Average QP and QS estimation in marine sediments using a dense receiver array

Abstract

A new spectral ratio method has been used to compute average P- and S-wave quality factors, [Formula: see text] and [Formula: see text], for the sedimentary sequence below the “V”-shaped Oseberg permanent reservoir monitoring (PRM) system. Quality factors are important for a more accurate characterization of the subsurface and to obtain additional information on the physical processes within the earth, such as fluid content and partial melt. However, few methods compute the average Q-value in sediments for P and S waves in a region, which can be used as a constraint for attenuation models in more sophisticated inversion techniques such as full-waveform inversion or attenuation tomography. To address this, a spectral ratio method using two receiving stations, one installed on sediment and one installed on bedrock, has been developed. This has resulted in quality factor estimations ranging from approximately 64 to 137 for P waves and 29 to 123 for S waves using a subset of the 172 ocean-bottom nodes in the Oseberg PRM system and a seismometer in Bergen, Norway. The [Formula: see text] values are more scattered and hence more uncertain than the [Formula: see text] values due to the P-wave signal being close to the background noise level. In the study area, local earthquakes of magnitude as low as [Formula: see text] 2.4 can be used to obtain [Formula: see text] values and earthquakes of magnitude lower than 2.1 to find [Formula: see text] values, with the possibility of using lower magnitude events if the epicentral distances to the receivers are similar and smaller. Using this method, the average quality factor in sediment packages, for a variety of regions, can be estimated using appropriate raypath configurations connecting source and receiver.