Quantification of marine sulfide deposits using marine electromagnetic methods

Electromagnetic methods are commonly employed in exploration for land based mineral deposits. A suite of airborne, land and borehole electromagnetic techniques consisting of different coil and dipole configurations have been developed over the last few decades for this purpose. In contrast, because the commercial value of marine mineral deposits has only recently been recognized, the development of suitable marine electromagnetic methods for mineral exploration at sea is still in its infancy. We examine conventional marine electromagnetic techniques (i.e. the electric dipole-dipole configuration) for their use in seafloor mineral exploration. Small scale targets such as sulfide deposits require short transmitter-receiver offsets to be properly imaged. Modeling and sensitivity analyses show that to obtain responses that are sensitive to sulfide structure at these small offsets, the transmitter must be positioned on or very close to the seafloor. This implies that conventional marine electromagnetic systems employing a source flown in the seawater column are not suitable for small scale exploration. One particularly interesting method which could be used to image a mineral deposit on the ocean floor is the central loop configuration. Central loop systems consist of a concentric transmitting loop of wire and a receiving coil. While these types of systems are frequently used in land-based or airborne surveys, to our knowledge neither system has been used for marine mineral exploration. The advantages of using central loop systems at sea are twofold: (1) simplified navigation, since the transmitter and receiver are concentric and (2) simplified operation, since only one compact unit must be deployed. In this paper we produce layered seafloor type curves for two particular types of central loop methods: the inloop and coincident loop configurations. In particular, we consider models inspired by real marine sulfide exploration scenarios consisting of overburdens 0m to 5m thick overlying a conductive sulfide body 5m to 30m thick. Modeling analysis shows that using a 50m
²
transmitting loop, these two configurations are useful tools to determine both the overburden and conductor thickness. In this case, absolute voltage errors on the order of ten nanovolts are required to resolve the base of a 30m thick sulfide deposit, while the noise floor may be as much as five orders of magnitude higher to determine the sedimentary overburden thickness to the conductor.