Environmental impact assessment has become an important issue for deep-sea resource mining. The International Seabed Authority has recently developed recommendations for guidelines on environmental assessment of resource mining effects. Several research and development groups have been organized to develop methods for environmental assessment of the seafloor and sub-seafloor under the “Zipangu in the Ocean program,” a part of the Cross-ministerial Strategic Innovation Promotion Program managed by the Cabinet Office of the Japanese government. One attempt planned for long-term environment and sub-seafloor structure monitoring uses a cabled observatory system. To support this observatory plan, we began development of a system to monitor the sub-seafloor resistivity and self-potential reflecting the physicochemical properties of ore deposits and the existence of hydrothermal fluid. The system, which mainly comprises an electro-magnetometer and an electrical transmitter, detects spatio-temporal changes in subseafloor resistivity and in self-potential. Because of the project’s policy changes, cabled observatory system development was canceled. Therefore, we tried to conduct an experimental observation using only a current transmitter and a voltmeter unit. Data obtained during three and a half months show only slight overall apparent resistivity variation: as small as 0.005 Ω-m peak-to-peak. The electrode pair closest to the hydrothermal mound shows exceptionally large electric field variation, with a semidiurnal period related to tidal variation. Results indicate difficulty of explaining electric field variation by seawater mass migration around the hydrothermal mound. One possibility is the streaming potential, i.e., fluid flow below the seafloor, in response to tides. However, we have not been able to perform rigorous quantitative analysis, and further investigation is required to examine whether this mechanism is effective. The system we have developed has proven to be capable of stable data acquisition, which will allow for long-term monitoring including industrial applications.
SMS (Seafloor Massive Sulfide) deposits, including rare and precious metals, have been often found out in the vicinity of hydrothermal active areas: for example in the Okinawa Trough, Izu-Bonin arc, and Azores triple junction in the mid-Atlantic ridge. Geophysical explorations with electromagnetic (EM) methods are recently carried out around the SMS deposits in Papua New Guinea because the SMS deposits are known as low resistivity material. The resistivity distribution below the seafloor can be a good indicator for the buried SMS deposits. However, the resistivity structure below hydrothermal active areas has not been clearly investigated. In this study, we developed a 2D inversion of a marine deep-towed DC resistivity survey as an effective tool to the exploration of SMS deposits. The finite-difference method is applied in the forward modeling calculation to solve potential difference between the electrodes at the electric current injection to the seawater. The real distributions of SMS deposits are concentrated at narrow (thin) zones. Therefore, we add a guided Fuzzy CMeans (FCM) clustering constraint into the objective function in the inversion procedure in order to obtain sharpchange of resistivity matching the petrophysical information. Although the two balance parameters in this objective function should be controlled in the inversion, the optimal ways to adjust these parameters have not been proposed. Therefore, we propose an algorithm to choose these parameters properly. The new algorithm is based on the two stages; the first stage with the ordinary Occam scheme, then the second stage with the FCM clustering constraint where the inverted model in the first stage is used as the initial model in the second stage. The other parameters are decided with the searching scheme. The refined inversion results show that the inversion can produce sharp boundaries in resistivity structure, which can also handle with the realistic petrophysical information. Finally, we applied the inversion code to the field data obtained at the Okinawa Trough; the inverted resistivity anomalies are consistent with the known geological investigations of hydrothermal fluid flows and the observed distributions of seafloor venting sites. This paper was withdrawn from the Technical Program. It was not presented at the 87th SEG Annual Meeting.
Japan has the sixth largest exclusive economic zone area in the world. It is becoming increasingly clear that seabed resources of the exclusive economic zone (EEZ) in Japan. They include mineral resources as well as important widely and abundantly distributed energy resources, such as methane hydrate. In recent years, autonomous underwater vehicles (AUV), which can obtain data of higher definition than survey vessels, are useful even for the survey of the ocean bottom resources. However, the survey area of an AUV is narrow and survey volumes are small.
In the present paper the writers discuss the stratigraphy, tectonics, geophysics, and geochemistry of the Cretaceous of the Ishikari coal fields.The area discussed here in is situated in the central part of Hokkaido. The Cretaceous consists of the Sorachi, Lower Ezo, Middle Ezo, Upper Ezo, and Hakobuchi groups in descending order, ranging Neocomian to Maestrichtian. They are overlain unconformably by the older Paleogene Ishikari group including coal measures and younger Paleogene marine Poronai group.In the northern area predominate pararell folds, of which the Sorachi anticline is the largest, with a general trend of N-S direction. In the southern area, on the contrary, a number of isoclinal or overturned foldings with the same general trend are developed by the more intense tectonic disturbance. Most of these folded structures, are well coinsident to high gravity anomalies.The Cretaceous may be promising for future oil production because of its occupying wide areal distribution and large amount of sediments accompanied with many oil seepages, although the sediments contain smaller content of residual hydrocarbons compared with those of the oil producing areas of Niigata prefecture.The MITT Sorachi, the first test boring well exploring the whole sequences of the Cretaceous and starting in June, 1970, will reveal the more detailed oil-geologic features of the area.
Detecting resistivity and self-potential (SP) anomalies is useful for the exploration of hydrothermal deposits.Using autonomous underwater vehicles (AUVs) can increase survey effectiveness because it allows stable posture control without a towing wire cable from a ship.We propose a new style for geophysical surveys using multiple AUVs without a towing electrode cable for marine direct current resistivity (MDCR) and SP survey.We used two AUVs for electrical signal transmission and their receiver.We successfully conducted MDCR and SP surveys in hydrothermal deposit areas using two AUVs with 20 m tow-rods.One AUV was assisted by an autonomous surface vehicle (ASV) for monitoring and controlling via satellite and the public broadband mobile communications radiowave.The survey covered an area of about 1 square kilometer, spending only 4 hours near the seafloor with the vehicle's speed maintained at 2 -2.5 knots at distance between the AUVs of 200 -300 m during most of the survey.The SP and apparent resistivity were calculated along the main survey line crossing known hydrothermal mounds of sulfide ore.The distribution of the negative SP anomalies obtained in the dive is similar to that obtained from our earlier survey using a deep-tow.The apparent resistivity is generally low (0.2 ohm-m or less) above the mounds.The averaged distance between the vehicles and the averaged altitude are respectively about 250 and 70 m.Therefore, the estimated apparent resistivities are the averaged value to several tens of meters below the seafloor.These positions show good agreement with the locations of known hydrothermal deposits.
