Abstract

It is of particular importance for structural geology, geophysical exploration and also obvious economical purposes to retrieve structures possibly hidden below salt domes. And these domes could trap hydrocarbon or gas. We thus propose a sensitivity analysis of seismic data in salt tectonic areas to identify different wavelengths associated with the geological structures under study involving salt domes. The wavelengths associated with the density or seismic velocities of the medium can give us information about the localization of shallow or deep geological structures surrounding salt domes in off-shore contexts. Seismic data can be more sensitive to density or to seismic velocities. Depending on the wavelengths associated with those two different properties, the dome shape and the different interfaces can be located and recovered at different depths. In a first approach, using velocity and density models from a salt tectonic region in the Gulf of Mexico we simulate a two-dimensional seismic data acquisition. Using these synthetic data, we aim at retrieving the salt dome shape as well as the surrounding and deepest geological layers. For this purpose, we propose to compute better imaging conditions by attenuating free surface multiples and introducing an adjoint theory-based reverse time migration (RTM) method, enhancing the limits of salt bodies and also the layers under salt structures. To obtain these imaging conditions, we compute the compressional and density sensitivity kernels \(K_\lambda\) and \(K_\rho\) using seismic sources activated separately. To attenuate the free surface multiples, the synthetic “observed” data computed with the free surface are introduced as adjoint sources and we replace the free surface condition by PML absorbing conditions in both the forward, backward and adjoint simulations needed to compute the kernels. We compare the quality of the kernels applying different strategies related to the normalization of kernels by the forward or adjoint energy, and different property parametrizations were tested to improve the imaging conditions. The specific wavelengths associated with the different (shallow to deep) interfaces are obtained using signal-to-noise ratios (SNRs) applied to both density and seismic velocity kernels. In some cases, density or seismic velocity kernels are more suited to retrieve the interfaces at different depths.

中文翻译：

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使用逆时偏移成像和灵敏度核波长分析的盐构造建模

摘要

对于构造地质学、地球物理勘探以及明显的经济目的而言，寻找可能隐藏在盐丘下的构造具有特别重要的意义。这些圆顶可以捕获碳氢化合物或气体。因此，我们建议对盐构造区的地震数据进行敏感性分析，以识别与正在研究的涉及盐丘的地质结构相关的不同波长。与介质的密度或地震速度相关的波长可以为我们提供有关离岸环境中盐丘周围浅层或深层地质结构定位的信息。地震数据可能对密度或地震速度更敏感。根据与这两种不同属性相关的波长，圆顶形状和不同的界面可以在不同的深度进行定位和恢复。在第一种方法中，我们使用墨西哥湾盐构造区域的速度和密度模型来模拟二维地震数据采集。使用这些合成数据，我们旨在检索盐丘形状以及周围和最深的地质层。为此，我们建议通过衰减自由表面倍数并引入基于伴随理论的方法来计算更好的成像条件逆时偏移（RTM）方法，提高了盐体和盐结构下层的界限。为了获得这些成像条件，我们计算了压缩和密度灵敏度内核\(K_\lambda\)和\(K_\rho\)使用单独激活的震源。为了衰减自由表面倍数，使用自由表面计算的合成“观察”数据作为伴随源引入，我们在计算内核所需的前向、后向和伴随模拟中用 PML 吸收条件替换自由表面条件。我们比较了应用不同策略的内核质量，这些策略与通过前向或伴随能量对内核进行归一化相关，并测试了不同的属性参数化以改善成像条件。使用应用于密度和地震速度内核的信噪比 (SNR) 获得与不同（浅到深）界面相关的特定波长。在某些情况下，