Swath bathymetry using phase difference: Theoretical analysis of acoustical measurement precision

The phase difference principle is widely applied nowadays to sonar systems used for sea floor bathymetry, The apparent angle of a target point is obtained from the phase difference measured between two close receiving arrays. Here we study the influence of the phase difference estimation errors caused by the physical structure of the backscattered signals. It is shown that, under certain current conditions, beyond the commonly considered effects of additive external noise and baseline decorrelation, the processing may be affected by the shifting footprint effect: this is due to the fact that the two interferometer receivers get simultaneous echo contributions coming from slightly shifted seabed parts, which results in a degradation of the signal coherence and, hence, of the phase difference measurement. This geometrical effect is described analytically and checked with numerical simulations, both for square- and sine-shaped signal envelopes. Its relative influence depends on the geometrical configuration and receiver spacing; it may be prevalent in practical cases associated with bathymetric sonars. The cases of square and smooth signal envelopes are both considered. The measurements close to nadir, which are known to be especially difficult with interferometry systems, are addressed in particular.

The pianosa contourite depositional system (northern Tyrrhenian sea): drift morphology and plio-quaternary stratigraphic evolution

The Pianosa Contourite Depositional System (CDS) is located in the Corsica Trough (Northern Tyrrhenian Sea), a confined basin dominated by mass transport and contour currents in the eastern flank and by turbidity currents in the western flank. The morphologic and stratigraphic characterisation of the Pianosa CDS is based on multibeam bathymetry, seismic reflection data (multi-channel high resolution mini GI gun, single-channel sparker and CHIRP), sediment cores and ADCP data. The Pianosa CDS is located at shallow to intermediate water depths (170 to 850 m water depth) and is formed under the influence of the Levantine Intermediate Water (LIW). It is 120 km long, has a maximum width of 10 km and is composed of different types of muddy sediment drifts: plastered drift, separated mounded drift, sigmoid drift and multicrested drift. The reduced tectonic activity in the Corsica Trough since the early Pliocene permits to recover a sedimentary record of the contourite depositional system that is only influenced by climate fluctuations. Contourites started to develop in the Middle-Late Pliocene, but their growth was enhanced since the Middle Pleistocene Transition (0.7–0.9 Ma). Although the general circulation of the LIW, flowing northwards in the Corsica Trough, remained active all along the history of the system, contourite drift formation changed, controlled by sediment influx and bottom current velocity. During periods of sea level fall, fast bottom currents often eroded the drift crest in the middle and upper slope. At that time the proximity of the coast to the shelf edge favoured the formation of bioclastic sand deposits winnowed by bottom currents. Higher sediment accumulation of mud in the drifts occurred during periods of fast bottom currents and high sediment availability (i.e. high activity of turbidity currents), coincident with periods of sea level low-stands. Condensed sections were formed during sea level high-stands, when bottom currents were more sluggish and the turbidite system was disconnected, resulting in a lower sediment influx.

Tectonic expression of an active slab tear from high-resolution seismic and bathymetric data offshore Sicily (Ionian Sea)

Subduction of a narrow slab of oceanic lithosphere beneath a tightly curved orogenic arc requires the presence of at least one lithospheric scale tear fault. While the Calabrian subduction beneath southern Italy is considered to be the type example of this geodynamic setting, the geometry, kinematics and surface expression of the associated lateral, slab tear fault offshore eastern Sicily remain controversial. Results from a new marine geophysical survey conducted in the Ionian Sea, using high-resolution bathymetry and seismic profiling reveal active faulting at the seafloor within a 140 km long, two-branched fault system near Alfeo Seamount. The previously unidentified 60 km long NW trending North Alfeo Fault system shows primarily strike-slip kinematics as indicated by the morphology and steep-dipping transpressional and transtensional faults. Available earthquake focal mechanisms indicate dextral strike-slip motion along this fault segment. The 80 km long SSE trending South Alfeo fault system is expressed by one or two steeply dipping normal faults, bounding the western side of a 500+ m thick, 5 km wide, elongate, syntectonic Plio-Quaternary sedimentary basin. Both branches of the fault system are mechanically capable of generating magnitude 6-7 earthquakes like those that struck eastern Sicily in 1169, 1542, and 1693.

VESPA cruise report. Volcanic Evolution of South Pacific Arcs. n/o L’Atalante, Nouméa – Nouméa, 22 May - 17 June 2015

VESPA was a successful 25 day research cruise on R/V l'Atalante that took place in May and June 2015. The main aim was to acquire new rock samples from extinct volcanoes on the Norfolk, Loyalty and Three Kings ridges, which connect New Caledonia and New Zealand. This was in order to test various hypotheses of Late Cretaceous-Miocene SW Pacific tectonic development relating to (i) nature and duration of magmatism on the ridges; (ii) timing of subduction initiation east of northern Zealandia; (iii) postulated subduction polarity changes. A total of 3400 km of 'sismique rapide' shallow reflection seismic data were acquired and processed onboard. The seismic lines provided a very useful structural-stratigraphic framework for the rock dredging. Combined with multibeam bathymetry data they allowed intelligent targeting of acoustic basement (lavas) and specific seismic reflectors (sedimentary strata) on rocky slopes and fault scarps. Different stratigraphic levels of the Loyalty and Three Kings Ridge volcanic piles were sampled by dredging at different water depths on the Cook Fracture Zone and Cagou Trough fault scarps. By the end of the cruise, 43 dredges had been attempted and 36 of them yielded igneous or sedimentary rocks potentially useful to the VESPA project. Onboard use of a portable X-ray fluorescence unit confirmed the presence of intraplate (but no arc) volcanoes on the Norfolk Ridge and presence of arc, intraplate and shoshonitic volcanoes on the Loyalty and Three Kings Ridges. A total of 770 kg of rock was retained for post-cruise analysis in New Caledonia, France and New Zealand. Future work will include micropaleontological dating of sedimentary rocks, U-Pb and Ar-Ar isotopic dating of igneous rocks, and whole rock geochemical and tracer isotope analyses. We are optimistic that many of the initial research hypotheses will be able to be tested.