Passive Seismic Survey Overview
Active reflective seismic exploration technologies provide data used to map the structure of the earth, including those types of formations frequently associated with hydrocarbon deposits. While these exploration methods have been useful in discovering large reservoirs, they have also been subject to error and they typically involve the use of heavy equipment, vibrators and explosions, all of which can cause extensive damage to the area being explored.
Naturally Occurring Noise.
Passive seismic exploration measures the naturally occurring low-frequency ambient seismic noises which continually occur within the earth. Generated by the movements of the oceans, microseismic movements within the earth and other natural activities, these sounds are well below the frequencies audible by human ears, but are measurable by sensitive seismometers operating within the range of .01 to 6 hertz.
Military and scientific applications of infrasonic technology have been used for decades to locate submarines and study geophysical formations, the later on earth and in space exploration. Research into monitoring low frequency signals for hydrocarbon exploration began in the middle of the 1990s.
Signal Amplification.
As a peculiarity, when the infrasonic energies from natural seismic sources pass through a hydrocarbon field, the amplitude of the sound waves increase enough to stand out significantly from the background seismic signals. This anomaly has occurred within passive seismic surveys conducted at thousands of points located in Texas, Oklahoma, Louisiana, Kansas, and Alabama by Acoustic Reservoir Imaging, LLC.
While the cause of the amplitude increase in infrasonic signals passing through a hydrocarbon deposit is subject to research and discussion, one theory suggests the hydrocarbon fields lengthen the frequencies of higher-frequency signals, adding their energy to the amplitude of the existing infrasonic signals. Whatever the cause, the increase in signal amplitude appears to be unique to infrasonic energy passing through hydrocarbon deposits and the junctures of these deposits with water. Hydrocarbon fields of larger thickness cause greater increases in signal amplitude.