Geophysical Methods for the Mapping of Submarine Massive Sulphide Deposits

摘要

In 1977, the first black smoker was discovered on the East Pacific Rise. Since this discovery, many morernhydrothermal vent occurrences have been discovered in the deep ocean. These vents are associated withrncompact high grade copper, gold, and zinc deposits that are being actively explored for by national andrnprivate organizations.rnExploration for these deposits usually begins with ship borne sonar mapping and towed waterrnchemistry samplers. From bathymetric maps, potential targets for more detailed mapping with underwaterrnvehicles are defined. Remotely operated vehicle (ROV) and autonomous underwater vehicle (AUV)rnmounted turbidity, pH, and oxidation reduction potential (ORP) sensors, magnetometers, electromagneticrn(EM) systems, high-resolution multibeam echosounder (MBES) bathymetry, sidescan sonar and subbottomrnprofiler surveys are used to delineate the extent and nature of the submarine massive sulfide (SMS)rndeposits.rnEM surveying can be used to determine the resistivity of near-surface and subsurface structure. SMSrndeposits fall into two categories; zinc rich non-conductive and conductive copper-gold deposits. Onernsystem is an ROV-mounted EM system that operates near the seafloor (Kowlaczyk, 2008). It comprisesrna transmitter coil wrapped around the ROV and an electric field sensor mounted close to the ROV. Thisrnsystem has successfully mapped high-conductivity zones corresponding to high grades of copper and gold.rnThis system has detected blind mineralization at a depth of several meters, but does not penetrate beyondrnfive or ten meters into the ocean floor. It does outline the limits of the system accurately, and has beenrnused to direct resource-definition drilling of SMS deposits.rnModelling shows that buried SMS deposits can be mapped using a controlled source electromagneticrn(CSEM) system. A CSEM system consists of an electrical transmitter and one or more electric fieldrnreceivers that are either on the seafloor or towed behind the transmitter. SMS deposits are conductive andrnwill channel electrical current. Since the conductivity of the ocean is constant, electrical fields are a proxyrnfor current density. Modelling shows that SMS targets produce a readily detectable electrical fieldrnanomaly. Using 3D inversion of the CSEM electrical field data, subsurface SMS deposits can be mapped.rnSMS deposits can also be mapped using seismic methods. The depth of water and the deposit geometryrnmake it difficult to deploy standard systems. A new system using a vertical cable array and a surfacernsource has successfully mapped SMS deposits in 3D.rnThis paper will review the use of EM and seismic methods successfully used to map SMS deposits.