Three-dimensional seismic refraction tomography of the crustal structure at the ION site on the Ninetyeast Ridge, Indian Ocean

Abstract

SUMMARY We have performed a 3‐D seismic refraction tomography of a 48 × 48 km2 area surrounding ODP site 757, which is planned to host an International Ocean Network (ION) permanent seismological observatory, called the Ninetyeast Ridge Observatory (NERO). The study area is located in the southern part of the Ninetyeast Ridge, the trail left by the Kerguelen hotspot on the Indian plate. The GEOMAR Research Centre for Marine Geosciences and the Federal Institute for Geosciences and Natural Resources acquired 18 wide‐angle profiles recorded by 23 ocean bottom hydrophones during cruise SO131 of R/V Sonne in spring 1998. We apply a first arrival traveltime tomography technique using regularized inversion to recover the 3‐D velocity structure relative to a 1‐D background model that was constructed from a priori information and averaged traveltime data. The final velocity model revealed the crustal structure down to approximately 8 km depth. Resolution tests showed that structures with approximately 6 km horizontal extent can reliably be resolved down to that depth. The survey imaged the extrusive layer of the upper crust of the Ninetyeast Ridge, which varies in thickness between 3 and 4 km. A high‐velocity anomaly coinciding with a positive magnetic anomaly represents a volcanic centre from which crust in this area is thought to have formed. A pronounced low‐velocity anomaly is located underneath a thick sedimentary cover in a bathymetric depression. However, poor ray coverage of the uppermost kilometre of the crust in this area resulted in smearing of the shallow structure to a larger depth. Tests explicitly including the shallow low‐velocity layer confirmed the existence of the deeper structure. The heterogeneity of the upper crust as observed by our study will have consequences for the waveforms of earthquake signals to be recorded by the future seismic observatory.

2-D and 3-D modelling of wide-angle seismic data: an example from the Vøring volcanic passive margin

“…Finally a constant uncertainty of 100 ms was assigned to all travel times picks, regardless of amplitudes or offsets. This value compares well with the constant uncertainty of 70 ms used by Schlindwein et al (2003) for offsets lower than 50 km and a simpler structure. line and OBS 10 is 800 m deeper than OBS 13, the first arrival is more than 1.5 s sooner on OBS 10 than on OBS 13.…”

“…Figure 8b shows the distribution of travel times versus offset (distance source-receiver). Compared to other studies (Zelt and Barton, 1998;Schlindwein et al, 2003), travel times are much more scattered, which reflects larger lateral velocity variations. Finally, only one fourth of the travel times picks were actually inverted (4239 picks) to reduce the computation time.…”

3D seismic refraction traveltime tomography at a groundwater contamination site

“…We derive a 3D P-wave velocity model to about 15 m depth by inverting first-arrival traveltimes using the regularized inversion algorithm of Zelt and Barton ͑1998͒. This algorithm typically has been applied to crustal-scale 3D data sets ͑e.g., Schlindwein et al, 2003;Ramachandran et al, 2004͒, although Deen and Gohl ͑2002͒ have used it in a mining application to approximately 100 m depth. This is the first time the Zelt and Barton ͑1998͒ algorithm has been applied in three dimensions to the nearsurface ͑Ͻ20 m͒ environment.…”

Crust and Lithospheric Structure - Active Source Studies of Crust and Lithospheric Structure

“…One of the most common forms of nonlinear model assessment for wide-angle data is the checkerboard test for estimating model resolution (e.g., Day et al, 2001;Evangelidis et al, 2004;Morgan et al, 2002;Schlindwein et al, 2003). In these tests, an alternating pattern of high and low anomalies is superimposed on a reference model, and synthetic data are calculated for the 'checkerboard' model.…”