Using diving waves for detecting shallow overburden gas layers

Abstract

We propose to use simple time shift analysis of diving waves to analyze shallow gas layers in a sedimentary overburden. By using simple equations for how the traveltime will change if a thin sand layer is charged by gas in a localized and constrained region, we show that such variations can be used to map and quantify the thickness of the gas layer. We use conventional 3D seismic data acquired close to a well where an unintended underground gas flow occurred in 1989. Raw seismic data are used as input and timeshifts are estimated for constant offsets for events that are interpreted as being predominantly diving waves. By assuming that the very shallow subsurface has a constant velocity gradient of 0.5 s-1, we find diving wave time shifts that fits an average thickness of the gas layer of approximately 3-4 m. This is the minimum gas thickness since it is assumed that the time shift analysis captures both the diving wave hitting the top and the base of the gas layer (sufficient dense offset sampling is important to achieve this). The outline and circumference of the close to circular gas anomaly around the well obtained by the diving wave analysis is confirmed by the 3D reflection mapping of the same anomaly.

Diving waves in acoustic factorized orthorhombic media

“…Lately, there has been an increasing interest in studying diving waves, motivated by their smooth behaviour and the massive amount of low‐frequency information they contain (Landrø et al., 2021; Stovas & Alkhalifah, 2014; Xu et al., 2016). The information contained within the low frequencies of the recorded seismic wave can be used to gain knowledge about the large‐scale velocity changes of the medium in which these propagate and are essential for focusing on the kinematics in the forward modelling (Plessix & Cao, 2011; Virieux & Operto, 2009).…”