dealt with the full character of elastic wave fields for several decades. In that time, the industry has developed advanced techniques for imaging underground structures and extracting important physical properties to better understand the subsurface.
Seismic waves are very analogous to GPR waves so similar processing and imaging techniques can be employed with GPR data. Until now, hardware limitations and cost have prevented GPR practitioners from taking advantage of these innovations.
Sensors & Software introduces the next-generation of SPIDAR® hardware to enable pulseEKKO® and Noggin® sensors to be integrated into distributed multi-frequency, multi-orientation and multi-field component deployments networked together.
The newest, most flexible and advanced component is the NIC 500X which allows concurrent receiver operation and brings a new dimension to GPR deployment. Historically, GPR was limited to the use of a single transmitter and receiver pair. Multiple channels of data were obtained by multiplexing pairs of transmitters and receivers. More complex surveys required fixing the transmitter and moving the receiver (or vice versa) to measure the wave field over a spatial area; this is both slow and inefficient.
Concurrent receiver operation enables multiple receivers to acquire the signal generated by a single transmitter. This capability makes it possible for rapid acquisition of the fields around the transmitter in space and time; emulating much of the capability that the seismic petroleum field has been able to exploit for many years.
The details can be complex so we will limit discussion here to a single example of the "WARR machine", as presented at the IWAGPR 2017 conference (paper available upon request). A WARR (wide angle reflection and refraction) sounding measures the GPR fields at differing transmitter and receiver separations as depicted in Figure 2. Such surveys allow analysis of ground velocity variations and variations of reflectivity with angle of incidence that provide valuable diagnostic information. While WARR surveys have been used for decades in the GPR field, data acquisition is slow because the receiving antenna had to be moved (usually manually) between each measurement point.
Figure 3 shows a 500 MHz WARR machine deployment which controls a single 500 MHz pulseEKKO® transmitter and seven 500 MHz receivers mounted in-line at fixed offsets. Deploying the system on a cart (Figure 3) or sled with odometer triggering allows full WARR data sets to be acquired at the same rate as traditional, single channel (one transmitter-receiver pair) GPR surveys.
Data processing and analysis are more complex with these types of concurrent receiver deployments and this will be addressed in future publications.
To put this benefit in context, 25 years ago, a skilled crew could acquire WARR soundings at a rate of 10 to 20 per hour. Even a few years ago, rates of acquisition had only doubled to 30/hour. The WARR machine can acquire 10,000 WARR soundings per hour (Figure 4). This massive increase in speed of acquisition opens the door to many interesting and advanced applications of GPR including routine generation of velocity and water content sections.
GPR will never be the same.
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