Exploration of the Lithosphere and Upper Mantle Using Passive Electro-Magnetic Receivers

Exploration of the Lithosphere and Upper Mantle
Using Passive Electro-Magnetic Receivers

Calif Tervo

24 April 2013

Prepared for ES767 Global Tectonics

Dr. J. Aber, Emporia State University


Deploying Ocean Bottom Electro-Magnetic Receiver (OBEM). Courtesy of Scripps Institute of Oceanography.

Contents:

AbstractBackgroundMT MethodMagma at RiftMagma Layer at Subduction ZoneMT ReceiverPatentsConclusionsReferences

Abstract

Exploration of the lithosphere and upper mantle has been improved with advancement in broadband passive marine magnetotelluric (MT) sensing using methods and receivers developed in 1996 at Scripps Institute of Oceanography (Scripps) at University of California, San Diego and refined since then. The MT method, using receivers deposited on the ocean floor, can estimate the electrical conductivity of the subsurface geology at depths to hundreds of kilometers, depending on the conductivity of the geology. Exemplary scientific uses of the MT method include the mapping in 2004 of magma beneath an undersea volcano on the East Pacific Rise off central America and the mapping in 2010 of a magma layer near the Cocos plate subduction off the coast of Nicaragua. The sensing device and methods are the subject of several patents.

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Background

Figure 1: Marine Controlled Source Electro-Magnetic Method (CSEM).


Perspective schematic of a CSEM method. Courtesy of EMGS, © EMGS (emgs.com)

One conventional method of sub-seafloor exploration, as shown in Figure 1, is a controlled source electro-magnetic (CSEM) survey wherein a deepsea tow vehicle , shown in yellow, towed by a ship so as to be near the seafloor, trails a long horizontal electric-field transmitter in the form of a dipole antenna, shown in black,. The antenna broadcasts a controlled EM field, typically of between 0.1 and 10 Hz, which penetrates the earth. This transmitted EM field is modified by the electrical resistance of subsurface layers. An array of electromagnetic receivers, placed on the seafloor at various ranges from the transmitter, receive and record signals indicative of this modified EM field in the subsurface. Subsurface formations containing hydrocarbons are highly resistive compared with surrounding formations. Therefore, a CSEM survey can indicate the presence of oil and gas. However, because CSEM has a typical penetration depth of only about 10 kilometers, it is of limited value in exploration of plate tectonics.

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MT Method

Magnetotellurics (MT) is a passive method using naturally occurring variations in the Earth’s electromagnetic fields resulting from interaction of the solar wind and Earth's magnetosphere to determine the electrical resistivity of subsurface geology. The interaction of the solar wind and Earth's magnetosphere generates a plane-wave magnetic field that propagates through the atmosphere and into Earth. As the magnetic field enters the Earth, it attenuates at a rate proportional to the electrical conductivity of the subsurface and induces an electric field, the strength of which also depends on the subsurface conductivity.


Deploying Ocean Bottom Electro-Magnetic Receiver. Courtesy of Kerry Key, Scripps.

An array of seafloor EM receivers, such as explained in more detail below in section “MT Receiver”, are deployed from a ship. The MT receivers may be very similar to CSEM receivers with the main difference being sensitivity to a different frequency range. The MT receivers record time series measurements of orthogonal components of the electric and magnetic fields of subsurface layers. This data is processed to calculate the impedance tensor of subsurface layers. This impedance, observed over a broad band of frequencies and over the surface, determines the electrical conductivity distribution beneath that surface. The impedance tensor is used to create a subsurface resistivity model. Resistivity is also affected by temperature, pressure, saturation with fluids, structure, texture, composition, and electrochemical parameters. Resistivity information may be used to map major stratigraphic units, determine relative porosity or support a geological interpretation. An MT survey may be performed in addition to seismic, gravity, and magnetic data surveys.

Lower EM frequencies penetrate deeper into the seafloor. Therefore, obtaining signals from greater depth requires measuring lower frequencies, which in turn requires longer recording times. It may take days or weeks to process meaningful data from the lower crust or upper mantle. MT receivers typically receive signals in the 10 kHz to 0.0001 Hz range and have depth sensitivity reaching hundreds of kilometers, depending on the conductivity of the geologic structures. In the higher frequency range of about 1 Hz to 10 Hz, the MT method may be used for hydrocarbon detection in acquisition depths. In the lower frequency ranges, the MT method may be used to probe the upper mantle. For two-dimensional MT surveys, the receivers are placed longitudinally and provide a vertical slice of subsurface resistivity. For three-dimensional MT surveys, the receivers are placed in a grid pattern and more processing is involved to create a three dimensional image of subsurface resistivity.

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Magma at Rift

Researchers at Scripps have been working for decades to develop low frequency EM receivers and data processing software to apply MT to exploration for oceanic hydrocarbon reservoirs. In 2004, Scripps researchers aboard the research ship Roger Revelle deployed MT receivers traversing the East Pacific Rise between the Clipperton transform fault and the Siqueiros transform fault off the coast of Central America. See Figure 2. The Roger Revelle is operated by Scripps and owned by the U.S. Navy. According to one of the researchers, Kerry Keys of Scripps, "This was the largest project of its kind, enabling us to image the mantle with a level of detail not possible with previous studies" (Scripps Institute of Oceanography, 2013a). The study was supported by the National Science Foundation and the Seafloor Electromagnetic Methods Consortium at Scripps. The development of the marine EM receivers began at Scripps in the 1960s mainly under the direction of Charles "Chip" Cox and has been advanced in recent years by Scripps researchers Steven Constable and Kerry Key (see Patents section, below) (Scripps Institute of Oceanography, 2013a).

Figure 2: Location of the MT survey across the fast spreading East Pacific Rise.


Twenty-nine sea-floor MT receivers, depicted as white circles, were deployed across the ridge axis at 9°30'N, about 1,000 km southwest of Central America (inset boxed in red). The Pacific and Cocos Plates diverge symmetrically (black arrows) while the entire ridge system migrates to the northwest relative to a fixed hotspot reference frame (grey arrow). The Clipperton and Siqueiros transform faults (TF) bound this ridge segment to the north and south. The color scale shows sea-floor topography and the 100-km scale bar indicates the half-aperture of the magnetotelluric array. Courtesy of Scripps Institute of Oceanography.

At mid-ocean ridges, upwelling magma melts and produces new oceanic crust. One theory regarding the actual mechanics of this is a passive-flow model that predicts the upwelling results from viscous drag from the separating tectonic plates. Another theory predicts that the upwelling is primarily the result of differentiated mantle pressure and of mantle temperature gradients. This asymmetric flow theory is supported by convective flow modeling using buoyancy and suggests that the upwelling would likely be asymmetric.

Figure 3: Magnetotelluric resistivity image of mantle upwelling beneath the East Pacific Rise.


The deep melting region where magma is generated in the mantle beneath the mid-ocean ridge is shown in cross-section. Green to red colors show regions of partially molten material. The linear array of seafloor receivers are shown as inverted triangles. Shaded colors in the upper panel show the adjacent seafloor topography. Courtesy of Scripps Institute of Oceanography.

Figure 3 shows the results of MT mapping to a depth of about 160 km. At depths of 20-90 km, a fairly symmetric region of high conductivity was found that is indicative of partial melting of upwelling mantle and is consistent with the predictions of the passive-flow model. The magma upwelling directly beneath the mid-ocean ridge occurs in the porous melting region and showed a deeper and broader melting region than previously thought. The scientists were surprised to discover that melting started much deeper in the mantle than expected (Scripps Institute of Oceanography, 2013a).

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Magma Layer at Subduction Zone

In 2010, scientists from Scripps and Woods Hole Oceanographic Institution, operating the research vessel Melville, deployed an array of seafloor MT receivers at the Middle America trench offshore Nicaragua to explore the sub-sea geology. The Melville is owned by the Navy and operated by Scripps. The National Science Foundation and the Seafloor Electromagnetic Methods Consortium at Scripps supported the research. This research project was nicknamed SERPENT (Serpentinite, Extension and Regional Porosity Experiment across the Nicaraguan Trench) (Scripps Institute of Oceanography, 2013b). Figure 4 shows the mapping area.

Figure 4: Map showing the location of the 2010 survey region.


Courtesy of Scripps Institute of Oceanography..

Earth scientists have proposed several theories regarding the forces and conditions that propel the Earth’s tectonic plates over the mantle. Some scientists have postulated that dissolved water in mantle rocks produces a more ductile mantle and facilitates tectonic plate motions. Laboratory studies seem to confirm this, but no field data has been available.

In Figure 5, below, the orange colored area enclosed by a dashed line denotes a magma layer that scientists believe is facilitating the motion of the Cocos plate off Nicaragua. The blue areas represent the Cocos plate sliding across the mantle and eventually diving beneath the Central American continent, while the black dots signify earthquake locations. The discovery was made by analyzing data collected by an array of seafloor MT receivers, shown as inverted triangles.

Figure 5: Diagram of subduction zone of 2010 survey.


Courtesy of Scripps Institute of Oceanography..

As seen in figure 5, a vast array of seafloor MT receivers was deployed. The scientists imaged a 25 kilometer-thick layer of partially melted mantle rock below the edge of the Cocos plate where it moves underneath Central America. The molten layer may be acting as a lubricant for the sliding motions of the plate. Kerry Key, a Scripps researcher, said, "This was completely unexpected. We went out looking to get an idea of how fluids are interacting with plate subduction, but we discovered a melt layer we weren't expecting to find at all. It was pretty surprising." (Scripps Institute of Oceanography, 2013b).

According to Samer Naif, a Scripps scientist, the discovery of the magma layer was counter to the above-mentioned dissolved water theory. "Our data tell us that water can't accommodate the features we are seeing. The information from the new images confirms the idea that there needs to be some amount of melt in the upper mantle and that's really what's creating this ductile behavior for plates to slide." (Scripps Institute of Oceanography, 2013b).

Considerable additional information on this 2010 voyage including insightful narrative and many photographs depicting life aboard a research vessel, additional search results, and information about the Scripps Marine Electromagnetics Laboratory is found at Research Project SERPENT.

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MT Receiver


Diagrammatic perspective view of the seafloor receiver unit.

MT receiver (10) has an aluminum frame (12) mounting a releasable concrete anchor (14), glass spheres (16) for flotation (when anchor (14) is released), a first waterproof pressure case (20) containing a digital data logging processor (22), magnetic field post-amplifiers, electric field amplifiers and a compass unit, a second waterproof pressure case (30) containing an acoustic navigation/release system (32) and the electric field, or telluric, receiver (40) including four booms (50) having Ag-AgCl electrolyte-filled electrodes (52) on their ends for detecting the electric field, and aluminum pressure cases tubes (60) housing magnetic induction coil sensors, i.e., magnetometers (62) for recording the magnetic field vector. Acoustic navigation/release system (32) serves to locate the system by responding to acoustic pings generated by a ship-board unit and also receives commands, including a release command that initiates detachment from the anchor (14) so that the buoyant remainder floats to the surface for recovery. Booms (50) with electrodes (52) are positioned in an "X" configuration to create two orthogonal dipoles to measure the complete vector electric field.

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Patents

Here are two selected related patents, one from early passive MT receiver developments and one more recent.

5,770,945, Title: Seafloor magnetotelluric system and method for oil exploration, Inventors: Steven C. Constable, Assignee: The Regents of the University of California (1998). The invention relates to the use of a magnetotelluric method and system for mapping electrical conductivity of the seafloor.

8,253,418, Title: Method and system for detecting and mapping hydrocarbon reservoirs using electromagnetic fields, Inventors: Steven C. Constable & Kerry W. Key, Assignee: The Regents of the University of California (2012). The invention relates to a method and system for detection and mapping of geologic formations such as seafloor hydrocarbon reservoirs by measuring electromagnetic fields and more particularly to measuring of the electric and/or magnetic field amplitude gradient and phase velocity on the seafloor during controlled source marine electromagnetic surveys.

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Conclusions

In the above rift survey, the melting at the mid-oceanic rift began much deeper than previously thought. The discovery of the size and depth of the magma region in the mantle below the mid-ocean rift provides a greater knowledge of the source of the ridge magma and advances our knowledge of plate tectonics and the workings of the mantle.

In the above subduction zone survey, the discovery of the magma layer provides further knowledge to earth scientists for a better understanding of forces and conditions affecting plate movement and of plate subduction and associated earthquakes. Electro-magnetics (EM) as a geological exploratory tool will likely continue to be enhanced improved and will expand in use. Controlled source electro-magnetics (CSEM) will be used more in exploration for hydrocarbons and to augment seismic techniques.

Because different rocks, sediments, and geological structures have a wide range of different electrical conductivities, further development and refinement of MT will allow more and more different materials and structures to be distinguished from one another. This will improve our knowledge of tectonic processes and geologic structures. EM may have utility discovering underground water supplies.

Passive magnetotelluric (MT) methods show great promise in exploration of the lithosphere and upper mantle. The last decade brought rapid advancement in MT receiver and data processing technology, as shown by the examples discussed above. MT can now be used to make meaningful mappings at greater depths than accessible with controlled source electro-magnetics (CSEM).

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References

Image Credits

  • Permission to use SIO credited material for this report was granted verbally in a teleconference with Shannon Casey, Marketing Manager, Institutional Marketing Coordinator for Scripps Institute of Oceanography on 4/5/13.

  • Permission to use EMGS credited image for this report from the www.emgs.com webpage was granted verbally in a teleconference with EMGS America, Houston, Texas on 4/25/2013.

  • Permission to use Kerry Key credited photographs for this report from the SIO released articles was granted verbally in a teleconference with Kerry Key on 4/16/2013.

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