Seismic footstep detection based systems for homeland security applications are an important additional layer to perimeter protection and other security systems. This article reports seismic footstep signal characterization for different signal to noise ratios. Various footstep signal spectra are analyzed for different distances between a walking person and a seismic sensor. We also investigated kurtosis of the real footstep signals under various environmental and modeled noises. We also report on the results of seismic signal summation from separate geophones. A seismic signal sum spectrum obtained was broader than that obtained from a single sensor. The peak of the seismic signal sum was broader than that from the footstep signal of the single sensor. The signal and noise spectra have a greater overlap for a seismic signal sum than that from a single sensor. Generally, it is more difficult to filter out the noise from the sum of the seismic signals. We show that the use of the traditional approach of spectrum technology and/or the statistical characteristics of signal to noise of reliable footstep detection systems is not practical.

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Tactical capabilities of single and three axis geophones for seismic detection and bearing estimation for homeland security and defense applications are described. It is shown that typically three axis geophones yield a high bearing estimation error. An alternate bearing estimation approach is based on using the time delay in footstep signal detection from three triangulated single axis vertical geophones. In this approach the standard deviation of the bearing estimation error is less than 12 degrees for a walking person distance of 10 to 70m and geophone distances of 8 to 9 m.We find that using the three-axis geophone approach makes it harder for path tracking and bearing estimation within the tactical zone area. We report that a single-axis geophone approach for triangulation of walking person is more effective. In addition, road monitoring is also more efficient using a single-axis geophone approach. We compare the relative and absolute improvement of bearing estimation probability for road monitoring using three single-axis geophones versus 1, 2 and 3 three-axis geophones. We will also discuss the use of single axis vertical geophone sets for monitoring various zone sizes.