Source Location and
Tracking Capability of a
Small Seismic Array
MARK L. MORAN AND DONALD G. ALBERT
ciples, these data constraints make it difficult to
1. INTRODUCTION
obtain reliable estimates of the spatial correlation
Acoustic and seismic sensor array processing
matrix, which is critical to the beamformer's per-
has many potential applications of interest to the
formance. The spectral bias and variance are the
Army. Recent military conflicts have clearly
statistical error parameters that describe the reli-
demonstrated the efficiency of smart weapons
ability of the resulting estimated frequency and
systems. A critical element in the effectiveness of
wavenumber spectra. Bias is a measure of the
many of these systems is their ability to discern
energy leakage from adjacent frequency bins that
and track a target in the presence of background
erroneously appears at an incorrect frequency;
noise. The Wide Area Mine (WAM) being devel-
this phenomenon is termed leakage and is caused
oped by the U.S. Army is heavily dependent upon
by the sampling distribution in the time (or space)
acoustic and seismic sensor information for bear-
domain. Variance is a measure of the stability in the
ing determination of targets. Weapon systems such
estimate and is a function of the number and
as the WAM are designed to recognize, track, and
degree of independence between samples avail-
destroy hostile military vehicles such as armor,
able to the estimation process.
mobile artillery, and heavy transport vehicles by
Beamforming complications may result from
deploying small arrays of passive acoustic and/or
source motion, acoustic-to-seismic coupling, in-
seismic sensors (microphones and geophones) to
homogeneous propagation environments, mul-
monitor local seismic and acoustic wavefields. A
tiple sources, and low signal-to-noise ratios (SNRs).
cartoon of the detectionattack scenario for the
Given the nature of the moving seismicacoustic
WAM device is shown in Figure 1.
sources being considered and the limited quantity
Other important areas where acoustic and seis-
of data available, it is important to recognize the
mic array processing may be applied include pe-
nonideality of the observed signals, so that those
rimeter defense and intrusion detection. In many
properties of the signal that have the highest de-
instances the effectiveness of line-of-sight systems
gree of coherency across the observation array can
such as radar, infrared, and motion detection sys-
be exploited fully.
tems are compromised or defeated by local topog-
The combined effect of these source-dependent
raphy, vegetation, and obscuring meteorological
characteristics and the problems arising from
conditions. In real-world settings, acoustic and
sparsely sampled short-duration signal vectors
seismic waves readily propagate along bending
has not been previously demonstrated on the beam
ray paths and are less affected by vegetation and
response function. In this report we show that
airborne obscurants. This allows coverage of zones
beamforming techniques can be reliably applied
currently outside the detection limits of systems
to these types of signal vectors as long as careful
that rely on line-of-sight propagation paths. Acous-
attention is paid to the spectral estimation param-
tic and seismic array-based systems are relatively
eters used in the Fourier transform process. We
inexpensive and may be produced and deployed
present a few rough guidelines that produce con-
in large numbers, providing a substantial perim-
sistently reliable wavenumber estimates from Ca-
eter defense or detection capability.
pon maximum-likelihood beamformers.
For small array processing systems to function
Historically, frequency wavenumber domain
reliably, it is necessary to be able to estimate accu-
(F-K) beamforming analysis has been applied to a
rately the wavefield's temporal frequency phase
variety of problems. In seismology, wavenumber
spectrum, even with limited spatial and temporal
beamformers using geophone arrays are routinely
wavefield samples. When using wavenumber es-
used to determine source parameters for regional
timation procedures based on beamforming prin-
and teleseismic events as well as to identify nuclear
Previous Page
