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Telemetered Arrays
Figure 20. Installation of a broadband sensor vault in southeastern
Tibet with local assistance.
to the permanent coverage provided by the GSN and other
networks. Many global tomographers make increasing and
extensive use of data from past PASSCAL deployments to
enhance their data sets. At the request of this community,
one station in each PASSCAL and EarthScope Flexible Array
experiment is now designated as “open” with the typical twoyear
data embargo waived.
Maintaining and operating the broadband instrument pool
consumes a significant portion of PASSCAL efforts. The
broadband sensors were not designed for portable operations
in the manner in which they are now employed; they
are sensitive to shock and vibration during shipping. When
being deployed in the field, care must be taken to ensure that
the vaults do not flood or retain moisture.
Using the same data-acquisition systems and sensors as in
stand-alone deployments, telemetered arrays can be supported
using specialized communications, software, and
computing equipment. In addition to on-site recording to
disk, data are telemetered to a central site and merged in
real time (the on-site disks provide a backup for telemetry
outages). The broadband telemetered array was developed
in the early 1990s in collaboration with the University of
California, San Diego, under the IRIS Joint Seismic Program
(JSP) for deployment in the former Soviet Union for nuclear
test-ban verification calibration tests. When the JSP program
was completed, the equipment and expertise necessary to
operate the array were transferred to PASSCAL. The original
PASSCAL broadband array consisted of 32 broadband sensors
and digitizers that telemetered the data via spread-spectrum
radios to a concentrator site located up to 80 km away.
This array was used for a number of experiments in locations
as diverse as the South African craton, and the Wyoming
province in the western United States. PASSCAL currently
is supporting a 22-station telemetered array in southern
Alaska. Telemetry expertise and new technologies developed
and implemented in EarthScope are being incorporated
into these systems.
A large fraction of broadband experiments is conducted
overseas in cooperation with foreign institutions. Foreign
operations usually require significant effort in making
arrangements for customs and shipping. While the PI is
responsible for the costs associated with getting the instruments
to the field, they usually rely on experienced PIC
personnel to make these arrangements.
Over PASSCAL’s lifetime, the average number of stations
per deployment has steadily increased and is now 30, with
many experiments exceeding 50, and several using more
than 75. One ongoing NSF Continental Dynamics program
experiment is fielding ~100 stations, 75 of which are from
PASSCAL and 25 are university-owned.
Figure 21. Radio telemetered broadband station on the Olympic
Peninsula, above the Cascadia subduction zone.
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