Using a Vertical array Receiver Function Method to Image the Subsurface of the Groningen Gas Field

Abstract

In this study, we determine the delay time of P-to-S conversions from earthquake sources using a vertical array-based receiver function method. This method was used to detect discontinuities below the Groningen gas field.

The signal that comes directly after the direct P-wave arrival, known as the P-wave coda, contains valuable information about the arrivals of additional waves. One of the waves that arrive in this coda is the PS-wave. The PS-wave is an S-wave with a P-wave origin; it has been converted at a strong contrast in velocity and/or density between two media. This contrast is called a discontinuity. Since S-waves travel slower than P-waves, we can measure the difference in P- and PS-wave arrival time to infer the depth at which this conversion took place.

Waves that arrive after the direct P-wave are usually obscured by the source time function of an earthquake. The source time function reflects the slip process at the source and depends on the magnitude and slip mechanism. The source time function overprints the coda wave arrivals due to its duration, and thus delayed phase arrivals cannot be directly identified from the seismogram. By doing a deconvolution of the P-wave coda of the horizontal component with the vertical component, we can remove the source time function and obtain a receiver function that reflects the P-to-S converted waves. This receiver function shows the delay times between the P-wave and subsequent PS-conversions.

In traditional receiver function studies, data from a single geophone at the surface are used to determine the depth of global scale discontinuities. In this study we use an array based method to determine the delay time between direct P-waves and their PS-conversions to infer small scale discontinuities. Using data from 3-component geophones placed in the abandoned borehole SDM-01 near Groningen, we have determined the depth of multiple discontinuities below the Groningen gas field.