17-10-2012, 04:20 PM
Seismic-Acoustic Hybrid Sensor & its Applications
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ABSTRACT
The need for instrumentation to gather seismic-acoustic signatures under
unusual conditions requires the use of low cost, simple design, and robust
sensors. In some instances, the sensors must adapt and mold to the
environmental medium to provide proper coupling for data collection
purposes. This paper explains how the idea of a hybrid seismic-acoustic
sensor originated and initially implemented. Testing shows the device
performs as a seismic and acoustic signature-gathering sensor. The data
product from the hybrid sensor is similar to those sensors available
through commercial sources. Further evolution of the hybrid sensor and
additional commercial applications for the device are discussed.
INTRODUCTION
The seismic-acoustic hybrid sensor addressed the need of a low-cost, simple
design and robust capability. The sensor was considered hybrid because it
reacted to seismic and seismic phenomena. The sensor was deployed in the
field in one of two ways. One method crested the sensor on top of the
ground. The second method required burying the sensor in the soil. Once
deployed, the hybrid sensor operated for long periods of time, nominally
for months.
Earlier designs of the hybrid sensor appeared in the mid-1970s. The basic
design consisted of 20 AWG Teflon insulated wire placed inside a copper
tube. Variations of design included different lengths, and diameters of
copper tubing to capitalize on signal detection. Coupled with detection was
analyzing the data. Preliminary findings, striking a plate with a
sledgehammer and generating sound from speakers, showed detection possible
and conventional signal processing techniques useful.
Follow-on testing involved sensor detection of different vehicles. One
significant observation regarding soil characteristics was noted.
Neighboring soil behaved similar to a low-pass filter. Since most vehicular
traffic of interest operated in the same frequency range, the observation
did not affect the collected data. Signatures from people walking or
running also fall within the same bandwidth making the device useful for
perimeter security purposes.
DESIGN AND OPERATION OF THE SEISMIC-ACOUSTIC HYBRID SENSOR
The mechanically flexible hybrid sensor easily adapts to various
environments. The device has inherent high noise characteristics similar to
those found in noise coaxial cables. Triboelectricity, current generated by
friction, is the chief cause of electrical noise in coaxial cables. Using a
standard length of RG-58 coaxial cable produces erratic bursts of noise
when acted upon by an external force. These bursts of erratic noise are
unmanageable and non-reproducible using basic signal processing techniques.