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 $0.5{\mathrm{s}}^{-1}$, 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.

中文翻译：

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利用潜水波探测浅表覆盖气层

我们使用潜水波的简单时移分析来分析沉积覆盖层中的浅层气层。如果薄砂层在局部和受限区域中被气体充填，通过使用简单的方程来说明走时将如何变化，我们表明这种变化可用于绘制和量化气层的厚度。我们使用在 1989 年发生意外地下气体流的井附近采集的常规 3D 地震数据。原始地震数据用作输入，并估计时移以获得解释为主要是潜水波的事件的恒定偏移量。通过假设非常浅的地下具有恒定的速度梯度$0.5{\mathrm{秒}}^{-1}$，我们发现潜水波时移适合大约 3-4 m 的气层平均厚度。这是最小的气体厚度，因为假设时移分析捕获了撞击气体层顶部和底部的潜水波（足够密集的偏移采样对于实现这一点很重要）。潜波分析得到的井周围近圆形气体异常的轮廓和周长通过同一异常的3D反射映射得到确认。