The CSEM method The RAMESSES Experiment III: Summary

Introduction

The electrical resistivity of solid, dry basalt exceeds that of molten basalt or seawater by orders of magnitude, so that seawater penetration into cracks, the presence of hydrothermal systems or the presence of melt will all decrease crustal resistivity. Electrical exploration methods, sensitive to these resistivity variations, thus provide information on the amount, distribution and temperature of fluid present, all important parameters in understanding of processes occurring at mid-ocean ridges.

Controlled source electromagnetic (CSEM) methods utilise time varying electric and magnetic fields from an artificial source. At frequencies sufficiently high that electromagnetic fields are attenuated rapidly in the seawater, energy detected by a receiver remote from the transmitter follows diffusion paths through the crust, and is therefore sensitive to its resistivity structure. The CSEM method provides resolution of resistivity structure on a crustal scale by using higher frequencies than is possible in conventional seafloor MT sounding, in which high frequencies from natural ionospheric sources are strongly attenuated by the water layer. Accounts of the theory and practice of marine electromagnetic methods are given by (for example) Cox (1980), Chave & Cox (1982), Edwards & Chave (1986) and Constable & Cox (1996).

Several CSEM experiments using a horizontal electric dipole source operated in the frequency domain have been performed to study the resistivity structure of normal oceanic crust (Young & Cox, 1981; Cox etal , 1986; Constable & Cox, 1996). The first experiment to be performed over the axis of a mid-ocean ridge was centred at 13°N on the fast spreading East Pacific Rise (Evans etal, 1994). No difference was observed between data collected at the ridge crest on zero age crust and those collected on the ridge flank over 100,000 year old crust. Lack of evidence for the presence of a low resistivity anomaly in the crust led to the conclusion that any melt present must be in the form of small isolated pockets, suggesting that at 13°N the East Pacific Rise is in a state of magmatic quiescence compared to other parts of the ridge.

The experiment described in this paper formed part of the RAMESSES (Reykjanes Axial Melt Experiment: Structural Synthesis from Electromagnetics and Seismics) study (Sinha etal, this issue), which was centred on an axial volcanic ridge segment of the Reykjanes Ridge at 57°45 ' N. The Reykjanes Ridge forms the northern part of the slow spreading Mid-Atlantic Ridge, extending from the Reykjanes peninsula on Iceland at 63°30 ' N to the Bight fracture zone at 56°50 ' N (inset in Figure. 1). The full spreading rate is 20 mm/ yr along 096 (DeMets , 1990), oblique to the overall trend of the ridge. The experiment was designed to study the processes of crustal accretion at a slow-spreading mid-ocean ridge, especially the question of melt delivery to and storage within the crust. A combination of CSEM sounding, MT sounding (Heinson, White & Constable, this issue) and both wide angle and normal incidence seismic profiling (Navin, Peirce & Sinha, this issue) were performed. With its sensitivity to crustal water and melt, the CSEM experiment was a key component of the study, and was centred on an AVR segment characterised by evidence of recent volcanism.

The CSEM method The RAMESSES Experiment III: Summary

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