A method of electromagnetic surveying of an area of seafloor that is thought or known to contain a subterranean hydrocarbon reservoir is described. The method includes broadcasting an EM signal from a horizontal electric dipole (HED) transmitter and obtaining vertical electric dipole (VED) response data at a remote receiver in response thereto. Survey data are analysed by comparing the VED response data with background data which are not sensitive to the postulated hydrocarbon reservoir. Accordingly, differences between the VED response data and the background data allow for the identification of buried hydrocarbon reservoirs. The background data may be provided by magneto-telluric surveying, controlled source electromagnetic surveying or from direct geophysical measurement. By employing VED response data in this way, surveys may be performed in shallower water than has previously been possible since the VED detector is not sensitive to air-wave components of the EM field induced by the HED transmitter at the VED detector.

Electromagnetic surveying for hydrocarbon reservoirs

The exploration community, from the earth scientist to the investment manager, has surely recognized the recent surge of interest in the use of controlled source electromagnetics for offshore hydrocarbon detection and assessment. The targets, petroleum, natural gas and gas hydrate, are resistive zones in an otherwise conductive background. I trace the academic and commercial development of marine methods from basic theory through experimental design to the few published relevant exploration case histories.

/pdf/marine-controlled-source-electromagnetics-principles-4ue18ngrqp.pdf

Marine Controlled Source Electromagnetics: Principles, Methodologies, Future Commercial Applications

Submarine permafrost degradation rates may be determined by a number of interacting processes, including rates of sea-level rise and coastal erosion, sea-bottom temperature and salinity regimes, geothermal heat flux and heat and mass diffusion, and flow processes within the sediment column. Observations of ice-bearing permafrost in shelf sediments are necessary in order to determine its spatial distribution and quantify its degradation rate. We tested the use of direct current electrical resistivity to ice-bearing permafrost in Elson Lagoon north-east of Barrow, Alaska (Beaufort Sea). A sharp increase in electrical resistivity was observed in profiles collected perpendicular to and along the coastline and is interpreted to be the boundary between ice-free sediment and underlying ice-bearing submarine permafrost. The depth to the interpreted ice-bearing permafrost increases from <2 m bsl to over 12 m bsl with increasing distance from the coastline. The dependence of the saline sediment electrical resistivity on temperature and freezing was measured in the laboratory to provide validation for the field measurements. Electrical resistivity was shown to be effective for detection of shallow ice-bearing permafrost in the coastal zone. Historical coastal retreat rates were combined with the inclination of the top of the ice-bearing permafrost to calculate mean vertical permafrost degradation rates of 1 to 4 cm a-1.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2011JF002088

Geoelectric observations of the degradation of nearshore submarine permafrost at Barrow (Alaskan Beaufort Sea)

SUMMARY In recent years, marine controlled source electromagnetics (CSEM) has found increasing use in hydrocarbon exploration due to its ability to detect thin resistive zones beneath the seafloor. Although it must be recognized that the quantitative interpretation of marine CSEM data over petroleum-bearing formations will typically require 2-D surveys and 2-D or 3-D modelling, the use of the 1-D approximation is useful under some circumstances and provides considerable insight into the physics of marine CSEM. It is the purpose of this paper to thoroughly explore the 1-D solutions for all four fundamental source types—vertical and horizontal, electric and magnetic dipole (VED, HED, VMD and HMD)—using a set of canonical reservoir models that encompass brine to weak to strong hydrocarbon types. The paper introduces the formalism to solve the Maxwell equations for a 1-D structure in terms of independent and unique toroidal and poloidal magnetic modes that circumscribe the salient diffusion physics. Green’s functions for the two modes from which solutions for arbitrary source current distributions can be constructed are derived and used to obtain the electromagnetic (EM) fields produced by finite VED, HED, VMD and HMD sources overlying an arbitrary 1-D electrical structure. Field behaviour is analysed using the Poynting vector that represents the time-averaged flow of energy through the structure and a polarization ellipse decomposition of the triaxial seafloor EM field that is a complete field description. The behaviour of the two EM modes using unimodal VED and VMD sources is presented. The paper closes by extending these results to the bimodal HED and HMD sources.

/pdf/on-the-electromagnetic-fields-produced-by-marine-frequency-55hs7laqdp.pdf

On the electromagnetic fields produced by marine frequency domain controlled sources

In reviewing seafloor induction studies conducted over the last seven years, we observe a decline in single-station magnetotelluric (MT) experiments in favour of large, multinational, array experiments with a strong oceanographic component. However, better instrumentation, processing techniques and interpretational tools are improving the quality of MT experiments in spite of the physical limitations of the band limited seafloor environment, and oceanographic array deployments are allowing geomagnetic depth sounding studies to be conducted. Oceanographic objectives are met by the sensitivity of the horizontal electric field to vertically averaged motional currents, providing the same information, at much greater reliability and much lower cost, as an array of continuously operating current meter moorings.

Marine electromagnetic induction studies

A novel variation of the geophysical technique known as MOSES, for Magnetometric Off-Shore Electrical Sounding, has been developed to map the electrical properties of the sea floor in Aretic regions. The particular target is the permafrost layer under the Beaufort Sea, a layer containing frozen or partially frozen sediment from 100 to 600 m thick underlying shallow sea water, typically 10 to 100 m deep, and several tens of metres of soft sediment. A detailed knowledge of the location and physical properties of the permafrost layer is essential for accurate interpretation of reflection seismic data. The permafrost can contain pockets, regions or layers of gas hydrate. The latter is both a possible resource and a hazard to drilling operations or hydrocarbon production. A local map of the permafrost zone is essential geotechnical information required prior to the construction of an offshore structure or pipeline.

P. A. Wolfgram

Papers

The ICE-MOSES experiment: mapping permafrost zones electrically beneath the Beaufort sea