Using the SPAC Microtremor Method to Identify 2D Effects and Evaluate 1D Shear-Wave Velocity Profile in Valleys

Abstract

The requirement of a layered-earth geology is a restrictive assumption when using the spatially averaged coherency spectra (SPAC) method. Numerical simulations of microtremors and SPAC observations recorded in the Tamar paleoval- ley, Launceston (Tasmania, Australia), are used to assess the potential of the SPAC method to identify two-dimensional (2D) effects and evaluate one-dimensional (1D) shear-wave velocity (SWV) profile in a valley environment. The Tamar Valley is approximately 250 m deep by 700-1000 m wide. It is filled with soft sediments from the Tertiary and Quaternary periods above hard dolerite bedrock of Jurassic age. Observed coherency spectra of the vertical component are analyzed at two sites in the Tamar Valley; using two 50-m-radius centered triangular arrays above the deepest point of the valley at site DBL, and above the east flank of the valley at site RGB. Simulated and observed coherency spectra suggest the propagation of Rayleigh waves of first higher mode at the SV frequency of resonance of the Tamar Valley affects the coherency spectra recorded with pairs of sensors perpendicular to the valley (transverse-COH). Simulated and observed coherency spectra recorded above the deepest point of the valley (site DBL) with pairs of sensors parallel to the valley axis (axial-COH) are not affected by these edge-generated Rayleigh waves and agree well with the theoretical coherency spectrum computed from the preferred 1D SWV profile. The simulated and observed results from this paper suggest that differences between the observed axial-COH and transverse-COH give an indication of the exis- tence of the 2D buried valley. Results also suggest that the observed coherency spectra recorded on pairs of sensors oriented parallel to the valley axis can provide a reliable evaluation of a 1D SWV profile above the deepest point of a deep and narrow valley, such as the Tamar Valley.