[论文解读] 3-D Magnetotelluric Investigations for geothermal exploration in Martinique (Lesser Antilles). Characteristic Deep Resistivity Structures, and Shallow Resistivity Distribution Matching Heliborne TEM Results

Nicolas Coppo, Jean-Michel Baltassat+10 more|arXiv (Cornell University)|Jan 26, 2015

Geophysical and Geoelectrical Methods参考文献 23被引用 10

一句话总结

本研究针对马提尼克的三个地热勘探区开展了三维磁电（MT）反演，揭示了与火山历史相关的显著深层电阻率结构。研究结果表明，尽管存在尺度和静态偏移差异，三维MT建模仍能准确再现浅层电阻率分布，且该结果得到了直升机载TEM数据的验证。

ABSTRACT

Within the framework of a global French program oriented towards the development of renewable energies, Martinique Island (Lesser Antilles, France) has been extensively investigated (from 2012 to 2013) through an integrated multi-methods approach, with the aim to define precisely the potential geothermal ressources, previously highlighted (Sanjuan et al., 2003). Amongst the common investigation methods deployed, we carried out three magnetotelluric (MT) surveys located above three of the most promising geothermal fields of Martinique, namely the Anses d'Arlet, the Montagne Pel{é}e and the Pitons du Carbet prospects. A total of about 100 MT stations were acquired showing single or multi-dimensional behaviors and static shift effects. After processing data with remote reference, 3-D MT inversions of the four complex elements of MT impedance tensor without pre-static-shift correction, have been performed for each sector, providing three 3-D resistivity models down to about 12 to 30 km depth. The sea coast effect has been taken into account in the 3-D inversion through generation of a 3-D resistivity model including the bathymetry around Martinique from the coast up to a distance of 200 km. The forward response of the model is used to calculate coast effect coefficients that are applied to the calculated MT response during the 3-D inversion process for comparison with the observed data. 3-D resistivity models of each sector, which are inherited from different geological history, show 3-D resistivity distribution and specificities related to its volcanological history. In particular, the geothermal field related to the Montagne Pel{é}e strato-volcano, is characterized by a quasi ubiquitous conductive layer and quite monotonic typical resistivity distribution making interpretation difficult in terms of geothermal targets. At the opposite, the resistivity distribution of Anse d'Arlet area is radically different and geothermal target is thought to be connected to a not so deep resistive intrusion elongated along a main structural axis. Beside these interesting deep structures, we demonstrate, after analyzing the results of the recent heliborne TEM survey covering the whole Martinique, that surface resistivity distribution obtained from 3-D inversion reproduce faithfully the resistivity distribution observed by TEM. In spite of a very different sampling scale, this comparison illustrates the ability of 3-D MT inversion to take into account and reproduce static shift effects in the sub-surface resistivity distribution.

研究动机与目标

• 利用三维磁电（MT）反演表征马提尼克地热勘探区的深层电阻率结构。
• 评估海岸效应和静态偏移对复杂火山地形中MT数据解释的影响。
• 将三维MT反演得到的电阻率模型与直升机载TEM数据进行对比，以验证浅层电阻率分布。
• 评估三维MT反演在存在静态偏移效应的情况下，对地下电阻率非均质性捕捉的有效性。
• 通过国家可再生能源计划中的综合地球物理建模，支持马提尼克的地热资源评估。

提出的方法

• 在安斯达雷、蒙特agne佩莱和皮顿杜卡贝特地区开展了三次MT测量，共获取约100个测站的多维响应数据及静态偏移信息。
• 采用远参考处理方法以减轻人为噪声影响，提升数据质量。
• 对全部四个阻抗张量分量执行三维MT反演，未进行预静态偏移校正，以保留天然电阻率分布特征。
• 在三维电阻率模型中整合了海床地形及海岸电阻率效应，模型延伸至距陆地200公里的海域，以考虑海陆效应。
• 通过正演模拟计算海岸效应系数，用于反演过程中调整MT响应，以实现更精确的数据对比。
• 利用覆盖全岛的近期直升机载TEM测量数据，对三维MT结果进行验证，以评估浅层电阻率分布的保真度。

实验结果

研究问题

• RQ1 三维磁电反演如何揭示马提尼克地热区在复杂火山历史背景下的深层电阻率结构？
• RQ2 在存在静态偏移效应和尺度差异的情况下，三维MT反演在多大程度上能准确再现浅层电阻率分布？
• RQ3 海岸及近海电阻率效应对火山岛环境中MT数据解释产生何种影响？
• RQ4 三维MT模型能否在浅层电阻率分布方面可靠地与直升机载TEM数据进行对比验证？
• RQ5 在马提尼克小安的列斯群岛的地质背景下，不同地热勘探区具有哪些独特的电阻率特征？

主要发现

• 蒙特agne佩莱地热区表现出广泛分布的导电层，电阻率呈单调分布，导致难以直接识别地热目标。
• 安斯达雷地区显示出沿主要构造轴线延伸的明显电阻性侵入体，表明可能存在浅层地热目标。
• 三维MT反演成功捕捉了蒙特agne佩莱和安斯达雷地区电阻率结构，反映了其不同的火山与构造历史。
• 尽管数据采集尺度和静态偏移效应存在差异，三维MT反演得到的浅层电阻率模型与直升机载TEM观测结果高度吻合。
• 引入海床地形和海岸效应建模显著提升了沿海区域三维MT反演结果的准确性。
• 本研究证实，三维MT反演能够有效模拟静态偏移效应，并在复杂地质环境中真实再现地下电阻率分布。