Transient electromagnetic assessment of seafloor mineral deposits: a model study

NS33B-1594 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. One particularly interesting electromagnetic 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 mineral exploration scenarios consisting of overburdens 0m to 5m thick overlying a conductive ore body 5m to 30m thick. Modelling and resolution analyses show that, using a transmitting loop with 20A of current and a 50 meter square area, these two configurations are useful tools to determine both the overburden depth to a conductive ore deposit and its thickness. In this case, absolute voltage errors on the order of ten nanovolts are required to resolve the base of a 30m thick ore deposit, while the noise floor may be as much as five orders of magnitude higher to determine the sedimentary overburden thickness to the conductor