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A Model for Small-Strain Damping for the Groningen Field Constrained by Vertical Array Measurements

Description: 

The incorporation of site effects into seismic hazard assessments requires the characterization of stiffness and attenuation (i.e., damping). This study presents the derivation of a model for small-strain damping for the seismic hazard assessment of the Groningen gas field in the Netherlands. The model is constrained by ground motion measurements over a large network of 200 m deep vertical arrays. Measurements of the seismic quality factor Q at the vertical arrays were conducted by first applying seismic interferometry by deconvolution to estimate the local transfer function. Two approaches were then used: the first by measuring the amplitude decay of the retrieved upgoing wave, and the second by measuring the amplitude difference between the upgoing and downgoing waves at a single geophone. The second approach has the advantage that corrections for elastic propagation are not needed; consequently, the use of this approach resulted in more robust measurements of Q. Moreover, the second approach is less sensitive to small differences in effective response functions of geophones at different depths because it relies on measurements at a single geophone. The measurements of Q at the vertical arrays were then used to build a model for the entire study area, which covers a region of approximately 40 by 50 km. The model was based on scaling small-strain damping models from the geotechnical engineering literature, which are based on geotechnical index properties, to match the measured Q values at the downhole arrays. This was possible because of the existence of a geostatistical model (the GeoTOP model) that provides estimates of geotechnical index properties for the entire study region. The scaling factor was determined by equating estimates of site kappa obtained using: a) the measured Q values and b) damping profiles obtained from the geotechnical damping models. We observed that the scaling factor is strongly correlated to the average shear wave velocity over the upper 30 m (V_S30). Q values at depths lower than the vertical arrays were derived from the attenuation of the microseism measured over the study region.

Session: High-frequency Ground Motion Measurements, Assessments and Predictions

Type: Oral

Date: 4/18/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Adrian Rodriguez-Marek

Student Presenter: No

Invited Presentation: Yes

Authors