Seismic Arrays presented at the WORKSHOP High Quality Seismic Stations and Networks for Small Budgets Volcan, Panama March 2004by Jens Havskov, Department of Earth Science University of Bergen Norway

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)

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

Arrray response function

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