Geophysical inversion seeks to recover models of the Earth's physical properties which can accurately explain field observations. Seismic full waveform inverison (FWI) and controlled source electromagnetic (CSEM) inversion can produce models of the subsurface characterized by and sensitive to the elastic and electrical properties, respectively. We present here strategies which leverage the highly sensitive CSEM inversion using the resolution at depth exhibited by the FWI, which can be better than standard seismic techniques, to be able to model more complex layered structure.
Determination of the long wavelength, or background, velocity structure is a critical step in prestack imaging. We have developed an efficient procedure for the solution of this non-linear inverse problem using a genetic algorithm (GA). In our approach velocities are described by splines; velocity values at spline nodes are the parameters in the inversion. We distinguish between primary nodes, affecting very long wavelength variation, and secondary nodes, describing more rapid change. The GA evolves a population of trial solutions, seeking out the globally fittest velocity model. Key to the method is the evaluation of fitness, which is carried out in three steps: (1) map migration of zero-offset traveltimes through the trial velocity model to identify the approximation locations of primary reflectors; (2) prestack Kirchhoff depth migration to generate image gathers in narrow depth windows centered on the predicted reflector locations; (3) calculation of horizontal semblance within the image gathers. For the correct velocity model, reflection events appear flat in each of the gathers; thus, by calculating horizontal semblance we obtain a measure of fitness for the GA. Since the migration is performed over a narrow depth range in the neighborhood of a given reflector, rays need only be traced from a small number of depth points for each gather; additional ray information can be efficiently obtained using paraxial approximations.
Elastic full waveform inversion of multichannel seismic data represents a data-driven form of analysis leading to direct quantification of the subsurface elastic parameters in the depth domain. Previous studies have focused on marine streamer data using acoustic or elastic inversion schemes for the inversion of P-wave data. In this paper, P- and S-wave velocities are inverted for using wide-angle multicomponent ocean-bottom cable (OBC) seismic data. Inversion is undertaken using a two-dimensional elastic algorithm operating in the time domain, which allows accurate modeling and inversion of the full elastic wavefield, including P- and mode-converted PS-waves and their respective amplitude variation with offset (AVO) responses. Results are presented from the application of this technique to an OBC seismic data set from the Alba Field, North Sea. After building an initial velocity model and extracting a seismic wavelet, the data are inverted instages. In the first stage, the intermediate wavelength P-wave velocity structure is recovered from the wide-angle data and then the short-scale detail from near-offset data using P-wave data on the [Formula: see text] (vertical geophone) component. In the second stage, intermediate wavelengths of S-wave velocity are inverted for, which exploits the information captured in the P-wave’s elastic AVO response. In the third stage, the earlier models are built on to invert mode-converted PS-wave events on the [Formula: see text] (horizontal geophone) component for S-wave velocity, targeting first shallow and then deeper structure. Inversion of [Formula: see text] alone has been able to delineate the Alba Field in P- and S-wave velocity, with the main field and outlier sands visible on the 2D results. Inversion of PS-wave data has demonstrated the potential of using converted waves to resolve shorter wavelength detail. Even at the low frequencies [Formula: see text] inverted here, improved spatial resolution was obtained by inverting S-wave data compared with P-wave data inversion results.
The Southwest Indian Ridge (SWIR) is an ultraslow spreading end‐member of mid‐ocean ridge system. We use air gun shooting data recorded by ocean bottom seismometers (OBS) and multibeam bathymetry to obtain a detailed three‐dimensional (3‐D) P wave tomographic model centered at 49°39′E near the active hydrothermal “Dragon Flag” vent. Results are presented in the form of a 3‐D seismic traveltime inversion over the center and both ends of a ridge segment. We show that the crustal thickness, defined as the depth to the 7 km/s isovelocity contour, decreases systematically from the center (∼7.0–8.0 km) toward the segment ends (∼3.0–4.0 km). This variation is dominantly controlled by thickness changes in the lower crustal layer. We interpret this variation as due to focusing of the magmatic activity at the segment center. The across‐axis velocity model documents a strong asymmetrical structure involving oceanic detachment faulting. A locally corrugated oceanic core complex (Dragon Flag OCC) on the southern ridge flank is characterized by high shallow crustal velocities and a strong vertical velocity gradient. We infer that this OCC may be predominantly made of gabbros. We suggest that detachment faulting is a prominent process of slow spreading oceanic crust accretion even in magmatically robust ridge sections. Hydrothermal activity at the Dragon Flag vents is located next to the detachment fault termination. We infer that the detachment fault system provides a pathway for hydrothermal convection.
O-19 SUB-BASALT IMAGING USING A FULL ELASTIC WAVEFIELD INVERSION SCHEME Abstract 1 As the use of wide-aperture seismic surveying is becoming more established in areas of hydrocarbon interest the importance of mode converted waves is increasing. Although the treatment of shear waves can make an efficient contribution to the understanding of the physical properties of the subsurface a conventional acoustic approach is inadequate to handle these elastic parameters. In this paper an elastic finite difference scheme has been implemented to perform an inversion of both P-wave and S-wave velocities. Until now such inversions have been limited to near offset seismic
We estimate the seismic structure of the slow spreading Lucky Strike segment of the Mid‐Atlantic Ridge, located approximately 300 km south of the Azores platform, using seismic reflection and seismic refraction data acquired in June 2005 as a part of the Seismic Study for Monitoring of the Mid‐Atlantic Ridge (SISMOMAR) survey. The three‐dimensional velocity model shows an upper crustal low‐velocity anomaly running parallel to the ridge axis, which is limited by the median valley bounding faults. The velocity models also show a low‐velocity anomaly underlying the axial melt lens reflector located at the segment center below the Lucky Strike volcano. This lower crustal low‐velocity region can be explained by elevated temperatures and possibly small amounts of melt. The lower crustal low‐velocity anomaly and the axial melt lens reflector constrain the geometry of the magma chamber responsible for the construction of the Lucky Strike volcano. The presence of this magma chamber and thick crust at the segment center are consistent with a focused melt supply to the segment center.
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