Contributed editorial appearing in
Scientific Computing & Instrumentation 19:9, August 2002, pg. 18.
Although justifiably supplanted by the media’s coverage of the war on terrorism, a large portion of pre-911 discussion centered on energy policy. According to a U.S. Department of Energy projection, our emerging click-and-mortar economy will consume four-times as much electricity in 2020 as in 1970. Without revolutionary breakthroughs in renewable technologies, coal, natural gas, and oil will continue to be the top three energy sources. Along with more efficient drilling and production methods, the National Energy Policy delivered to the President by the National Energy Policy Development Group named 3-D seismic technology as pivotal to our ability to meet future energy demands.Since fossil fuels are hidden from view beneath tons of earth and perhaps additional miles of water, the first step in their recovery is to know where to drill. Moving beyond divining rods and gut feelings, modern geologists employ sound waves to survey the subterranean landscape. Similar to their electromagnetic cousins, sound waves are characterized by frequency, phase, polarization, and intensity and propagate through material with associated processes of reflection, refraction, diffraction, and absorption. When used to locate objects in water, the process is termed sonar, while the terms seismic and seismology are used when earth is the propagation medium. As with most spectroscopic techniques, seismology utilizes a source of energy that interacts with the sample and a transducer to convert the detected energy into an electronic signal for subsequent processing. On land, truck-mounted vibrators or dynamite are used as sources of sound waves while compressed air guns are often employed for aquatic surveys. The choice of sound wave detector depends on the medium and is known as a microphone, hydrophone, and geophone for air, water, and earthen media, respectively.
The analog geophone has been in use for over 50 years. Its basic design incorporates a conductive coil, known as the proof mass, that surrounds a permanent magnet. The proof mass is supported by springs that permit the magnet to move relative to the coil when the geophone is vibrated, thereby inducing an electric current proportional to the velocity of the movement. Improved frequency response can be achieved by measuring vibration as a function of position when the proof mass is configured to be one side of a capacitor. Increased frequency response, sensitivity, and signal-to-noise ratio can be achieved by measuring proof mass acceleration as a function of vibration. Input/Output, Inc. has recently marketed the VectorSeis digital geophone that incorporates MEMS acceleration sensors similar to those used by automobile manufacturers for airbag deployment. The device consists of a bulk micro-machined moving proof mass suspended by springs from a surrounding silicon frame structure. The MEMS accelerometer is attached to a custom, mixed-signal, closed-loop, force-feedback ASIC that keeps the proof mass stationary while the device is vibrating. The commercial VectorSeis Module incorporates three accelerometers arranged in an orthogonal corner-cube configuration giving the capability to resolve sound waves traveling in the x, y, and z spatial dimensions. Traditional geophones must be operated within a few degrees of vertical, while the VectorSeis’s force-feedback stabilization compensates for operation at any angle.
When many geophones are deployed in a regular spatial matrix, the distributed detector system can be used to generate 3-D images of subterranean features from the collected data. The same 3-D volume elements, or voxels, generated in CAT scans and 3-D ultrasound diagnostics used in the medical industry can be calculated from the data streams collected from the geophone array. Increasingly, 3-D seismology is used to visualize changes in production drilling operations to monitor the health of the well. This time-profiled 3-D seismology has been coined “4-D” technology. As is the case with all modern data acquisition, 3-D seismology is turning to the field of informatics for strategies to manage the influx of data from geophone arrays containing thousands of data streams. The oil and gas industry has developed storage and compression formats for 3-D seismic data sets routinely larger than 20 GB. Until we perfect cold fusion in a cup, let’s hope that acquisition technologies such as 3-D seismic visualization assist in meeting our technological society’s energy needs.
