In support of projects for monitoring geomagnetic hazards for electric power grids, we develop a simple mathematical formalism, consistent with the time causality of deterministic physics, for estimating electric fields that are induced in the Earth's lithosphere during magnetic storms. For an idealized model of the lithosphere, an infinite half‐space having uniform electrical conductivity properties described by a galvanic tensor, we work in the Laplace‐transformed frequency domain to obtain a transfer function which, when convolved with measured magnetic field time series, gives an estimated electric field time series. Using data collected at the Kakioka, Japan observatory, we optimize lithospheric conductivity parameters by minimizing the discrepancy between model‐estimated electric field variation and that actually measured. With our simple model, we can estimate 87% of the variance in storm time Kakioka electric field data; a more complicated model of lithospheric conductivity would be required to estimate the remaining 13% of the variance. We discuss how our estimation formalism might be implemented for geographically coordinated real‐time monitoring of geoelectric fields.

S U M M A R YSubmarine mud volcanos at the seafloor are surface expressions of fluid flow systems within the seafloor. Since the electrical resistivity of the seafloor is mainly determined by the amount and characteristics of fluids contained within the sediment's pore space, electromagnetic methods offer a promising approach to gain insight into a mud volcano's internal resistivity structure. To investigate this structure, we conducted a controlled source electromagnetic experiment, which was novel in the sense that the source was deployed and operated with a remotely operated vehicle, which allowed for a flexible placement of the transmitter dipole with two polarization directions at each transmitter location. For the interpretation of the experiment, we have adapted the concept of rotational invariants from land-based electromagnetics to the marine case by considering the source normalized tensor of horizontal electric field components. We analyse the sensitivity of these rotational invariants in terms of 1-D models and measurement geometries and associated measurement errors, which resemble the experiment at the mud volcano. The analysis shows that any combination of rotational invariants has an improved parameter resolution as compared to the sensitivity of the pure radial or azimuthal component alone. For the data set, which was acquired at the 'North Alex' mud volcano, we interpret rotational invariants in terms of 1-D inversions on a common midpoint grid. The resulting resistivity models show a general increase of resistivities with depth. The most prominent feature in the stitched 1-D sections is a lens-shaped interface, which can similarly be found in a section from seismic reflection data. Beneath this interface bulk resistivities frequently fall in a range between 2.0 and 2.5 m towards the maximum penetration depths. We interpret the lens-shaped interface as the surface of a collapse structure, which was formed at the end of a phase of activity of an older mud volcano generation and subsequently refilled with new mud volcano sediments during a later stage of activity. Increased resistivities at depth cannot be explained by compaction alone, but instead require a combination of compaction and increased cementation of the older sediments, possibly in connection to trapped, cooled down mud volcano fluids, which have a depleted chlorinity. At shallow depths (≤50 m) bulk resistivities generally decrease and for locations around the mud volcano's centre 1-D models show bulk resistivities in a range between 0.5 and 0.7 m, which we interpret in terms of gas saturation levels by means of Archie's Law. After a detailed analysis of the material parameters contained in Archie's Law we derive saturation levels between 0 and 25 per cent, which is in accordance with observations of active degassing and a reflector with negative polarity in the seismics section just beneath the seafloor, which is indicative of free gas.

SUMMARY Marine natural source electromagnetic data acquired on continental margins are often of considerable scientific and commercial interest. However, the large conductivity contrast between the ocean and coast causes this type of data to be severely distorted. For a 2‐D coastal model, this distortion is most pronounced for the marine magnetotelluric and geomagnetic response function derived from induced currents flowing parallel to the coast. A maximal distortion occurs for a given period at a specific distance from the coast and causes severe anomalies in the magnitude and phase of the response functions. Based on a modelling study, we empirically relate the characteristic period and characteristic distance to physical parameters such as the ocean depth and the host resistivity. Based on a simple analytical approach, we test these approximations and show that maximum distortion occurs when destructive interference between the ocean and host response is at its highest. While the coast effect causes a large distortion in the marine responses we show through a resolution analysis that it does not mask subsurface conductivity anomalies but in fact increases the sensitivity to the seafloor.

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, although the commercial value of marine mineral deposits has been recognized for decades, 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 concentric transmitting and receiving loops of wire. 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

The self-potential (SP) method detects naturally occurring electric fields which may be produced by electrically conductive mineral deposits such as massive sulfides. Recently, there has been increasing interest in applying this method in a marine environment to explore for seafloor massive sulfide (SMS) deposits which may contain economic resources of base and precious metals. While SMS sites that are associated with active venting and are not buried under sediment cover are known to produce an SP signal, the effectiveness of the method at detecting inactive and sediment-covered deposits remained an outstanding question. We built an instrument capable of recording SP data in a marine setting. We carried out a test of the instrument at the Palinuro Seamount in the Tyrrhenian Sea. Palinuro is one of only a few known sites containing an SMS occurrence which is buried under sediment and not associated with active hydrothermal venting, although diffuse seepage of hydrothermal fluids is known to occur at the site. Elevated electric field strengths recorded in and near the site of previously drilled massive sulfide samples are on the order of 1-3 mV/m. A second zone of high field strengths was detected by us to the north of the drilling area where gravity coring later confirmed the existence of massive sulfides. Our observations indicate that an SP signal can be observed at the site of SMS mineralization even when the mineralized zone is shallowly buried and active hydrothermal venting is not present. These observations could aid in the planning of future marine research expeditions which use the SP method in the exploration of seafloor massive sulfides.