Seismic Arrays

presented at the

WORKSHOP

High Quality Seismic Stations and Networks for Small Budgets  Volcan, Panama 8-13. March 2004by

Jens Havskov, Department of Earth Science

University of Bergen

Norway

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A plane wavefront arrives with apparent slowness p to a rectangular shaped array (the intersection of the wavefront with the surface, where the array is installed, is shown). The angle with respect to North defines the ray arrival azimuth. Right: The waveforms recorded are ideally identical, except for the relative delays. In practice, they present differences due to the added noise and to the local ground heterogeneity

A projection on the incidence vertical plane. As the wavefront advances a distance cΔt, the intersection with the horizontal plane advances Δx, with apparent velocity v. The relations between v and c can be seen from the geometry in the figur as v = c/sin(i)

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ARCES is a small aperture array with 25 stations covering four concentric circles. The diameter of the external ring is about 3 km. The radial spacing of the stations is not uniform in order to deal with different wavelength

Determination of azimuth and apparent velocity with Chiriqui array

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Arrray response function

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Block diagram of a portable array. It is composed of several modules, one of which is represented. A central box contains an 8-channels antialias filter, a 16 bit-digitizer, a GPS receiver and a power supply for all. The digitizer is controlled by a PC through the parallel port and is synchronized to the GPS time via the serial port. The seismometers have a preamplifier with a low-impedance differential output, so that long connection cables may be used. The recording is made on the PC hard disk.

Potential use of a small seismic array

Improved detection of weak signals

Automatic detection of P and S-wave arrivals

Determination of azimuth

Automatic location

Location of weak emergent arrivals like volcanic tremor

Building a regional location capability in a small area