Geology and Geophysics pages

Geology & Geophysics Group

Seafloor electromagnetic geophysics

Contact: Prof. Martin Sinha

Controlled Source Electromagnetic (CSEM) surveying is a powerful geophysical tool for mapping electrical resistivity in geological structures beneath the sea floor.  The method is based on towing a high-powered, low frequency electromagnetic transmitter (such as NOC’s DASI-2 system) close to the sea bed; and measuring the resultant signals at an array of autonomous ocean-bottom receivers.  As the signals propagate through the sea floor, their amplitude and phase are modified depending on the resistivity of the rocks through which they pass.  By measuring these characteristics of the signals at numerous recivers, and for many transmitter positions, it is possible to build up a two- or three-dimensional image of the electrical resistivity structure of the underlying crust. The depth of investigation can be up to 5 km beneath the sea floor, and our instrument systems can operate in water depths up to 6,000 m.

CSEM survey cartoon

Images above - top: Schematic diagram of a CSEM survey; bottom: The layout of a CSEM survey - receiver positions and transmitter tows - superimposed on the bathymetry of the Lucky Strike segment of the Mid-Atlantic Ridge (Cruise CD120).

The resistivity of rocks is a particularly useful physical property to determine in areas where fluids determine either the dynamic behaviour of a geological system or the economic value of a formation.  This is because in most geological settings, crustal resistivity is governed primarily by the nature and distribution of pore fluids. 

The mid-ocean ridge ridge (or indeed any volcanic setting) is an example of a dynamic, fluid-rich geological setting in which conductive fluids play a dominant role.  Here, crustal resistivity is chiefly governed by the accumulation of conductive molten material in magma chambers, or its distribution in regions of partial melt; and by the ingress of conductive sea water which is then heated, modified chemically and ultimately vented through the process of hydrothermal circulation. 

CSEM modelling

Above left: Geophysical effective medium modelling.  The surfaces of misfit for seismic and electrical properties in terms of fracture porosity and fracture aspect ratio, for a location on the Valu Fa Ridge, Lau Basin, SW Pacific. Above right: 3D modelling of electric field amplitudes.  This shows a vertical cross section through a sedimentary structure.  The transmitter is located at the centre, and a resistive block beneath the sea floor – offset to the right relative to the transmitter – represents a buried hydrocarbon reservoir.

A second example is the accumulation of hydrocarbons (which might be oil, natural gas or methane hydrates) in sedimentary formations.  Submarine sediments typically have quite high porosities, and this is usually saturated with saline formation water, making the rocks quite conductive (they typically have resitivities two to ten times that of sea water).  However hydrocarbons have much higher resitivities, leading to a large increase in the bulk resistivity of reservoir formations.  This can be detected and mapped by the CSEM method, making this an important tool both in commercial exploration for oil and gas and in research aimed at investigating the formation and distribution of submarine methane hydrates.  Other resistive formations – such as basalt layers, salt bodies and some carbonate formations – can also be usefully investigated for exploration purposes using CSEM surveying.

The Seafloor EM research team at NOC is active in all of these areas.  We have excellent facilities for carrying out CSEM survey work at sea, and our research projects include:

  • Investigating mid-ocean ridge systems, and the relationship between magmatic and hydrothermal processes
  • Developing improved imaging and inversion methods for the analysis and interpretation of CSEM data
  • Developing CSEM methods for the appraisal and monitoring of reservoirs, and the study of methane hydrates
  • Developing improved laboratory-based and theoretical models for the relationships between electrical resistivity, seismic properties and the distribution and nature of pore fluids in multi-phase geological formations
  • Developing improved instrumentation systems and survey design methods for CSEM surveying

In 2002, our research team’s work in developing the CSEM method over many years and in applying it to hydrocarbon exploration led to the formation of a spin-out company, Offshore Hydrocarbon Mapping plc (OHM).  We continue to work closely with OHM on a number of research projects. In 2005, the team who founded OHM were finalists in the Royal Academy of Engineering’s competition for the MacRobert Award – the UK’s most prestigious prize for innovation – for our work on the industrial application of CSEM surveying methods.

 

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