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Seismic Array
An L-shaped seismic array of vertical component broad-band
seismometers in shallow boreholes with 20 elements at approximately 2.5
km spacing supplemented by 4 additional sites near the crossing of the two arms (fig 2.1). The site WB2 has a three-component broadband seismometer. The sites are serviced by two main tracks along each arm which involve a number of creek crossings.
Figure 2.1: Seismic Array Configuration
The seismic array has been upgraded to meet the requirements of the CTBT with
Nanometrics Inc. digitising, telemetry and central processing equipment.
The individual stations in the array use solar power and transmit data to
the central recording site by digital radio-telemetry.
Each set of equipment is protected by a fence to prevent animal incursion.
For more details on seismic sites click here.
The full set of data from the stations is recorded locally, and is transmitted
by VSAT link to the International data Centre for the CTBTO in Vienna.
The data is also sent to Geoscience Australia (GA) in Canberra via satellite
link. The central processing equipment in the laboratory associated with the
Seismic Array is illustrated in Plate 2.2.
Plate 2.1: Central processing equipment for the WRA Seismic Array
An array configuration map is shown in fig 2.1. The nearly north-south
arm is designated the 'Blue' arm with site names B1-B10 and the
east-west arm is the 'Red' arm with site names R1-R10. The new sites
are named as C1-C4 and are indicated in green. The 3-component sensor
will be placed at site B2. The theoretical response of the array is
illustrated in fig 2.2. The additional stations should help to improve
analysis for the many events at distances between 12 and 20 degrees
from the array.
A separate broad-band seismometer (Guralp CMG-3) has been operated
near the main station building since 1990; this has been superseded by the
new three-component station at the site B2 at the completion of the
upgrade.
Figure 2.2: WRA Seismic Array response
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WRAB - broadband system
A broad-band seismometer installation operated on behalf of the IRIS-IDA
project (U.S.A) at the University of California, San Diego.
This site is approximately 500m from the main station
building and comprises a 100 m borehole into granite with a KS-54000
seismometer and a surface vault with an STS-2 seismometer. Power at the
site is provided by solar panels and data transmitted to the central
recording site by optical fibre. Communication to the San Diego data centre
is now by satellite link.
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Infrasound array
A set of pressure sensors have been operated since the 1970's for research
into the complex atmospheric wave phenomena in Northern Australia.
The infrasound array was reconstructed in 2000 to CTBT standards
as a telemetered 8 element array with 4 long-period sensors and
4 short-period sensors (figure 2.3). The array lies in an area with
significant vegetation to the west of seismic site C1.
One high and one low frequency element (H0 and L0)
are co-located at the centre point of the array. Each remote array site
comprises of an equipment vault, solar power system, telemetry mast and
a noise reducing pipe array. The apertures for the high frequency pipe
arrays are 18 meters and 70 metres for the low frequency arrays.
All 8 remote elements are located in the vicinity of the existing analog
Infrasound array, thus placing them within 3 kilometres of the central
recording facility.
Figure 2.3: WRA Infrasound Array configuration
Each of the sensors is accompanied by a radiating
pipe array designed to minimise the effects of wind by averaging
over an array around the sensor.
For more details on infrasound sites click here
Figure 2.4: Response of array for a vertically incident signal
centred around 1 Hz
The response of the array for a vertically incident signal with a frequency
around 1 Hz is illustrated in fig 2.4. The response for a monochromatic signal
has very large side lobes, but these are suppressed somewhat when the
bandwidth is increased.
