GP51C-01 INVITED
On one-dimensional mantle conductivity modeling of 3-dimensional Earth
It is well known that the Earthfs conductivity increases by about three orders of magnitude from the surface to the upper-lower mantle boundary, due to vertical changes in physical conditions. When the structure in the mantle transition zone is concerned, phase changes of constituent minerals are considered to be the major controlling factor. It is recently found that changes in water solubility will also significantly influence the conductivity of transition zone minerals. Therefore one-dimensional (1-D) modeling of the mantle conductivity is useful in obtaining information on the mean states in terms of these conditions. A set of MT response functions at a single station is usually employed for 1-D conductivity modeling, and GDS response functions from the same station are also used to extend the sounding curve to longer periods, in order to resolve transition zone or deeper conductivity. However, our recent study revealed that such a treatment is not allowed in general 3-D case, though it is well known that the GDS responses are equivalent to the MT responses under a 1-D situation. Kuvshinov (2008) also indicated that 3-D effects by ocean induction are not negligible up to the period of 20 days. It is possible to assume 1-dimensionality, but the assumption has to be verified. There are a number of 1-D models obtained in this way, and these models show different features not only in shallow but also in deep conductivity distribution in the transition zone. This suggests that the assumption is not applicable. Possible reasons are: (1) deep conductivity distribution is 3-dimensional or (2) surface conductivity heterogeneity causes 3-D inductional effects. Either indicates a simple approach given above is not appropriate. Here we show that the approach proposed by Utada et al. (2003) is one of the possible methods to estimate a reliable 1-D conductivity distribution in case of general 3-D setting including both ocean-land distribution and deep seated heterogeneity. In this paper, we revisit the dataset from the northwestern Pacific Ocean with additional time series and present a new model of 1-D conductivity for the region which can be used as a reference model for semi-global inversion of 3-D conductivity distribution in the region.
GP51C-02
The adjoint method of electromagnetic induction for CHAMP magnetic data
Martinec & McCreadie (2004) developed a time-domain spectral-finite element approach for the forward modelling of electromagnetic induction vector data as measured by the CHAMP satellite. Here, we present a new method of assessing the sensitivity of the CHAMP electromagnetic induction data on the Earth's m antle electrical conductivity, which we term the adjoint method. Both the forward and adjoint methods are performed in the time domain, such the boundary-value data at the satellite's altitude are respectively the X magnetic component measured by the CHAMP vector magnetometer along satellite tracks and the difference between the measured and predicted Z magnetic component. The squares of these differences summed up over all CHAMP tracks determine the misfit function. The sensitivity of the CHAMP data, that is the partial derivatives of the misfit function with respect to mantle conductivity parameters, are then determined by the scalar product of the forward and adjoint solutions, multiplied by the gradient of the conductivity and integrated over all CHAMP tracks. Such exactly determined sensitivities are checked against numerical differentiation of the misfit function, and very good agreement is obtained. The attractiveness of the adjoint method lies in the fact that the sensitivities are calculated for little cost, regardless of the dimension of the conductivity vector. However, since the adjoint solution proceeds backwards in time, the forward solution must be stored at each time step, leading to memory requirements that are linear with respect to the number of steps undertaken. Having determined the gradient of the misfit function, we apply the conjugate gradient method to infer 1-D and 2-D conductivity structures for the Earth based on the CHAMP residual time series (after the subtraction of static field and secular variations by the CHAOS model) between the years 2001 and 2008. We show that this time series is capable of resolving both 1-D and 2-D structures in the upper mantle and upper part of the lower mantle, while it is not sufficiently long to resolve a conductivity structure in the lower part of the lower mantle in a reliable way.
GP51C-03
Towards Realistic Global Geomagnetic Induction Modeling Using Scripted Finite Element Methods
We present recent progress in forward modeling the global induction problem with realistic external field structure, 3D Earth conductivity, and rotation. The modeling is performed in the time and frequency domains via a commercially-available, general-purpose, finite element modeling package called FlexPDE, and has been validated against known solutions to 3D steady state and time-dependent problems. The induction problem is formulated in terms of the magnetic vector potential and electric scalar potential, and mesh density is managed both explicitly and through adaptive mesh refinement. The modeling routine allows for arbitrary conductivity and external field structure. The strength of the external magnetic field generated by the magnetospheric ring current is known to vary as a function of local time, giving it an asymmetric spatial structure. Electromagnetic c-responses estimated from satellite data are known to be biased with respect to local time. We investigate the influence that Earth's rotation through the non-uniform external field should have on these c-responses.
GP51C-04
MCMT3DID - a finite-element, 3D MT inversion code using local coordinates for each site and solving for distortion parameters
In recent years different 3D inversion codes for magnetotelluric data, using finite differences, finite elements or integral equations, have been developed, and some have become available. All of these codes are designed to accommodate different aspects (e.g., topography, marine applications, including Tipper, ...). Our inversion program presented here uses a finite-element forward engine, and differs from previous ones by solving for galvanic distortion parameters as well Earth structure, and by using individual, local coordinate systems for each MT site instead of one global coordinate system. The reason for using local coordinate systems is fairly simple: Using a global coordinate system means that measured data need to be rotated from the coordinate system in which the data were recorded to the common, global modelling system. Therefore a rotation will be applied to the complex 2x2 impedance matrix which results in transferring noise of one component into the others - a huge disadvantage that can cause loss of information, especially if only one component is strongly affected by noise, but the others are not. Hence our philosophy is to leave measured data in the acquisition coordinates they are and not to apply any rotations to them. Of course there will be a global coordinate system used for the inversion itself, but for comparison between measured data and synthetic data, the noise-free, synthetic data will be rotated to the local coordinate system of each site to avoid the noise propagation into all components. Another common problem of magnetotelluric field data is galvanic distortion due to charges on small-scale, near-surface conductivity anomalies. Since the magnetic effects of the galvanic distortion decay rapidly with increasing periods they can be neglected in the MT case and only the electric effects, which persist to all periods, will be taken into account. Therefore the relation between the measured field data (Zmeasured), the impedance tensor of the true structure (Ztrue) and the distortion matrix (C) can be written as: Zmeasured = C Ztrue Inverting for conductivities only would result in an image of a structure related to Zmeasured rather than being an accurate representation of the subsurface. To gain a result being more alike the true structure, we invert for conductivities and distortion parameters simultaneously to accommodate these small-scale, near- surface anomalies causing the galvanic distortion. We will present the 3D finite-element code with the focus on some of the more interesting aspects (e.g., local coordinate system and distortion). We will also show some preliminary inversion results obtained from the first test data sets.
GP51C-05 INVITED
Recent Progress of WSINV3DMT
In early of 2006, WSINV3DMT which is a three-dimensional inversion software for Magnetotelluric (MT) data, was released to the MT community. In its original version, WSINV3DMT was capable of inverting only the components of the impedance tensor Z. Currently, WSINV3DMT has been upgraded to invert the vertical magnetic transfer function (VTF) and also a joint inversion of both types of data. In addition, bathymetry and topography can be incorporated to make the model more realistic, and to allow for bathymetric/topographic effects. To efficiently reduce both computational time and memory requirements, WSINV3DMT has also been coded with the Message Passing Interface (MPI) so that the data from each period is distributed and processed on one computer node. Here, we present inversion results from EXTECH data inverting VTF data alone, and jointly with impedances. Advantage of joint inversion will be provided. We also show inversion results from OBEM data collected in the Philippine Sea with real bathymetry included in the inversion. Some discussion about future research on WSINV3DMT will be included.
GP51C-06 INVITED
Electrical Conductivity and Non-volcanic Tremors: Implications for the Structure and Dynamics of the San Andreas Fault System
Deep non-volcanic tremors (NVT) have been observed SW of the San Andreas Fault Observatory at Depth (SAFOD), where the San Andreas Fault (SAF) zone changes its mechanical behavior from creeping in the NE to being locked in the SW. Two-dimensional inversion of magnetotelluric (MT) data acquired across the transitional-to-creeping segment of the SAF reveal a sub-vertical channel connecting a high conductivity region in the upper mantle and lower crust with the upper-crustal, brittle deformation zone of the SAF. We interpret this high conductivity as a zone where fluids can migrate into the SAF system. Recent seismological array observations confirm a depth range of approximately 30-50 km for the NVT but suggest that their source region is offset from the surface trace of the SAF by about 15 km to the SW, coincident with the region of high conductivity. We speculate that both observations could be related with a confined region of trapped fluids at mantle depth. Depending on the permeability state of the crust, fluid migration and pressure release into the SAF system may be possible and find its expression in a sub-vertical channel of high conductivity and generally low or lacking tremor activity. Whether this along-strike variability in the deep hydraulic system depends on or controls the changing dynamics of the SAF is an open question. Our observations are based on more than 250 MT sites, collected over the past three years and covering the entire transitional segment of the San Andreas Fault (SAF) near Parkfield and Cholame. We present data from seven 130 km long profiles extending from the Pacific coast into the Central Valley, crossing the San Andreas Fault Observatory (SAFOD) near Parkfield and the source region of non-volcanic tremors (NVT) near Cholame.
GP51C-07 INVITED
An emerging view of the crust and mantle of tectonic North America from EMScope: a mid- term progress review of Earthscope's magnetotelluric program
EMScope, the MT component of the Earthscope project has completed its final year of infrastructure
construction, and its third annual campaign of regional magnetotelluric array operations in the western USA.
Seven semi-permanent "backbone" MT observatories have been installed in California, Oregon, Montana,
New Mexico, Minnesota, Missouri and Virginia, designed through installation in 2 m deep, insulated
underground vaults and with long, buried electric dipole detectors using stable electrodes, to provide
extremely long-period magnetotelluric data meant to provide a set of regional, deep structural "anchor points"
penetrating into the mid-mantle, in which a series of denser and more uniform regional, transportable MT
networks can be tied.
A total of 160 "transportable array" MT stations have been occupied in Oregon, Washington, Idaho,
northernmost-California, and Montana. These were located on a 70 km quasi-regular grid, with coverage of
Cascadia, parts of the Basin and Range, the Rockies and the Snake River Plain, the zone above a putative
mantle plume that is hypothesized to serve as the magma source for both the Yellowstone supervolcano and
a chain of volcanic features extending westward into Oregon. It is anticipated that in 2009 the transportable
array will sweep eastward through the Yellowstone region, following which a set of regional transects at sites
of special geodynamic interest will be staged.
The transportable array stations are typically occupied for three weeks, providing MT response functions
extending from 2-10,000 s or in cases as great as 20,000 s period. These stations are anchored at longer
periods (extending as close to 100,000 s periods as possible) by the network of 7 backbone stations, to be
operated continuously for up to five years. We present an initial set of 3-d inverse models from the EMScope
data sets There is substantial coherence between the resulting 3-d conductivity model and the known
boundaries of major physiographic provinces, as well as seismically delineated mid-to-lower crustal and
upper mantle features.
A combination of telemetry from backbone stations and frequent batch transmission of data from the
transportable array field sites, followed by rapid data quality control procedures and generation of MT
response functions provides a data set of use to all interested researchers. All EMScope data are made
available freely through the IRIS Data Management Center or via the EMScope data portal. For transportable
array sites these data are available typically within two weeks of acquisition.
http://www.emscope.org
GP51C-08
Preliminary results of 3D inversion of the EarthScope Oregon MT data using the integral equation method
In this paper we apply 3D inversion to MT data collected in Oregon as a part of the EarthScope project. We use the integral equation method as a forward modeling engine. Quasi-analytical approximation with a variable background (QAVB) method of Frechet derivative calculation is applied. This technique allows us to simplify the inversion algorithm and to use just one forward modeling on every iteration step. The receiver footprint approach considerably reduces the computational resources needed to invert the large volumes of data covering vast areas. The data set, which was used in the inversion, was obtained through the Incorporated Research Institutions for Seismology (IRIS). The long-period MT data was collected in Eastern Oregon in 2006. The inverted electrical conductivity distribution agrees reasonably well with geological features of the region as well as with 3D MT inversion results obtained by other researchers. The geoelectrical model of the Oregon deep interior produced by 3D inversion indicates several lithospheres' electrical conductivity anomalies, including a linear zone marked by low-high conductivity transition along the Klamath Blue Mountain Lineament associated with a linear trend of gravity minima. High electrical conductivity values occur in the upper crust under the accreted terrains in the Blue Mountains region.