IRIS/GSN PROJECT INITIATION FORM
PIF Reference:
Title:
Authors:
Date:
PIF-GSN-2012-03
Prototype efforts toward an Array of Arrays
Kent Anderson and Chuck Ammon
15 February 2012
Project Description
Install a prototype broadband seismic array.
Project Purpose and Business Benefit
A recommendation from the NRC report “New Research Opportunities in the Earth
Sciences (pg 98 - Prepublication)” states
EAR should pursue the development of facilities and capabilities that will improve
spatial resolution of deep structures in the mantle and core, such as dense seismic
arrays that can be deployed in various favorable locations around Earth, enhanced
computational software and hardware to enable increased resolution of threedimensional geodynamical models, and improved high-resolution experimental and
theoretical mineral physics investigations. This will provide definitive tests of many
hypotheses for deep Earth structure and evolution advanced over the past decade. The
large scope of such facilities will require a lengthy development and review process,
and building the framework for such an initiative needs to commence soon.
In addition, the “Seismological Grand Challenges” report recognizes that seismic
arrays offer great potential for resolving important questions regarding such
diverse topics as the nature of the lithosphere-asthenosphere boundary, how
temperature and compositional variations control mantle and core convection, and
how Earth’s internal boundaries are affected by dynamics. Moreover, arrays can be
used to greatly improve earthquake detection capabilities on a global scale. While
events as large as magnitude 5.5 can hide from current networks, a global array of
arrays would lower detection thresholds by one to two magnitude units. Complete
and accurate earthquake catalogs are a fundamental dataset for addressing several
of the Grand Challenges. Whereas some of these questions may be answered with
temporary PASSCAL portable array deployments, the others will require long-term
to semi-permanent monitoring and hence fit within a framework that bridges the
gap between GSN’s permanent global observatories and PASSCAL’s higherresolution temporary deployments.
Arrays have several advantages over three-component stations. An array provides
directional information on an arriving wavefield, including both azimuth and
“slowness” (inverse apparent velocity of the wave), and individual sensor channels
can be combined as a beam to improve signal to noise and to focus on aspects of the
wavefield. There are diverse designs for arrays, depending upon the particular
purpose, which include high-frequency and broadband elements, as well as threecomponent and only vertical elements. The aperture (array width) and the
organization and spacing of array elements can enhance or attenuate features of the
wavefield being viewed. Whereas a GSN station occupies a relatively small footprint,
extending this framework for an array may be constrained by local host
considerations and can limit collocation with existing GSN sites. Finally, the array is
a passive sensor—like the GSN station, it records seismic phenomena that
propagate to it.
Four Affiliate arrays are part of GSN, installed and operated by AFTAC or
DOE/Southern Methodist University, which are also IMS arrays. There are 18
additional IMS primary arrays, but unfortunately the CTBTO confidential data policy
limits scientific community access to these valuable resources. Open access has been
obtained on a bilateral basis with Canada, Australia, Germany, Kazahkstan, and
Norway. Efforts continue for more open release of array data from the other 11 IMS
primary arrays, in coordination with FDSN. Nonetheless, most of these arrays have
been narrowly designed for their sole purpose—to detect and monitor nuclear
explosions. The Southern Hemisphere has only two Australian arrays. “Sweet-spots”
for viewing a particular feature may require an array installed at an entirely new
site. To use the array for specific imaging of Earth structure, the geometry of the
earthquake sources, the array, and the lithosphere- asthenosphere-mantle-core
structures to be illuminated must be refined.
Project Scale and Duration
This project would look to install a 40 element broadband array in either the
Eastern US (designed within and around the 1N4 remaining TA distribution in
region) or could be considered to be built within the TA Alaska project station
deployments, taking advantage of the logistics resources being planned for that
deployment. We hope to have a working 40-element array by the end of the 5 year
2013-18 cooperative agreement.
In addition to procurement and deployment, there will be technical and scientific
workshops associated with array design and targets in the early years of the CA.
Estimated Project Cost
Hardware for a broadband array element would be based on a TA type deployment
with Q330 DAS and STS-2 or equivalent VBB seismometer. Hardware costs for a
station would include one DAS ($10,000), one seismometer (~$20,000), power,
comms, infrastructure (~$10,000) for a total of ~$40,000. Siting, permitting and
construction costs would be modeled after TA costs in the particular areas (Eastern
US - ~$20K/site, Alaska - ~$40K/site … NEED TO GET SYNC COST ESTIMATES
WITH TA/ALASKA- BUSBY. ) Therefore, a 40 element array in the Eastern US could
cost ~$2.4M and an Alaska Array could cost ~$3.2M (VERY ROUGH ESTIMATES). If
station equipment from the TA can be repurposed for this project, the cost could be
reduced by the hardware investment (~40*$40K = $1.6M)