GP13A-01 INVITED 13:40h
Possible Link Between Numerical Modeling of The Lithospheric Deformation and MT Research Fields
There is almost no connection established yet between two rapidly developing fields of the lithospheric research, namely between numerical simulation of the thermo-mechanical processes and MT studies. To compensate for this gap I focus here on the numerical modeling of the strain localization processes at the lithosphere-scale continental transform faults and on prediction of the possible electrical conductivity structures associated with these processes. First, I use a finite-element thermo-mechanical modelling technique in simplified 3D approximation to study effect of lithospheric rheology on localization of the strike slip deformation in continental lithosphere in general case. The numerical model allows for realistic temperature-, stress- and strain- dependant visco-elasto-plastic rheology as well as for the spontaneous self-generation of faults in brittle upper crust and strain localization in the ductile lower crust and upper mantle. The modeling shows that in the case of brittle upper crust and temperature-independent Newtonian rheology of the lower crust and mantle, the strike-slip deformation localizes in the vertical transform fault in the upper crust and within the broad zone (several tens km) in the ductile lower crust and mantle lithosphere, which rapidly widens with depth. No connectivity of the conducting inclusions in the lower crust and upper mantle can be expected in this case. In the case of temperature-dependent viscosity in the lower crust and upper mantle, more strain localization occurs in the ductile part of the lithosphere. If, in addition the viscosity is stress dependent (dislocation creep deformation mechanism), than the strain localization zone in the lower crust and mantle lithosphere becomes relatively narrow (20-30) km with ca. 5 km wide high-strain core in the middle. In this high-strain core the connectivity percolation threshold for the high-conductive inclusions may be achieved. Finely, I discuss the thermo-mechanical models and associated predicted electrical conductivity structures for the two particular continental transform faults, the Dead Sea Transform between the Dead Sea and the Read Sea as well as the San-Andreas Fault system in the San Francisco Bay area.
GP13A-02 INVITED 14:10h
Structure classification from the joint interpretation of seismic and magnetotelluric models
Magnetotelluric (MT) and seismic methods provide information about the conductivity and velocity structure of the subsurface on similar scales and resolutions. The independent electrical and seismic tomograms can be combined, using a classification approach, to map lithologic, tectonic, and hydrologic boundaries. The method employed is independent of theoretical/empirical relations linking electrical and seismic parameters, and based solely on the statistical correlation of physical property models in parameter space. Regions of high correlation (classes) can in turn be examined in the spatial domain. The spatial distribution of these clusters, and the boundaries between them, provide structural information not always evident from the individual models. The method is applied to coincident seismic velocity and electrical resistivity models from two active transform margins. Along the San Andreas Fault, classification studies reveal the strong lithological contrast across the fault, suggesting it is sub-vertical in the upper crust throughout central California. A possible hydrologic boundary is further identified to the northeast of the fault. Classification studies along the Dead Sea Transform reflect the dominant lithologies surrounding the fault, and suggest the fault is again vertical in the upper crust, but offset to the east of the surface trace. There are indications that the basement is uplifted by $\sim 2$ km east of the fault. These results suggest a quantitative, joint interpretation of MT and seismic data can greatly improve our ability to delineate lithologic, tectonic, and hydrologic boundaries, thus overcoming some of the resolution limitations inherent to the MT and seismic methods.
GP13A-03 14:25h
Conductivity Structure Associated with the Yellowstone-Snake River Hotspot Track
The Snake River Plain (SRP) in Idaho marks the path of an active hotspot with a terminus under Yellowstone National Park in Wyoming. In the 2003 and 2004 field seasons we collected magnetotelluric (MT) data at 36 sites in the Snake River Plain (SRP) region to map the conductivity structure of this region. Our objective is to detect conductive anomalies associated with the Yellowstone hotspot, and ultimately, to place limits on the temperature and melt fraction of these features. To this end, we collected long period electrical and magnetic data at 28 sites along a northwest-southeast line stretching from near Challis to the southeast corner of Idaho, at an average spacing of 13 km, and 8 more sites along a perpendicular transect along the SRP, at an average spacing of 30km. We used long period MT equipment from the ElectroMagnetic Study Of the Continents (EMSOC) instrument pool, and broadband field data at 2 sites using SIO's MT equipment. Parkinson vectors for this area point in the direction of the Snake River Plain but in a direction slightly to the southwest of the transect, and not to the northeast where the Yellowstone hotspot currently lies. Initial results of inversions of these data suggest that the mantle in this region is unusually conductive, suggesting that it is anomalously warm, or has a high melt fraction in this region. Detailed mantle structure in this region is, however, partially masked by the presence of highly conductive features at depths from approximately 10-35km. Our preliminary interpretation is that the shallow, conductive features correlate with the subsurface dikes that trend across the SRP and are associated with the extension of the Snake River Plain; deeper conductive features correspond to melt associated with the passage of the Yellowstone hotspot. A better understanding of the complex interaction between the Yellowstone hotspot track and the dynamics of crustal extension at the surface will require a full 3D analysis of MT data, thus we are proposing to collect data at 1 or 2 more transects across the Plain.
GP13A-04 INVITED 14:40h
Broadband Marine Magnetotelluric Exploration of the Crust at a Petroleum Prospect and a Mid-Ocean RIdge
Broadband marine magnetotelluric (MT) instrumentation developed at Scripps Institution of Oceanography enables resolution of electrical resistivity structure at much shallower depths than previously attainable. While marine seismic reflection surveys have routinely surveyed crustal structure on the continental shelves and mid-ocean ridges, traditional marine MT sensors were only capable of measuring long period fields and so MT experiments were limited to studying mantle structure. The introduction of low-noise sensors allows the broadband MT instrument to now measure the shorter period fields that contain information about crustal resistivity structure. We present two case studies of using the broadband MT system at areas previously surveyed with seismic methods. The joint interpretation of both seismic and MT models for these case studies leads to an improved geological interpretation. At Gemini Prospect in the northern Gulf of Mexico we have collected 42 MT sites in a grid over a three-dimensional (3D) resistive salt structure associated with the petroleum prospect. Depth migrated seismic reflection profiles from a 3D seismic survey at Gemini allow for the verification of two-dimensional (2D) MT inversion models. Combined images of the MT resistivity and seismic reflection profiles show that 2D MT can recover that salt body despite its 3D shape. A steeply dipping and overhanging resistive feature correlates with a previously uninterpreted strong reflection and illustrates how MT can constrain structure in regions where the seismic method performs poorly. A thin and shallow resistive feature shown outside the seismic salt volume may indicate a change in porosity or pore fluids associated with a natural trap in the sediments. At the East Pacific Rise near 9{$^\circ50'$}N, a pilot survey using the broadband instruments shows sensitivity to structure at shallower depths than previous ridge MT experiments. Two-dimensional inversion of data from 4 MT sites shows a high conductivity zone located in the crust and shallow mantle that is associated with the ridge magmatic system and that agrees well with seismic tomography studies of a nearby section of the ridge. Resistivities beneath the ridge imply a crustal partial melt fraction of about 1-20%, in accordance with the seismic results. Nearly vertical isotherms calculated from the seismic model imply deep hydrothermal circulation acts to convectively cool the flanks of the partial melt region, however, the MT model suggests that this region is probably limited to within a few kilometers of the ridge axis. We augment this study with a preliminary look at data from our experiment this past February, where we collected 69 additional MT sites along two transects at the EPR near 9{$^\circ$} N.
http://marineemlab.ucsd.edu
GP13A-05 14:55h
A Pilot Marine EM Study of Hydrate Ridge, Oregon.
It has long been proposed (e.g. Nigel Edwards, U.\ Toronto) that EM methods may be able to detect and map gas hydrate, which is more resistive than host sediments and thus provides an electrical target. While the base of hydrates often produces a distinctive seismic signature (the bottom-simulating reflector, or BSR), the gradational upper surface is less well imaged using seismology, and some hydrates are known to exist without a prominent BSR. In August of 2004 the Scripps Institution of Oceanography Marine EM Lab collected magnetotelluric (MT), dipole-dipole controlled source electromagnetic (CSEM), and controlled source magnetotelluric (CSMT) data across Hydrate Ridge, about 70 km offshore Newport, Oregon. Three component electric field data and two component magnetic field data were recorded at 25 evenly spaced sites along a 14.4 km east-west line that coincides with 2D and 3D seismic transects and ODP Leg 204 well-log data. Forward calculations of models with conservative resistivity contrasts suggest that radial mode CSEM electric fields at frequencies of 5 Hz (and up to the 7th square wave harmonic of 35 Hz) will give a measurable electric field response to shallow hydrates at source-receiver ranges between 500 m to 2500 m. An important part of this experiment is to compare well log data with electrical conductivity estimates of hydrate {\it in situ}, unmodified by either drilling or sample collection. We will present the CSEM and (CS)MT data from this new experiment along with preliminary interpretations.
http://www.marineemlab.ucsd.edu
GP13A-06 15:10h
Hydrocarbon Exploration Using Marine Controlled-Source EM
The seafloor dipole-dipole controlled-source EM (CSEM) method, pioneered over 20 years ago by Charles Cox, has recently been applied to the exploration of offshore hydrocarbon deposits. This timing has been driven mostly by the deeper water exploration environment, which favors the CSEM method, and the high cost of deep-water wells, which has reduced the potential barrier associated with non-seismic methods in the industry. The CSEM method is sensitive to thin resistive layers (hydrocarbon reservoirs, but also possibly other, less economical, geological targets) largely (but not totally) because of a galvanic response to vertical electric currents generated by the dipole source. Under industry sponsorship we have developed and built low power, low weight, highly reliable ocean-bottom electromagnetic (OBEM) recorders, and a deep-towed EM transmitter (Scripps Undersea Electromagnetic Source Instrument, SUESI). We have participated in several of the early trials of this method, and continue to provide instrumentation for industrial surveys. We will review the principles of the method and instrumentation, the sensitivity of CSEM to targets of interest, and present representative data sets.
http://www.marineemlab.ucsd.edu
GP13A-07 15:25h
Anisotropy of Point Defect Mechanisms and Electrical Conduction in Single Crystal Olivine
Understanding how the relative mobility of major electrically conducting point defects in olivine varies with orientation is crucial in interpreting conductivity-depth profiles of anisotropic regions of the upper mantle. We performed electrical conductivity and thermopower measurements, over mantle temperatures (1100-$1400\deg$C) and controlled oxygen fugacity, on San Carlos olivine samples, oriented along the three principle crystallographic directions cut from a single crystal. Fe-soaked Pt electrodes were used to prevent significant Fe exchange or loss in the samples; electron probe measurements before and after experimental runs prove the effectiveness of this method. Conductivity results, for various f$_{O2}$ values at $1200\deg$C, range between 1.49x10$^{-3}$ to 6.63x10$^{-3}$ S/m, which is about 2.5 times higher than previous studies on single crystal San Carlos olivine in which Ir or pure Pt electrodes were used. In accordance with previous studies, our measurements generally indicate that \sigma$$_{[001]}$ $>$ \sigma$$_{{[100]}$ $>$ $\sigma$$_{[010]}$. At a given set of conditions, conductivity varied 50-100% higher in $\sigma$$_{[001]}$ than for $\sigma$$_{[010]}$. Our thermopower measurements, ranging between 3.06x10$^{-4}$ to -0.79x10$^{-4}$ V/K, can be modeled to obtain valuable information about point defect mechanisms. Thermopower of olivine along the [001] direction decreases with increasing temperature more quickly than for other directions, crossing to negative values at log(f$_{O2}$) between -6.3 to -5.9 log units (atm) for $1315\deg$C. Negative thermopower indicates that a negatively charged defect dominates electrical conduction, which confirms previous predictions of mixed-conduction models of polaron and magnesium vacancies. This work was supported by Laboratory Directed Research and Development funding, and was performed by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.