Marine dipole-dipole controlled source electromagnetic and coincident-loop transient electromagnetic experiments to detect seafloor massive sulphides: effects of three-dimensional bathymetry
Marine dipole-dipole controlled source electromagnetic and coincident-loop transient electromagnetic experiments to detect seafloor massive sulphides: effects of three-dimensional bathymetry
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DOI：
10.1093/gji/ggy398
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发表时间：
2018-12-01
影响因子：
2.8
通讯作者：
Murton, Bramley J.
中科院分区：
文献类型：
--
作者：
Haroon, Amir;Hoelz, Sebastian;Murton, Bramley J.
海底块状硫化物（SMSs）被认为是一种潜在的未来资源，可以满足全球对铜、锌和金等金属日益增长的需求。除了从海底开采和回收大量有益的块状硫化物外，目前的挑战是探测和描绘重要的SMS积累，这些积累通常位于海洋中脊附近以及海底火山弧和弧后扩张中心。目前，一些地球物理技术正在开发中，用于检测和量化通常与周围围岩在物理参数上存在可测量差异的短粒岩。在这里，我们使用了一个短的、固定偏移的可控源电磁（CSEM）系统和一个重合环瞬变电磁（TEM）系统，理论上，由于浅层海底的电导率与周围环境形成鲜明对比，这两种系统允许在浅层海底检测SMS。2016年，在跨大西洋Geotraverse热液区附近的几个地点进行了CSEM和TEM实验，以调查海底以下块状硫化物的浅层分布。测量是在一个区域进行的，该区域包含不同的SMS站点，位于离大西洋中脊几公里的轴线上，其中一些仍然与热液活动有关，而其他热液活动已经停止。根据获得的数据质量，两次实验都是成功的。然而，数据分析表明，研究区域粗糙水深的三维（3D）效应造成了偏差，因此，数据解释仍然具有挑战性。因此，我们研究了三维测深对海洋CSEM和TEM实验的影响，重点研究了位于丘状结构下的浅层三维导体。我们分析了与CSEM和TEM数据的3D失真相关的属性的综合反演模型，这些属性在传统的1D （TEM）和2D （CSEM）解释方案中没有充分考虑。在对该地区的SMS进行充分量化之前，需要研究这些3D效应，以避免使用所获得的EM数据对SMS进行高估/低估。通过三维正演模拟研究了CSEM和TEM对测深的敏感性，然后在实际误差条件下对合成数据进行了一维（TEM）和二维（CSEM）反演。随后，对合成三维数据的反演模型进行了分析，并与从测量数据中得出的模型进行了比较，以说明在记录数据集中三维变形是明显的。
Seafloor massive sulphides (SMSs) are regarded as a potential future resource to satisfy the growing global demand of metals including copper, zinc and gold. Aside from mining and retrieving profitable amounts of massive sulphides from the seafloor, the present challenge is to detect and delineate significant SMS accumulations, which are generally located near mid-ocean ridges and along submarine volcanic arc and backarc spreading centres. Currently, several geophysical technologies are being developed to detect and quantify SMS occurrences that often exhibit measurable contrasts in their physical parameters compared to the surrounding host rock. Here, we use a short, fixed-offset controlled source electromagnetic (CSEM) system and a coincident-loop transient electromagnetic (TEM) system, which in theory allow the detection of SMS in the shallow seafloor due to a significant electrical conductivity contrast to their surroundings.In 2016, CSEM and TEM experiments were carried out at several locations near the Trans-Atlantic Geotraverse hydrothermal field to investigate shallow occurrences of massive sulphides below the seafloor. Measurements were conducted in an area that contains distinct SMS sites located several kilometres off-axis from the Mid-Atlantic ridge, some of which are still connected to hydrothermal activity and others where hydrothermal activity has ceased. Based on the quality of the acquired data, both experiments were operationally successful. However, the data analysis indicates bias caused by three-dimensional (3D) effects of the rough bathymetry in the study area and, thus, data interpretation remains challenging. Therefore, we study the influence of 3D bathymetry for marine CSEM and TEM experiments, focusing on shallow 3D conductors located beneath mound-like structures. We analyse synthetic inversion models for attributes associated with 3D distortions of CSEM and TEM data that are not sufficiently accounted for in conventional 1D (TEM) and 2D (CSEM) interpretation schemes. Before an adequate quantification of SMS in the region is feasible, these 3D effects need to be studied to avoid over/underestimation of SMS using the acquired EM data. The sensitivity of CSEM and TEM to bathymetry is investigated by means of 3D forward modelling, followed by 1D (TEM) and 2D (CSEM) inversion of the synthetic data using realistic error conditions. Subsequently, inversion models of the synthetic 3D data are analysed and compared to models derived from the measured data to illustrate that 3D distortions are evident in the recorded data sets.