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FIGURE 2. Flow chart for the processing of the electrical data. (1) Instrumentation of the porous cement block, (2) Data acquisition
system, (3) Signal conditioning of the raw data, (4) Mapping the voltage response using ordinary kriging (geostatistical inferences), (5)
Localization of the causative sources in the block.

researcher in this evolving branch of        and consortia, forming a critical mass of     events, while the inverse modeling did
geophysics. “If you put electrodes on        expertise in the fast-growing field.          the opposite: The inverted simulations
your head, you can monitor the electrical    	                                             calculated all of the possible parameters
activity in areas of interest within your                                                  of the fracking events from the synthetic
brain, and make informed inferences.         èFORWARD AND INVERSE                          sensor data. The differences between the
We are trying to do the very same thing      MODELING                                      forward and inverted results narrowed
underground.”                                                                              with repeated adjustments and iterations
	 Revil is an associate professor of         “We found that fracking releases both         until a match was obtained.
geophysics at CSM, one of the world’s        seismic and electromagnetic energy, and       	 The next step was to determine how
leading colleges of mineral engineering,     so we developed theories about what the       well the simulations corresponded to
and also a faculty member with the           electrical activity associated with these     reality. The team designed a laboratory
Centre National de la Recherche              events should look like,” Revil said. “To do  experiment in which they would
Scientifique (CNRS) in France.               this, we used the COMSOL Multiphysics®        repeatedly hydraulically fracture a porous
	 His team has done a great deal of          software to simulate fracking events and      cement cube by pumping in saline water
forward modeling and some inverse            to produce synthetic seismograms and          under high pressure. The purpose was to
modeling with multiphysics software.         electrograms. We coupled MATLAB® and          see if electric signals could be passively
The team is part of the Unconventional       COMSOL® to conduct many iterations of         recorded and then inverted to pinpoint
Natural Gas Institute (UNGI) at the CSM.     forward and inverse modeling thanks to        the positions of actual fluid leakages over
UNGI serves as a focal point to facilitate   LiveLink™ for MATLAB®. COMSOL saved           time. They drilled several 10-mm-diameter
and promote research and development         us a great deal of time because all of        holes, or “wells,” at varying depths in the
in all areas of unconventional natural gas,  the necessary underlying physics, such as     30.5 x 30.5 x 27.5 cm cube (see Figure 1).
encompassing coal bed methane, tight gas     hydromechanical equations, are very well      	 Various ways to seal the well bores
sands, shale gas, and gas hydrates. UNGI     defined in the software and we didn’t         were tested, and stainless steel tubing
combines the resources of seven academic     have to spend time calculating them.”         with a 9.5 mm outside diameter was
departments and 11 research centers          	 The forward modeling generated              placed into the holes to simulate well
                                             synthetic seismograms and electrograms
                                             corresponding to simulated fracking

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