RESOLUTION OF PROMINENT CRUSTAL SCATTERERS NEAR THE NORESS SMALL-APERTURE ARRAY

Our goal is to identify prominent scatterers near seismic arrays so th at the secondary phases they generate can be anticipated and recognize d as being of local, not teleseismic, origin. Toward this end we have developed a technique to scan seismic coda recorded by arrays for phas es generated locally by scattering of teleseismic energy from large to pographical and crustal heterogeneities. In this paper we test the tec hnique by using the NORESS small-aperture array in southern Norway to image free surface secondary R(g) sources excited by incoming teleseis mic P waves. In essence, our technique suppresses the teleseismic ener gy and migrates the residual array records to image-coherent local sou rces. To assess the azimuthal and radial resolution that the NORESS ar ray provides we have analysed point-source synthetics. A preliminary a nalysis of recorded data provides an initial view of the scatterers in the vicinity of NORESS and identifies limitations of imaging using th e full coda of individual events. Single events can be used to image s catterers but more robust images, with higher azimuthal resolution, ca n be obtained by using a broadly distributed suite of events. Radial r esolution of scatterers can be enhanced by deconvolution. A composite image generated by simultaneously considering a suite of 31 deconvolve d teleseisms indicates that two secondary R(g) sources are present at, or near, the free surface near NORESS. One, 27 km to the south-west o f the array, appears to be related to dramatic topographic relief at t he south-west edge of Lake Mjosa. Another, roughly 10 km to the east o f the array is judged by a bootstrap-resampling analysis to be only ma rginally significant. This apparent source does not coincide with obvi ous topography and is likely buried. Although the time delay that sepa rates the scattered and 'parent' phases depends somewhat on the angle of approach of the incoming waves, in general these secondary sources are contributing wave trains to the seismic coda that begin approximat ely 4 and 11 s, respectively, after the primary onset.