Automatic cross-well tomography by semblance and differential semblance optimization: theory and gradient computation

A technique for automatic cross-well tomography based on semblance and differential semblance optimization is presented. Given a background velocity, the recorded seismic data traces are back-propagated towards the source, i.e. shifted towards time zero using the modelled traveltime between the source and the receiver and corrected for the geometrical spreading. Therefore each back-propagated trace should be a pulse, close to time zero. The mismatches between the back-propagated traces indicate an error in the velocity model. This error can be measured by stacking the back-propagated traces (semblance optimization) or by computing the norm of the difference between adjacent traces (differential semblance optimization).
It is known from surface seismic reflection tomography that both the semblance and differential semblance functional have good convexity properties, although the differential semblance functional is believed to have a larger basin of attraction (region of convergence) around the true velocity model. In the case of the cross-well transmission tomography described in this paper, similar properties are found for these functionals.
The implementation of this automatic method for cross-well tomography is based on the high-frequency approximation to wave propagation. The wavefronts are constructed using a ray-tracing algorithm. The gradient of the cost function is computed by the adjoint-state technique, which has the same complexity as the computation of the functional. This provides an efficient algorithm to invert cross-well data. The method is applied to a synthetic data set to demonstrate its efficacy.     

Slope‐to‐basin stratigraphic evolution of the northwestern Great Bahama Bank (Bahamas) during the Neogene to Quaternary: interactions between downslope and bottom currents deposits

Multichannel high‐resolution seismic data along the northwestern margin of the Great Bahama Bank (GBB), Bahamas, detail the internal geometry and depositional history of a Neogene‐Quaternary carbonate slope‐to‐basin area. The stratigraphic architecture through this period evolves from (i) a mud‐dominated slope apron during the Miocene, (ii) a debris‐dominated base‐of‐slope apron during the Late Pliocene and then (iii) return to a slope apron with very short prograding clinoformal aprons during the Pleistocene. This geometric evolution was broadly constrained by the development of the Santaren Drift by bottom current since the Langhian. The drift expands along the northwestern GBB slope, forming a continuous correlative massive feature that shows successive phases of growth and retreat and influenced the downslope sediments distribution. Indeed, Late Pliocene deposits are confined into the moat, forming a strike‐continuous coarse debrites belt along the mid‐slope, preventing their free expansion into the basin. The occurrence of basinal drift that operated since 15 Ma showed a significant upslope growth around 3.6 Ma and is interpreted as resulting from the closure of the Central American Seaway which also coincides with a global oceanographic re‐organization and climate changes in the Northern Hemisphere.     

An enriched mantle source for potassic basanites: evidence from Karisimbi volcano,Virunga volcanic province,Rwanda

Lavas from Karisimbi, the largest volcano in the Virunga province in the Western Branch of the African rift on the Zaire-Rwandan border, constitute a suite of mafic potassic basanites and more evolved potassic derivatives. All of the lavas are potassic with K₂O/Na₂O1, and enriched in incompatible elements, with chondrite normalised (La/Yb)_n>18 and Nb/Zr>0.25. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios reflect these enriched compositions, varying from 0.7052 and 0.51258 respectively in the K-basanites to 0.7132 and 0.51226 in the most evolved K-trachyte, although at MgO abundances >4% there is no systematic variation of isotope ratios with fractionation. At >4% MgO, lava compositions were controlled by assimilation and fractional crystallization in a sub-volcanic magma chamber. Trace-element and isotope variations in the more mafic lavas appear to reflect mixing between a primitive K-basanite (PKB) magma and a Sr-rich end-member, similar to melilite nephelinites from the neighbouring volcano, Nyiragongo. Both endmembers are mantle-derived and isotopically distinct, with the PKB being characterised by ⁸⁷Sr/⁸⁶Sr up to 0.707 and ¹⁴³Nd/¹⁴⁴Nd as low as 0.51236. Alternatively, isotope variations may be the time-integrated response to trace-element fractionations in a variably enriched mantle source. The Pb isotope variations within Karisimbi are complex. In the more evolved lavas all three ratios increase coherently with fractionation, whereas in the mafic varieties ²⁰⁶Pb/²⁰⁴Pb remains roughly constant at 19.2 while ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb vary from 15.67 to 15.78 and 39.49 to 40.80 respectively, defining sub-vertical trends, consistent with PKB-nephelinite magma mixing. The Nd and Sr isotopes indicate trace-element fractionation in the PKB source at 1 Ga, similar to ages derived from the overlying crust and suggesting a lithospheric origin. Elevated ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁸Pb^*/²⁰⁶Pb^* values of the PKB are also consistent with Th/U fractionation at a similar time. However, this 1Ga age contrasts with that derived from the elevated ²⁰⁷Pb/²⁰⁴Pb ratios which indicate U/Pb fractionation during the Archaean. Crustal contamination can be excluded as the major control of Pb isotope variation in the PKB because their high Ce/Pb ratios (27) are similar to those typical of oceanic basalts. Parent/daughter trace-element fractionation and the high Ti, Nb and Ta abundances of the PKB lavas are all consistent with enrichment of a lithospheric source region by small-degree silicate melts at 1Ga. Comparison between measured and time-integrated trace-element ratios suggests that the degree of melting associated with recent magmatism was 5%. These data show that significant Th/U and Rb/Sr fractionation can be produced by intra-mantle melting processes and that high ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁸Pb^*/²⁰⁶Pb^* values can evolve within the upper mantle and do not necessarily require the recycling of crustal material. Comparable isotope features in continental flood basalts and DUPAL ocean island basalts may be explained in a similar way.     

Sediment distribution and evolution of sedimentary processes in a small sandy turbidite system (Golo system, Mediterranean Sea): implications for various geometries based on core framework

The Golo Margin in eastern Corsica is dissected by four canyons and two gullies which fed turbidite systems. Study of the dispersal of surficial sediments and flow dynamic in the Golo system is based on Kullenberg and interface cores interpreted in relation to a previously published seismic dataset. Cores were described in detail and interpreted within a sedimentary and stratigraphic framework. During the last 42,000 years, gravity processes which occurred in the large systems with a canyon source were mainly slide-induced, differentiated turbulent surges and hyperpycnal flows. Processes occurring in the small system with a gully source are mainly hyperconcentrated and concentrated flows. Deposits from the Corsican Margin can intercalate with products of processes triggered on the Pianosa Ridge located in the eastern part of the basin. During relative sea-level lowstands or during periods of rapid or high-amplitude sea-level fall, only large canyons (South and North Golo) are supplied by carbonate-rich hyperconcentrated and concentrated flows which are channelled in incised valleys on the shelf. During periods of slow or low-amplitude sea-level fall and during sea-level rise, sediments are trapped on a shelf delta and intensely winnowed by shelf hydrodynamic processes. Sand-rich hyperconcentrated and concentrated flows occur. All the systems fed by a canyon are active simultaneously. Gullies form and are active only during periods of sea-level rise. During relative highstands of sea level (Holocene), all the system is draped by hemipelagic sediments. Relative sea-level changes and canyon location relative to river mouths have a strong influence on the nature of sediment input, and the initiation and type of gravity flows which, in turn, control morphology and geometry.     

Contourite drift construction influenced by capture of Mediterranean Outflow Water deep-sea current by the Portimão submarine canyon (Gulf of Cadiz, South Portugal)

The margin of the Gulf of Cadiz is swept by the deep current formed by the Mediterranean Outflow Water (MOW) flowing from the Mediterranean to the Atlantic. On the northern margin of the Gulf (Algarve Margin, South Portugal), the MOW intensity is low and fine-grained contourite drifts are built up with an alongslope development. From new sedimentological data, this study emphasizes the presence of two types of contourite drifts separated only by a deep submarine canyon incising the slope with a north-south orientation (Portimão Canyon). High-resolution seismic and bathymetry interpretation shows that on the eastern side of the canyon, the MOW forms a thick and large detached drift (Albufeira Drift) prograding toward both north and west, as shown in seismic profiles, with a high sedimentation rate. On this side of the canyon, the MOW intensity is high enough to erode the slope forming a moat channel (Alvarez Cabral). On the western side of Portimão Canyon, the MOW energy is lower, preventing moat channel erosion. Only flat and thin drift develops (Portimão and Lagos Drifts) with slow aggradation and a low sedimentation rate. This difference in drift development is due to the presence of the canyon which generates an important change in hydrodynamic of the MOW, confirmed by temperature-density measurements showing that MOW flows down Portimão Canyon. The canyon is responsible for the deviation of the direction of the MOW as it partly catches the deep-sea current flowing westward (i.e. capture phenomenon). It creates, thus, a decrease of the flow energy, competency and capacity between the east and west sides of the canyon. Through this phenomenon of MOW deep-sea current capture, the canyon constitutes a morphologic feature generating an important change in the contourite deposition pattern.In addition to already known climatic and oceanographic influences, our results show the role of canyons on contourite drift building. This study provides new elements on autocyclic factors influencing the contourite sedimentation, which are important to consider in future sedimentary paleo-reconstruction interpretations.     

The French Atlantic margin and deep-sea submarine systems

The sedimentary infill of the Bay of Biscay off Ireland, UK, France and Spain took place in four phases. The last one (35 Ma to present) is characterised by gravitational, pelagic, contouritic and glacigenic processes leading to the setup of three deep sea systems. To the North, the Celtic and Armorican fans are fed by a “canyon-dominated” margin and its connection with the “Manche” palaeoriver, which drained a large area of western Europe. To the South, the Cap-Ferret fan results from the evolution of a “tectonic-dominated” margin and the erosion of the Pyrenee mountains.