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Using an autonomous underwater vehicle (AUV) equipped with a three-axis electric field receiver we were able to make measurements of self potential (SP) fields over a hydrothermal prospect in the Iheya area of the Okinawa Trough, southwest of Japan. Additionally, by deploying battery-powered controlledsource EM (CSEM) transmitters on the seafloor, we were able to make CSEM measurements out to ranges of about 800 m. We observed negative SP anomalies associated with seafloor mounds, localized close to the seafloor and probably associated with hydrothermal venting. Apparent conductivities computed from the CSEM data were as high as 30 S/m, and were not correlated with the largest SP anomalies. A preliminary 3D inversion yielded a compact, 10 S/m body extending 100 m below the seafloor. The high conductivities may be associated with hydrothermal systems, but more likely are a manifestation of submarine massive sulfide (SMS) deposits.
Presentation Date: Thursday, October 18, 2018
Start Time: 8:30:00 AM
Location: 213A (Anaheim Convention Center)
Presentation Type: Oral
Barak, Ohad (Stanford University) | Key, Kerry (Scripps Institution of Oceanography) | Constable, Steven (Scripps Institution of Oceanography) | Milligan, Paul (Seabed Geosolutions) | Ronen, Shuki (Stanford University, Dolphin Geophysical)
There are currently no widely available rotation sensors that can operate on the ocean-bottom. We derive rotation data on the ocean bottom from two surveys that were not originally designed to record them: 1) from geophone recordings in the Moere Vest ocean-bottom survey by differencing adjacent geophones; and 2) from magnetometer recordings in the SERPENT CSEM ocean-bottom survey by extrapolating from the deviations in magnetic field projections on the magnetometer components.
Rigid bodies in a three dimensional world have six degrees of freedom: three components of linear motion and three components of rotation. In the subsurface, the linear motions are the particle velocities and the rotations are the pitch, roll and yaw, as shown in the following table:
where vi are particle velocities along the axis, and ri are rotation rates around the axis.
In ocean-bottom node acquisition, multicomponent geophones that are coupled to the seafloor record the vertical and the two horizontal components of particle velocity. Additionally, a hydrophone records the divergence of the wavefield P=k (V·u), where u are particle displacements and k is the bulk modulus of the water to which the hydrophones are coupled. Rotations are a measurement of the curl of the wavefield r = ½ (V ×v ), and are a recording of the anti-symmetric strains of the medium (Cochard et al., 2006).
Vassallo et al. (2012) use hydrophones together with pressure gradient sensors in marine streamer acquisition to interpolate the pressure wavefield in the crossline direction, between streamer cables. Similarly, the rotational components can be used to interpolate vertical geophone data (Edme et al., 2014), and spatial aliasing of high-wavenumber arrivals can thus be mitigated. Barak et al. (2014b) show that rotation data are extra information, are independent of geophone data, and how they can be used in conjunction with geophone data to identify and separate wave-modes on land using singular-value decomposition polarization analysis.
As of yet there are no industry-grade solutions for recording rotational motion on the ocean bottom, though a few such recording stations have been deployed previously by Pillet et al. (2009).The objective of this paper is to show how rotation data can be extracted from existing ocean-bottom recordings.
This work presents a parallel goal-oriented adaptive finite element method for accurate and efficient electromagnetic modeling of complex 3D structures. An unstructured tetrahedral mesh allows this approach to accommodate arbitrarily complex 3D conductivity variations and a priori known boundaries. Accuracy of the finite element solution is achieved through adaptive mesh refinement that is performed iteratively until the solution converges to the desired accuracy tolerance. Refinement is guided by a goal-oriented error estimator that uses a dual-weighted residual method to optimize the mesh for accurate EM responses at the locations of the EM receivers. Our algorithm is parallelized over frequencies, transmitters and receivers, where the adaptive mesh refinement is performed in parallel on subsets of these parameters. We validate the newly developed algorithm by comparison with controlledsource EM solutions for a 1D layered model. A 3D model with significant seafloor bathymetry variations and a heterogeneous subsurface demonstrates the code’s ability to model complex features.
During the past decade, the commercial investment in marine EM exploration has motivated many innovations in numerical techniques (Constable, 2010; Key, 2012). In particular, adaptive finite element methods (Key andWeiss, 2006; Li and Key, 2007; Ren et al., 2013) have received considerable attention since their unstructured modeling grids can readily accommodate highly complex geological features, such as seafloor topography and stratigraphic horizons. In addition to the ease of handling topography, another key benefit of unstructured grids is the ability to efficiently discretize multiple-scale structures.
Starting with a coarse model discretized by a grid of tetrahedral elements, adaptive methods seek to increase the solution accuracy by iteratively refining the grid, where each iteration consists of selecting a subset of elements for refinement based on an estimate of their contribution to the solution error, and then refining the grid by creating new smaller elements in these regions.
To measures the global contribution to the local error, we develop a goal-oriented error estimation approach using a sensitivity functional that measures how the error in one portion of the model corrupts the solution at the regions of interest, an effect referred to as pollution (Oden and Prudhomme, 2001). Refinement only occurs where the solution is inaccurate and where that inaccuracy corrupts the solution at the receivers’ locations.
Between 1997 and 2003 the Gemini Prospect, Gulf of Mexico, was used as a test ground for development of broadband marine magnetotellurics (MT). In 2003 controlled source electromagnetic (CSEM) data were acquired over Gemini using a prototype 200A transmitter on a profile that included earlier MT sites. In 2013 and 2014 these data were revisited as part of the evaluation of the recently developed Scripps 2D EM inversion/forward modeling code, MARE2DEM, developed largely by K. Key. A portion of the results of this evaluation are presented here with the objective that of examining aspects of the application of the inversion algorithm to real CSEM and MT data, rather than a detailed re-interpretation of the Gemini data set. Of order of 100 inversions were carried out as part of this study. The processed CSEM data were inverted for each of three frequencies individually and then for the three frequencies jointly, the MT data alone were inverted, and finally the multi-frequency CSEM and MT data were jointly inverted. Each of these data sets was subject to isotropic inversion, and then the joint CSEM/MT data were the subject of anisotropic inversion. Target misfits were varied and the results compared. Inversions were carried out with a half-space starting model as well as with more complex starting models. The results provide insight into the inversion outcome based on the effects of choice of data component to be used as input, the choice of starting model, target misfit selected, and the degree of anisotropy. The results imply that for CSEM and MT data collected at multiple frequencies, inverted models can change radically as different subsets of data are inverted individually or in combination, and with varying target misfit and/or anisotropy penalty.
Abstract An extensive marine controlled source electromagnetic (CSEM) data set was collected in the Gulf of Mexico targeting gas hydrate deposits at four geologically distinct areas within water depths that varied from 900 to 3000 m. The Joint Industry Project 2 drilled two of these locations, Green Canyon 955 and Walker Ridge 313, providing ground truth for the CSEM data. A third survey location, Mississippi Canyon 118 (MC118), is a designated hydrate observatory. We collected both traditional ocean bottom electromagnetic receiver data as well as continuous profiling data using a novel towed three-axis electric field receiver. The towed receiver measured data at a fixed offset of 300 m behind the transmitter, while the ocean bottom receivers recorded data over a range of offsets as the transmitter passed by each receiver. We inverted the CSEM data using a newly developed parallel goal-oriented adaptive finite element-modeling algorithm for efficient 2.5D imaging of the CSEM data. 2D inversions from MC118 are consistent with 1D apparent resistivity pseudosections and reveal resistive areas associated with the carbonate/hydrate mound. An extensive conductive region exists below this, associated with conductive brines sourced from a deeper more resistive salt body. Away from the mound the resistivity structure is a fairly uniform and homogeneous. We present new 2D inversions of the CSEM data that consider electrical anisotropy and constrained inversions that leverage structural control using key seismic horizons along the profiles.
Key, Kerry (Scripps Institution of Oceanography)