Summary
In time-domain electromagnetic (TDEM) measurements, secondary fields which contain information on conductive targets in the seafloor can be measured in the absence of strong primary fields. The TDEM system is advantageous to the development of compact, autonomous instruments which are well suited to submersible-based surveys. Since electrical conductivity of subseafloor materials can be frequency dependent, these induced polarization (IP) effects may affect the reliability of TDEM data interpretation. In this paper, we investigate IP effects on TDEM responses of deep-sea hydrothermal deposits with a thin sediment cover. Time-domain target signals are larger and appear earlier in horizontal magnetic fields than in vertical ones. IP effects cause transient magnetic fields to enhance initially, to decay rapidly and then to reverse the polarity. The DC conductivity and IP chargeability in Cole-Cole parameters influences the time of sign reversal and the enhancement of the target response, simultaneously. The reversal time is almost invariant with the time constant while the target signal is almost invariant with the frequency exponent.
Introduction
A marine controlled-source electromagnetic (CSEM) survey has already become a popular tool for hydrocarbon exploration (Constable, 2010). Possible targets of the marine CSEM survey, other than hydrocarbon, may be a shallow hydrothermal mineral deposit under the deep sea. Active seafloor massive sulfide (SMS) deposits are found in as many as a dozen different tectonic settings such as the mid-ocean ridge, volcanic arcs, and back-arc spreading systems (Hoagland et al., 2010). In contrast, inactive SMS deposits may be a difficult survey target to find because they have no detectable emissions (Dover, 2011).
For the exploration of hydrothermal deposits, there was a recent trial using a bi-static EM sensor mounted on a remotely operated vehicle (Goto et al., 2011). The timedomain EM (TDEM) system may be useful to the development of compact, autonomous instruments which are well suited to submersible-based surveys (Nakayama and Saito, 2014). Jang et al. (2014) demonstrated the possibility of applying an in-loop TDEM system to the detection of marine hydrothermal deposits with a thin sediment cover through a one-dimensional (1D) modeling and inversion study.
When interpreting most EM data, electrical conductivity can normally be assumed to be independent of frequency or delay time. In most cases this assumption is valid for the frequency band utilized in typical EM systems (approximately 10 Hz to 10 kHz). Where variation in electrical conductivity with frequency exists, the conductivity tends to increase with increasing frequency. These induced polarization (IP) effects may affect the reliability of TDEM data interpretation (Nabighian and Macnae, 1991). It is well-known that marine hydrothermal deposits reveal an obvious IP effect (Goto et al., 2011).
