Technology
PSTT®
Passive seismic transmission tomography, PSTT®, creates 3-D images using the observed travel time of seismic signals originating from micro-earthquakes occurring below the target.
A sparse array of independent seismographs is established above the target. The array usually consists of 20 to 100 stations each recording the output of a 3-component geophone. Typical imaging areas for such an array are 300 to 1500 km2. The 3-component phones are placed 10 to 30 m below the surface to get away from the noisy surface environment. The stations may store their data locally, but often are linked to the processing center by some form of telemetry.
In PSTT®, micro-earthquakes occurring below the targetserve as the seismic source. 3-component geophones on the surface record the arrival time of p and s waves from these tremors. Travel time inversion is used to estimate the p and s velocity distribution in the target. As more events are observed, the velocity distribution can be estimated to a finer resolution.
With the array established, the survey proceeds by simply listening. Assuming an initial velocity model, the observed micro-earthquakes are located in time and space using long-standing location algorithms based upon picks of the "p" and "s" phase arrival times at each observation station. Once a number of events has been located one flips the process, assumes the origin time and hypo-centers of the events are known, and uses some form of travel time inversion to estimate a new velocity model. The 3-component nature of the observations allows for estimation of the Vp and the Vs velocity structures. As more events are added to the dataset, finer estimates of the velocity structure can be achieved. The process proceeds in this boot-strapping fashion until the desired resolution is reached.
Most of us are surprised to find that there are enough micro-earthquakes occurring to make this a viable tool. We are conditioned to think of earthquakes in terms of life threatening, concrete crushing events that thankfully happen only rarely. Such events have a local magnitude (mL) of 3 or greater. Earthquakes are observed to be log-normally distributed as to their magnitude. This means that there will be, on a statistical basis, 10 times as many magnitude 2 earthquakes as magnitude 3, and 10 times as many magnitude 1 as 2, and so on. The micro-earthquakes used for passive seismic transmission tomography are typically all those down to magnitude 0 or even smaller. Since the processing of the data proceeds in near real time, it is possible to monitor the effectiveness of the survey and cease field operations when the particular needs of the survey have been met. Survey times of 6 to 12 months are to be expected.
Where and when does such an approach to imaging become cost effective? Certainly in flat, open country a more conventional reflection survey is probably a better solution. But in mountainous terrain passive can be as much as an order of magnitude less expensive. In environmentally sensitive areas the benign environmental impact of passive means that a survey that might otherwise never get permitted becomes possible. In highly cultured areas, the low impact presence of passive again makes the likelihood of obtaining permits much higher.
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