Marine magnetotellurics for petroleum exploration Part I: A sea‐floor equipment system

Abstract

Induction in electrically conductive seawater attenuates the magnetotelluric (MT) fields and, coupled with a minimum around 1 Hz in the natural magnetic field spectrum, leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1000 s. For this reason the marine MT method traditionally has been used only at periods of 103 to 105 s to probe deep mantle structure; rarely does a sea‐floor MT response extend to a 100-s period. To be useful for mapping continental shelf structure at depths relevant to petroleum exploration, however, MT measurements need to be made at periods between 1 and 1000 s. This can be accomplished using ac-coupled sensors, induction coils for the magnetic field, and an electric field amplifier developed for marine controlled‐source applications. The electrically quiet sea floor allows the attenuated electric field to be amplified greatly before recording; in deep (1-km) water, motional noise in magnetic field sensors appears not to be a problem. In shallower water, motional noise does degrade the magnetic measurement, but sea‐floor magnetic records can be replaced by land recordings, producing an effective sea‐surface MT response. Field trials of such equipment in 1-km-deep water produced good‐quality MT responses at periods of 3 to 1000 s; in shallower water, responses to a few hertz can be obtained. Using an autonomous sea‐floor data logger developed at Scripps Institution of Oceanography, marine surveys of 50 to 100 sites are feasible.

Asperity along the North Anatolian Fault imaged by magnetotellurics at Düzce, Turkey

“…geophysical studies such as geothermal systems (Ç aglar andİşseven, 2004), active volcanoes (Ogawa et al, 1998;Aizawa et al, 2005), petroleum exploration (Constable et al, 1998), crustal structures (Wei et al, 2001;Wannamaker and Doerner, 2002;, marine studies (Key and Constable, 2002), mantle properties (Jones, 1999;Simpson, 2002), and active faults (Honkura et al, 2000;Unsworth et al, 2000;Ritter et al, 2005;Tank et al, 2005).…”

Parallel goal-oriented adaptive finite element modeling for 3D electromagnetic exploration

“…Along with the rapid development of instrumentation [Constable et al, 1998] and data acquisition techniques, many innovations in the data processing have also been motivated. These newly developed modeling and inversion methods have been applied to various datasets both in 2D and 3D [Börner, 2009, Key, 2012b.…”

Controlled-source electromagnetic and seismic delineation of subseafloor fluid flow structures in a gas hydrate province, offshore Norway

“…They are predominantly manifested by a resistivity contrast of a few Ωm increase compared to the background resistivity (Collett et al 1998). Consequently, the marine controlled-source electromagnetic (CSEM) technique (Constable et al 1998;Constable 2006;Key 2012) was modified to use higher transmitted frequencies and a fixed offset three-axis electric field receiver, to extend the CSEM capability to detect shallow and subtle resistivity anomalies with high resolution (Weitemeyer & Constable 2010). CSEM sounding is highly sensitive to volumetric changes in resistivity (Edwards 2005;Constable 2010), mainly related to pore fluid properties; therefore, it can detect robustly fluid migration pathways and quantify porosity (Harris & MacGregor 2006;MacGregor & Tomlinson 2014;Naif et al 2015).…”