Using diving waves for detecting shallow overburden gas layers

Abstract

We have used 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 time shifts 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 [Formula: see text], we find diving wave time shifts that fit an average thickness of the gas layer of approximately 3–4 m. This is the minimum gas thickness because it is assumed that the time-shift analysis captures the diving wave hitting the top and the base of the gas layer (sufficiently 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 are confirmed by 3D reflection mapping of the same anomaly.