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Joint seismic/electrical effective medium modelling of hydrate-bearing marine sediments and an application to the Vancouver Island margin

机译：含水合物海洋沉积物的联合地震/电有效介质模拟及其在温哥华岛边缘的应用

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Remote determination of the hydrate content of marine sediments remains a challenging problem.udIn the absence of boreholes, the most commonly used approach involves the measurement of Pwaveudvelocities from seismic experiments. A range of seismic effective medium methods hasudbeen developed to interpret these velocities in terms of hydrate content, but uncertainties aboutudthe pore-scale distribution of hydrate can lead to large uncertainties in this interpretation. Whereudborehole geophysical measurements are available, electrical resistivity is widely used as a proxyudfor hydrate content, and the measurement of resistivity using controlled source electromagneticudmethods shows considerable promise. However, resistivity is commonly related to hydrateudcontent using Archie’s law, an empirical relationship with no physical basis that has been shownudto fail for hydrate-bearing sediments. We have developed an electrical effective medium methodudappropriate to hydrate-bearing sediments based on the application of a geometric correction to theudHashin-Shrikman conductive bound, and tested this method by making resistivity measurementsudon artificial sediments of known porosity. We have adapted our method to deal with anisotropicudgrains such as clay particles, and combined it with a well-established seismic effective mediumudmethod to develop a strategy for estimating the hydrate content of marine sediments based on audcombination of seismic and electrical methods. We have applied our approach to boreholeudgeophysical data from Integrated Ocean Drilling Program Expedition 311 on the VancouverudIsland margin. Hydrate saturations were determined from resistivity logs by adjusting theudgeometric factor in areas of the log where hydrate was not present. This value was then used overudthe entire resistivity log. Hydrate saturations determined using this method match well thoseuddetermined from direct measurements of the methane content of pressurized cores.
机译：远程确定海洋沉积物中的水合物含量仍然是一个具有挑战性的问题。 ud在没有钻孔的情况下，最常用的方法涉及通过地震实验测量P波的波速。已经开发了一系列地震有效的介质方法来解释水合物含量方面的这些速度，但是关于水合物的孔尺度分布的不确定性可能导致这种解释中的很大不确定性。在可以进行井眼地球物理测量的地方，电阻率被广泛用作水合物含量的替代物，并且使用可控源电磁方法测量电阻率显示出了可观的前景。但是，根据阿奇定律，电阻率通常与水合物含量/过剩度有关，这是一种没有物理基础的经验关系，已证明对于含水合物沉积物是无效的。我们基于对 udHashin-Shrikman导电边界的几何校正应用，开发了一种适用于含水合物沉积物的电有效介质方法，该方法有效，并通过对已知孔隙度的人工沉积物进行电阻率测量来测试该方法。我们已经调整了处理各向异性 udgrains（例如粘土颗粒）的方法，并将其与公认的地震有效介质 udmethod结合起来，以开发基于地震和电学结合的海洋沉积物水合物含量估算策略方法。我们已将我们的方法应用于来自温哥华 udIsland边缘的“综合海洋钻探计划”远征311的井眼/预算物理数据。通过调整测井仪中不存在水合物的区域的测压系数，从电阻率测井仪确定水合物饱和度。然后将该值用于整个电阻率测井。使用这种方法确定的水合物饱和度与直接测量加压岩心甲烷含量所确定的饱和度非常匹配。

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