Feasibility study of marine controlled-source electromagnetic sounding for submarine massive sulphides explorations

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  • 人工信号源電磁探査法の海底熱水鉱床への適用可能性
  • ジンコウ シンゴウ ゲン デンジ タンサホウ ノ カイテイ ネッスイ コウショウ エ ノ テキヨウ カノウセイ

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Abstract

 In conventional marine controlled source electromagnetic (CSEM) survey, we need a vessel towing an electromagnetic (EM) transmitter near the seafloor using a long cable. However, in practice, we must tow cables at some height from seafloor because of rough topography (e.g, chimneys) around submarine massive sulphides (SMS). Therefore, it is difficult to get information about shallow sub-seafloor structure. In this research, we propose a new marine CSEM method to solve this problem using two autonomous underwater vehicles (AUV). Using this method, we could keep the diving two AUVs much closer to the seafloor than in the conventional marine CSEM, and we can effectively carry out the exploration of the SMS.<br>  In CSEM method, the behavior of electric and magnetic fields are determined by the arrangement of source dipoles against survey target. Therefore, it is important to consider where to place the transmitter and the receivers to the structure including low resistivity anomaly. In this study, we discussed the feasibility of our new two-AUV CSEM method employing a 2.5-D FEM forward program for solving EM propagation near the seafloor.<br>  Our analysis of electromagnetic field has revealed that the received electric and magnetic fields become weakened steeply as the offset from the transmitter to the receiver increases when the source dipole is placed near the SMS. We have also found that the change in the amplitude of electromagnetic anomalies decreases in proportion to the thickness of the SMS. Even in the contamination of noise, we found that it is possible to detect the electromagnetic field for about 200m offset. Our numerical calculations yield the following considerations: (1) the location of source dipole and the change in the amplitude of electromagnetic anomalies could be used to detect the rough horizontal extent of the SMS, and (2) the attenuation in the electromagnetic field could indicate the rough thickness of the SMS. Therefore, our study implies that the proposed two-AUV CSEM method would bring a way to detect the electromagnetic anomalies caused by the existence of the SMS.<br>

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