Active Electromagnetics At The Mid-Ocean Ridge
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Author(s): Everett, Mark E | Abstract: The 59,000 km long global mid-ocean ridge system is the site of formation of 20 km3 of oceanic crust yearly. Two-thirds of all heat loss from the interior of our planet is through the ocean floors, 40% of this amount is focused through the ridge. Activity involves complex interactions among a number of processes occurring over wide ranges of depths and lateral distances, including melting of the earth's mantle, delivery of the molten rock to a crustal magma chamber, cooling of the magma intrusion by hydrothermal circulation and volcanic eruption, chemical exchange between hot rock surrounding the magma chamber and the overlying seawater, and even the establishment of exotic biological communities near hydrothermal vents at the ridge axis. These features justify the expanding scientific interest in the study of the ridge.Transient controlled-source electromagnetics (CSEM) is a geophysical exploration technique capable of determining the electrical conductivity beneath fast-spreading segments of the mid-ocean ridge. Geological structure beneath the mid-ocean ridge that is readily accessible to transient CSEM exploration is located at crustal levels and includes the axial magma chamber and its associated zones of partial melt and hydrothermal activity. Seismic images of the top several kilometers beneath the fast spreading East Pacific Rise (EPR) between 9-13°N have already been obtained. Multi-channel reflection profiles place strong constraints on the geometry of the top of the axial magma chamber but refraction data provide only coarse estimates of the sub-surface temperature, distribution of partial melt and porosity, parameters required to distinguish between proposed petrological models of the ridge. Electrical conductivity is a strong indicator of all these critical parameters and therefore CSEM methods are well-suited to improve the estimates and help characterize the ridge environment.In this thesis, a pair of forward modeling computer programs have been developed to design ridge-going experiments and assist interpretation of mid-ocean ridge transient CSEM data sets, as they become available. The programs may also be used to evaluate the transient CSEM technique as it might be applied to investigate other tectonically active regions of the seafloor. One program rapidly computes the theoretical response, as a function of time, of an arbitrary, two dimensional earth to a sudden switch-on of electric current in a line source of electromagnetic energy. The other program is more advanced, requires more computer time, and is referred to as a 2.5-D program because it can handle excitation of the earth by a more realistic, finite source.The programs solve the forward problem as follows. Electromagnetic boundary value problems based on the governing Maxwell's equations are solved by the finite element method in the Laplace frequency s-domain. The calculated electromagnetic field components are then transformed into the time domain by means of the Gaver-Stehfest algorithm. In the 2.5-D program, Maxwell's equations are additionally Fourier transformed in the direction parallel to the strike of the 2-D conductivity structure, and field components are computed in the along-strike wavenumber q-domain. Following the calculation, inverse transforms are performed to obtain the along-strike spatial variations of the field components. The codes have been validated through comparisons with known analytic solutions in which the earth is modeled as a uniformly conducting half-space. Convergence of the finite element approximation is found to be O(h), where h measures the size of the triangles comprising the finite element mesh. An extrapolation formula is described by which numerical solutions on progressively finer meshes are combined. The formula permits great accuracy to be attained in the computed field components, using relatively coarse meshes.A numerical study of the performance of an idealized transient CSEM system at the East Pacific Rise has been carried out using the 2-D code. The system consists of an infinite source located 5 km west of the ridge axis, and seafloor magnetic field sensors placed at various distances across the ridge crest. The source is oriented with respect to the strike of the ridge so as to produce only the H-polarization mode of electric current flow. The results indicate that this system can detect the axial magma chamber and the associated zones of hydrothermal activity and partial melt by monitoring two electromagnetic response parameters, the diffusion time T and the response amplitude B max , as a function of transmitter/receiver separation. These response parameters are easily extracted from measured data and are diagnostic of the sub-surface electrical conductivity. The presence of a highly conductive magma chamber slows and attenuates signals diffusing beneath the ridge, increasing T and decreasing Bmax. Hydrothermal circulation in the highly fractured, extrusive basalt layer has the same effect on the data for receivers placed within 3 km of the ridge axis, but very little effect elsewhere. Inferences made from the numerical results suggest that a horizontal electric dipole (HED) of moment 10 4 A•m and receivers sampling the seafloor magnetic field at 10-25 Hz with a sensitivity of 1 pT/s over a time window extending to 10 s are sufficient to detect these crustal targets.Interpretation of transient CSEM data requires forward modeling using a more realistic, finite source. The 2.5-D code is capable of achieving this. Sample field patterns produced in the vicinity of the ridge by a sudden switch-on of electric current in a horizontal electric dipole (HED) are computed. The patterns illustrate diffusion, in three spatial dimensions and time, of various along-strike electromagnetic field components through typical mid-ocean ridge structures. The results demonstrate the utility of the 2.5-D code, i.e. its potential for interpreting data from a transient CSEM ridge-going experiment.Keywords:
Magma chamber
Seafloor Spreading
Electromagnetics
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Dharwar Craton
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Shear wave splitting observations are a commonly used tool for inferring anisotropy and flow within the Earth's interior. Here we present the development and validation of a new technique for imaging anisotropy in the upper mantle using local events—shear wave splitting tomography (SWST). The mantle is parametrized as a 3-D block model of crystallographic orientations with the elastic properties of olivine and orthopyroxene, and both orthorhombic and hexagonal symmetries are tested. To efficiently forward calculate splitting, the Christoffel equation is used to progressively split the horizontal components of a synthetic wavelet in each block of the model, and predicted shear wave splitting parameters are obtained with an eigenvalue minimization technique. Numerically calculated partial derivatives are utilized in a linearized, damped least-squares inversion to solve for a best-fitting model of crystallographic orientations. To account for the non-linear properties of shear wave splitting, the inversion is applied iteratively and partial derivatives are recalculated after each iteration. A starting model that incorporates information from predicted splitting parameters is found by spatially averaging fast directions and the ratio of observed-to-predicted splitting times. Models from inversions utilizing this average starting model reach lower misfit levels than do inversions with a random or uniform starting model. Modelling results using synthetic data from several anisotropic structures (i.e. sharp lateral and vertical variations in anisotropy) both within an idealized and a real (Nicaragua–Costa Rica) subduction zone illustrate the capabilities and limitations of SWST. With a station spacing of 25 km in an idealized subduction zone containing uniformly spaced events down to 225 km, both the azimuth and dip of crystallographic axes are resolvable to a depth of 100–150 km and lateral heterogeneities in anisotropy on a scale of 50 km at arc and forearc distances from the trench are retrieved. Spatial resolution of anisotropy at scales of 75 km is possible further into the backarc above 150 km depth. The geometry of stations and observed seismicity in the Nicaragua–Costa Rica subduction zone yields partial to good resolution at scales of 50–75 km beneath the forearc, arc and limited regions of the backarc down to 100 km, and resolution at coarser scales is possible in wider regions beneath the backarc. Given the distributions of seismic sources within many subduction zones and the advances in broad-band seismic array deployments, this new method offers a powerful means with which to constrain the orientation of anisotropic fabric in the upper mantle.
Shear wave splitting
Seismic anisotropy
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Seismogram
P wave
Receiver function
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In a previous study, models of the crust and magma chamber at 12°50′N on the East Pacific Rise were derived with a two‐dimensional ray‐tracing analysis of rise‐normal refraction lines. The best model featured a 4 km wide axial magma chamber, but the data contained diffraction effects that were not modeled by geometric ray theory. This raised questions as to whether this model would reproduce the waveform data and whether diffraction might be obscuring a much larger magma chamber. We have tested the model with a finite element calculation of synthetic seismograms. This method incorporates full wave effects, including diffraction. The synthetics accurately reproduce the data and exhibit diffraction phenomena similar to those in the data. The previous travel time analysis was largely successful and was only slightly compromised by diffraction. Diffraction in rise‐normal refraction experiments is, therefore, not obscuring a large, low velocity magma chamber.
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Seismogram
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Studies of seismic propagation through oceanic crust have contributed enormously to
our understanding of the generation and evolution of oceanic crust However, such work
has largely been confined to the seismic velocity structure. In this thesis we present
results from a study of seismic attenuation using a data set collected for three-dimensional
tomographic imaging of a fast-spreading ridge. The experiment location at 9°30'N on the
East Pacific Rise is the site of a strong mid-crustal seismic reflector which has been
inferred to be the roof of a small axial magma chamber at about 1.6 km depth.
A spectral method is used to estimate t*, a measure of the integrated attenuation along
a wave path. Such a method asswnes that the dominant frequency-dependent component
of propagation is intrinsic attenuation. A logarithmic parameterization is then used to
invert t* measurements for Q-1 structure asswning that the velocity structure is given from
earlier studies. To evaluate the method of Q tomography a full-waveform finitedifference
technique which does not include attenuation is used to calculate solutions for
seismic propagation through a two-dimensional velocity model. The results show a
complex pattern of seismic propagation in the vicinity of the axial magma chamber. The
first arrival always passes above the magma chamber. However, for paths of significant
length that cross the rise axis the amplitude of this arrival is very small, and the first phase
with significant amplitude is a diffraction below the magma chamber. High-amplitude
Moho turning and PP arrivals may also be important secondary arrivals. Synthetic
inversions show the importance of selecting time windows for power spectral estimation
which are dominated by a single phase and of using wave paths which closely
corresponds to that of the selected phase.
A comparison of the finite difference solutions and the predictions of the a twodimensional,
exact ray-tracing algorithm with record sections obtained during the
tomography experiment significantly improves our understanding of seismic propagation
across the East Pacific Rise. The results enable an objective choice of the position and
length of the time window fort* estimation. Moreover, additional constraints are
incorporated into an approximate three-dimensional ray-tracing algorithm used in the
inversion so that the wave paths more closely correspond to those of the desired phase.
The full data set to be inverted comprises about 3500 t* estimates and includes crustal
paths which do not cross the rise axis, diffractions above and below the axial magma chamber, and Moho-turning phases. Wave paths for the Moho-turning phases cross the
rise axis at a wide range of lower crustal depths.
The Q-1 models resulting from two-dimensional and three-dimensional tomographic
inversions show that the attenuation of seismic waves on the East Pacific Rise is
dominated by two regions of low Q; one in the upper 1 km of crust, and one at depths
greater than about 2 km below the rise axis.…
Magma chamber
Reflector (photography)
Anelastic attenuation factor
Shadow zone
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SUMMARY We apply the Automated Multimode Inversion of surface and S-wave forms to a large global data set, verify the accuracy of the method and assumptions behind it, and compute an S vvelocity model of the upper mantle (crust‐660 km). The model is constrained with ∼51 000 seismograms recorded at 368 permanent and temporary broadband seismic stations. Structure of the mantle and crust is constrained by waveform information both from the fundamentalmode Rayleigh waves (periods from 20 to 400 s) and from S and multiple S waves (higher modes). In order to enhance the validity of the path-average approximation, we implement the automated inversion of surface- and S-wave forms with a three-dimensional (3-D) reference model. Linear equations obtained from the processing of all the seismograms of the data set are inverted for seismic velocity variations also relative to a 3-D reference, in this study composed of a 3-D model of the crust and a one-dimensional (1-D), global-average depth profile in the mantle below. Waveform information is related to shear- and compressional-velocity structure within approximate waveform sensitivity areas. We use two global triangular grids of knots with approximately equal interknot spacing within each: a finely spaced grid for integration over sensitivity areas and a rougher-spaced one for the model parametrization. For the tomographic inversion we use LSQR with horizontal and vertical smoothing and norm damping. We invert for isotropic variations in S- and P-wave velocities but also allow for S-wave azimuthal anisotropy—in order to minimize errors due to possible mapping of anisotropy into isotropic heterogeneity. The lateral resolution of the resulting isotropic upper-mantle images is a few hundred kilometres, varying with data sampling. We validate the imaging technique with a ‘spectral-element’ resolution test: inverting a published global synthetic data set computed with the spectral-element method using a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately. This test confirms both the accuracy of the implementation of the method and the validity of the JWKB and path-average approximations as applied in it. Reviewing the tomographic model, we observe that low-S v-velocity anomalies beneath mid-ocean ridges and backarc basins extend down to ∼100 km depth only, shallower than according to some previous tomographic models; this presents a close match to published estimates of primary melt production depth ranges there. In the seismic lithosphere beneath cratons, unambiguous high velocity anomalies extend to ∼200 km. Pronounced low-velocity zones beneath cratonic lithosphere are rare; where present (South America; Tanzania) they are neighboured by volcanic areas near cratonic boundaries. The images of these low-velocity zones may indicate hot material—possibly of mantle-plume origin—trapped or spreading beneath the thick cratonic lithosphere.
Seismic Tomography
Inverse theory
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Abstract : One strategy for discriminating between explosions and natural events depends on accurate determinations of event locations, including focal depths. If a seismic event could be reliably determined to have a focal depth greater than a few kilometers, one could be confident that the event is not an explosion. But to determine focal depths accurately, one must first have a fairly accurate model of the crustal structure in the vicinity of the event. Unfortunately, sufficiently accurate models do not exist for many regions of interest to the nuclear explosion monitoring community. Our previous work focused on developing and evaluating strategies for locating events using a single three-component seismic station; velocity models were obtained via crustal receiver function modeling, and waveform correlation methods were used to determine focal depths for which synthetics fit the data best. However, for an event at a regional distance from a given station, the sampling provided by the teleseismic phases used for receiver functions is not ideal. These waves tend to approach the station at a steep angle, sampling just a narrow cone beneath the station. Better sampling is provided by shear-coupled PL (SPL) phases, which sample the crust over 1000 km or more as they approach the station. This sampling provides a better lateral average of the crust and more closely resembles the sampling of phases emanating from seismic events at regional distances. Our current research centers on modeling SPL phases using a novel modeling algorithm that uses the reflectivity method to compute synthetic seismograms while holding deeper portions of the mantle fixed, in terms of pre-computed and stored reflectivity and transmission matrices. Layers of the crust and upper mantle are allowed to vary over broad ranges and the entire algorithm is powered by a variant of simulated annealing, a global optimization method.
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Focal mechanism
Receiver function
Hypocenter
Earth structure
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We propose a modification to the Non-linear Asymptotic Coupling Theory (NACT), a normal mode coupling method used for synthesizing seismograms and computing sensitivity kernels in 3D Earth. The modification is aimed to meet the computational challenges for NACT when approaching higher frequencies, which is required for obtaining finer scale images of the Earth's upper mantle. The new scheme is numerically validated. We present new constraints on the topography of the 410 and 660, two important seismic discontinuities in the Earth's upper mantle. The data used are the SS precursors recorded by the US Transportable Array. We first demonstrate a case of 3D mantle heterogeneity interplaying with discontinuity depth. We show observations from one event for which a large scale heterogeneity away from the SS precursor bounce point region produces an artificial precursor . This new discovery raises a caution for identifying and interpreting SS (and perhaps PP) precursors. We then present high resolution maps of 410 and 660 discontinuity topography across a large area of the Pacific Ocean, derived from SS precursors. A new filtering tool called the Local Slant-Stack Filtered (LSSF) is employed to successfully clean up the SS precursor record sections, leading to robust precursor travel time measurements with increased spatial resolution. Good agreements are observed between our discontinuity images and a very recent upper mantle tomography model SEMum2. Besides the new constraints on the discontinuities, we also work on refining the elastic structure of the Earth's upper mantle in the context of regional/continental scale full-waveform tomography. We propose a framework for a hybrid adjoint tomography, in which the gradient of the misfit function is numerically computed using the adjoint method with the highly accurate Regional Spectral Element Method (RegSEM) code, while the Hessian is computed approximately using NACT. We present results from tests on a dataset for imaging the North American continent. Finally, we study the infra-gravity wave induced long-period noise on an ocean bottom broadband seismometer, MOBB, deployed offshore in the Monterey Bay, California, using ~10 years' continuous recording data. Strong correlation between the vertical component seismogram and the pressure record is observed. We define and calculate the transfer function between the two channels, and demonstrate that the transfer function is stable over time. We then utilize the average transfer function to remove pressure-correlated noise from the vertical component seismogram, and show that the cleaned MOBB waveforms help to better constrain the moment tensors for regional near-shore earthquakes on the San Andreas Fault system in Northern California.
Classification of discontinuities
Discontinuity (linguistics)
Seismogram
Earth structure
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This paper applies nonlinear Bayesian inversion to marine controlled source electromagnetic (CSEM) data collected near two sites of the Integrated Ocean Drilling Program (IODP) Expedition 311 on the northern Cascadia Margin to investigate subseafloor resistivity structure related to gas hydrate deposits and cold vents. The Cascadia margin, off the west coast of Vancouver Island, Canada, has a large accretionary prism where sediments are under pressure due to convergent plate boundary tectonics. Gas hydrate deposits and cold vent structures have previously been investigated by various geophysical methods and seabed drilling. Here, we invert time-domain CSEM data collected at Sites U1328 and U1329 of IODP Expedition 311 using Bayesian methods to derive subsurface resistivity model parameters and uncertainties. The Bayesian information criterion is applied to determine the amount of structure (number of layers in a depth-dependent model) that can be resolved by the data. The parameter space is sampled with the Metropolis–Hastings algorithm in principal-component space, utilizing parallel tempering to ensure wider and efficient sampling and convergence. Nonlinear inversion allows analysis of uncertain acquisition parameters such as time delays between receiver and transmitter clocks as well as input electrical current amplitude. Marginalizing over these instrument parameters in the inversion accounts for their contribution to the geophysical model uncertainties. One-dimensional inversion of time-domain CSEM data collected at measurement sites along a survey line allows interpretation of the subsurface resistivity structure. The data sets can be generally explained by models with 1 to 3 layers. Inversion results at U1329, at the landward edge of the gas hydrate stability zone, indicate a sediment unconformity as well as potential cold vents which were previously unknown. The resistivities generally increase upslope due to sediment erosion along the slope. Inversion results at U1328 on the middle slope suggest several vent systems close to Bullseye vent in agreement with ongoing interdisciplinary observations.
Accretionary wedge
Magnetotellurics
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This study attempts to discuss the possible causes of the crustal low resistive zone based on the magnetotelluric observations in the Western Foothills, Taiwan.The depth and resistivity of this low resistive zone (LRZ) have the values, on the average, of 9 km and 30 ohm-meters.According to the independently geological data, the possible causes of the LRZ are re lated to the high C02 activity in Taiwan and the dehydration reactions.The existence of a significant amount of HC03 in crustal fluid would produce a consequent impact on resistivity.
Foothills
Resistive touchscreen
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