Joint two-dimensional cross-gradient imaging of magnetotelluric and seismic traveltime data for structural and lithological classification

Collocated magnetotelluric (MT) and seismic profiling is emerging as a necessary combined approach for deep and near-surface imaging but the resulting experimental data are typically interpreted separately since no production programs exist for multidimensional joint inversion of MT and seismic data. We present a joint 2-D inversion approach for imaging collocated MT and seismic refraction data with cross-gradient structural constraints. We describe the main features of the algorithm and first apply it to synthetic data generated for a hypothetical complex geological model. For the synthetic data, we find that the scheme leads to models with remarkable structural resemblance and improved estimates of electrical resistivity and seismic velocity. We apply the scheme to near-surface field data to test the consistency of a previously suggested resistivity–velocity interrelationship and its potential use for subsurface lithofacies discrimination or structural classification. The MT-seismic relationship is found to be in excellent accord with that derived previously for DC resistivity and seismic data set at the test site. Our results suggest that joint MT-seismic cross-gradient imaging leads to improved characterization of heterogeneous geological targets at near-surface to mantle depths.     

Crustal structure across the Grand Banks–Newfoundland Basin Continental Margin – II. Results from a seismic reflection profile

New multichannel seismic reflection data were collected over a 565 km transect covering the non-volcanic rifted margin of the central eastern Grand Banks and the Newfoundland Basin in the northwestern Atlantic. Three major crustal zones are interpreted from west to east over the seaward 350 km of the profile: (1) continental crust; (2) transitional basement and (3) oceanic crust. Continental crust thins over a wide zone (∼160 km) by forming a large rift basin (Carson Basin) and seaward fault block, together with a series of smaller fault blocks eastwards beneath the Salar and Newfoundland basins. Analysis of selected previous reflection profiles (Lithoprobe 85-4, 85-2 and Conrad NB-1) indicates that prominent landward-dipping reflections observed under the continental slope are a regional phenomenon. They define the landward edge of a deep serpentinized mantle layer, which underlies both extended continental crust and transitional basement. The 80-km-wide transitional basement is defined landwards by a basement high that may consist of serpentinized peridotite and seawards by a pair of basement highs of unknown crustal origin. Flat and unreflective transitional basement most likely is exhumed, serpentinized mantle, although our results do not exclude the possibility of anomalously thinned oceanic crust. A Moho reflection below interpreted oceanic crust is first observed landwards of magnetic anomaly M4, 230 km from the shelf break. Extrapolation of ages from chron M0 to the edge of interpreted oceanic crust suggests that the onset of seafloor spreading was ∼138 Ma (Valanginian) in the south (southern Newfoundland Basin) to ∼125 Ma (Barremian–Aptian boundary) in the north (Flemish Cap), comparable to those proposed for the conjugate margins.     

Crustal model of the Bransfield Rift, West Antarctica, from detailed OBS refraction experiments

The first detailed deep seismic refraction study in the Bransfield Strait, West Antarctica, using sensitive OBSs (ocean bottom seismographs) was carried out successfully during the Antarctic summer of 1990/1991. The experiment focused on the deep crustal structure beneath the axis of the Bransfield Rift. Seismic profile DSS-20 was located exactly in the Bransfield Trough, which is suspected to be a young rift system. Along the profile, five OBSs were deployed at spacings of 50-70 km. 51 shots were fired along the 310 km profile. This paper gives the first presentation of the results. A detailed model of the crustal structure was obtained by modelling the observed traveltimes and amplitudes using a 2-D ray-tracing technique. The uppermost (sedimentary?) cover, with velocities of 2.0-5.5 km s⁻¹, reaches a depth of up to 8 km. Below this, a complex with velocities of 6.4-6.8 km s⁻¹ is observed. The presence of a high-velocity body, with V _p= 7.3-7.7 km s⁻¹, was detected in the 14-32 km depth range in the central part of the profile. These inhomogeneities can be interpreted as a stage of back-arc spreading and stretching of the continental crust, coinciding with the Deception-Bridgeman volcanic line. Velocities of 8.1 km s⁻¹, characteristic of the Moho, are observed along the profile at a depth of 30-32 km.