A three-dimensional (3D) seismic reflection survey was carried out during the SISMOMAR 2005 experiment covering an area of 18x3.8 km, which includes the Lucky Strike volcano and associated hydrothermal vent sites, part of the graben on top of the volcano, and extends out to the median valley bounding faults. The survey consisted of 39 lines shot at 100 m spacing using a 4.5 km-long streamer resulting in a sixty-fold coverage and 6.25 m CDP spacing. We present here the resulting 3D time migrated volume that shows a bright reflector at about 3 km depth beneath the volcano, which is interpreted as the roof of a magma chamber, along with the base of layer 2A. We were also able to image faults on the volcano and bounding the median valley, some of which penetrate down to the vicinity of the magma chamber. We also provide the 3D geometry of the magma chamber roof and base of layer 2A, converted to depth using the velocities from the 3D refraction survey (Seher et al., same session), and a highresolution seafloor bathymetric map derived from the picks of the seafloor arrival.
SUMMARY The lithosphere–asthenosphere boundary (LAB) separates the rigid lithospheric plate above with the ductile and convective asthenosphere below and plays a fundamental role in plate tectonic processes. The LAB has been imaged using passive geophysical methods, but these methods only provide low-resolution images. Recently, seismic reflection imaging method has provided high-resolution images of the LAB, but imaging of the LAB at younger ages has been difficult. Here, we present the image of the LAB using wide-angle seismic reflection data covering 11–21 Ma old lithosphere in the equatorial Atlantic Ocean. Using ocean bottom seismometers (OBSs), we have observed wide-angle reflections between 150 and 400 km offsets along with crustal and mantle refraction arrivals. We first performed traveltime tomography to obtain the velocity in the crust and upper mantle. The Pn arrivals provide the information about P-wave velocity down to 4 km below the Moho. The disappearance of Pn arrivals beyond 130 km offset suggests that vertical P-wave velocity gradient is negligible or negative below this depth. We extended these velocities down to 90 km depth and then applied two imaging techniques to wide-angle reflection data, namely traveltime mapping of picked reflection arrivals and pre-stack depth migration of full wavefield data. We find that these reflections originate between 34 and 67 km depth, possibly from the LAB system. We have carried out extensive modelling to show that these reflections are real and not artefacts of imaging. Comparison of our results with coincident passive seismological and magnetotelluric results suggests that wide-angle imaging technique can be successfully used to study the lithosphere and the LAB system. We find that the LAB gradually deepens with age, but becomes very deep at 17–19 Ma, which we interpret to be due to the anomalous geology along this part of the profile.
The obtained 3-D P-wave and S-wave velocity models in the Romanche eastern ridge-transform intersection, using arrival times from 514 microearthquakes recorded by a recent temporary array of seafloor seismometers. Related article: Yu, Z., Singh, S. C. & Maia, M. (2023). Evidence for low Vp/Vs ratios along the eastern Romanche ridge-transform intersection in the equatorial Atlantic Ocean. Earth and Planetary Science Letters, 621, 118380.
Geological sequestration of CO2 is considered to be an important greenhouse gas mitigation technology (Washington et al. 2009). Current public resistance to onshore sequestration sites makes offshore sites an attractive alternative. The Sleipner gas field in the North Sea is such a site and has been the worlds first industrial scale CO2 storage project. An effective CO2 storage requires monitoring and verification over large areas. Time lapse seismic monitoring is an integral tool to do so. Here we use time lapse elastic 2D full wave form inversion (FWI) to monitor CO2 in the Sleipner aquifer reservoir and retrieve the amount of free CO2. Inverted P-wave velocities (Vp) are related to gas saturations using a fluid substitution model employing Gassmann theory assuming patchy saturation. We find that a considerable amount of the injected CO2 could be in dissolved form by 2006 and hence saline aquifers could be a natural reservoir for CO2 storage.
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