First, let us imagine that the truck removing the dirt from the end of a distant tunnel opening must use Mexican roads to transport these “tailings” to its dumping area. All roads have imperfections. Mexican roads have more imperfections than many. The physical movement of the dirt-hauling vehicle over a pot-hole or bump in the road 3,000 feet from a seismic sensor will be detected by that sensor.
Each such thump or bump occurs in one moment in time. The moment of occurrence can be detected by each of the sensors along the border that are within two miles. The event will cause a shockwave which moves outward at a finite speed. This ground speed is far greater than 1,000 feet per second.
The two problems are: determining a “signature” of the seismic bump and computing the time of arrival.
It is easy to survey the roads below the border and measure the intensity of the bumps and thumps created by the uneven roadways. Each bump has its own signature for a given type and size tire passing over it. The speed and weight of the passing vehicle can be estimated by the size and duration of the seismic event.
The time of arrival can be estimated and refined over time.
The seismic data from each sensor is stored. The seismic events occurring within any five second window can be compared and aligned for all the emplaced sensors. The constant seismic activity from the urban area’s other trucks and transit busses and machinery all combine to create a cacophony of seismic noise. Through this noise one still “sees” the localized seismic activity with a resolution of 100 feet or better at more than a mile.
The seismic data can be recorded continuously for hours, days, weeks, or forever. Simple computer processes are used to constantly align the seismic data in time to “point’ all the sensors in one direction — to focus the attention of hundreds of sensors on one single spot.
This single spot of attention is then moved in east and west and north and south and data is processed.
What is critical to understand is that the data for the entire border area is being collected all at the same moment. While the computer might look at only one spot at a time, the data for the entire border area and for as long as data has been collected is available all at the same time.
What is derived from this process is a set of seismic event density maps. You can think of them as red colored areas overlaid on the city’s street map. You might also imagine that as the day begins and traffic increases that the red spots on the city road map will increase in intensity and then decrease as the day comes to an end. These spots seem to “breathe” and trails of heavy traffic and heavy vehicles can be discerned.
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