Guy R. Cochrane

Use of acoustic classification of sidescan sonar data for mapping benthic habitat in the Northern Channel Islands, California

Highly reflective seafloor features imaged by sidescan sonar in nearshore waters off the Northern Channel Islands (California, USA) have been observed in subsequent submersible dives to be areas of thin sand covering bedrock. Adjacent areas of rocky seafloor, suitable as habitat for endangered species of abalone and rockfish, and encrusting organisms, cannot be differentiated from the areas of thin sand on the basis of acoustic backscatter (i.e. grey level) alone. We found second-order textural analysis of sidescan sonar data useful to differentiate the bottom types where data is not degraded by near-range distortion (caused by slant-range and ground-range corrections), and where data is not degraded by far-range signal attenuation. Hand editing based on submersible observations is necessary to completely convert the sidescan sonar image to a bottom character classification map suitable for habitat mapping.

Transect across the West Antarctic rift system in the Ross Sea, Antarctica

Abstract In 1994, the ACRUP (Antarctic Crustal Profile) project recorded a 670-km-long geophysical transect across the southern Ross Sea to study the velocity and density structure of the crust and uppermost mantle of the West Antarctic rift system. Ray-trace modeling of P- and S-waves recorded on 47 ocean bottom seismograph (OBS) records, with strong seismic arrivals from airgun shots to distances of up to 120 km, show that crustal velocities and geometries vary significantly along the transect. The three major sedimentary basins (early-rift grabens), the Victoria Land Basin, the Central Trough and the Eastern Basin are underlain by highly extended crust and shallow mantle (minimum depth of about 16 km). Beneath the adjacent basement highs, Coulman High and Central High, Moho deepens, and lies at a depth of 21 and 24 km, respectively. Crustal layers have P-wave velocities that range from 5.8 to 7.0 km/s and S-wave velocities from 3.6 to 4.2 km/s. A distinct reflection (PiP) is observed on numerous OBS from an intra-crustal boundary between the upper and lower crust at a depth of about 10 to 12 km. Local zones of high velocities and inferred high densities are observed and modeled in the crust under the axes of the three major sedimentary basins. These zones, which are also marked by positive gravity anomalies, may be places where mafic dikes and sills pervade the crust. We postulate that there has been differential crustal extension across the West Antarctic rift system, with greatest extension beneath the early-rift grabens. The large amount of crustal stretching below the major rift basins may reflect the existence of deep crustal suture zones which initiated in an early stage of the rifting, defined areas of crustal weakness and thereby enhanced stress focussing followed by intense crustal thinning in these areas. The ACRUP data are consistent with the prior concept that most extension and basin down-faulting occurred in the Ross Sea during late Mesozoic time, with relatively small extension, concentrated in the western half of the Ross Sea, during Cenozoic time.

Negative‐polarity seismic reflections along faults of the Oregon accretionary prism: Indicators of overpressuring

Either thrusting of higher over lower impedance sediment or reduction of impedance locally in the fault zone can produce negative-polarity reflections along the protothrusts and the frontal thrust of the Oregon accretionary prism. The vertical displacement of protothrusts averages only 27 m, is uncorrelated to the occurrence of polarity reversals, and is insufficient to generate the impedance contrasts associated with the negative-polarity reflections. The negative-polarity reflections from the protothrusts are probably due to localized impedance reduction in the fault zone. Waveform modeling of these negative-polarity reflections indicates they develop due to a single interface with negative impedance contrast. This seismic signal could be produced by a fault zone with a sharp upper boundary with low impedance fault material and a transition down section to sediment of higher impedance. An impedance inversion due to the 1-km throw on the frontal thrust could create the observed negative-polarity reflections. Interval velocities do not decrease across the frontal thrust, but core-scale velocity measurements indicate a velocity decrease in the frontal thrust. Therefore the negative-polarity reflections are apparently produced by a localized decrease in impedance. Waveform modeling of frontal thrust reflections suggests the low impedance interval is about 20 m thick with discrete upper and lower boundaries. Moderately dipping faults are an efficient path for fluid expulsion across the turbidites with low equivalent vertical permeability. Fluid migration from the over-pressured section up the faults can locally reduce the velocity sufficiently to cause the negative-polarity reflections.

Velocity and inferred porosity model of the Oregon accretionary prism from multichannel seismic reflection data: Implications on sediment dewatering and overpressure

A two-dimensional model of seismic velocity derived from multichannel seismic data collected off Oregon in 1989 shows that as sediments are carried from Cascadia Basin into the accretionary prism, there are measurable changes in velocity-depth profiles. In the seaward most area of the basin, where no thrust faults are observed, there is a landward (and downward) increase of velocity in the sedimentary section. We attribute the velocity increase in the basin to a reduction of porosity resulting from consolidation and cementation, accompanied by diffusive flow of pore water driven by lateral tectonic as well as gravitational stress. Near the base of the slope there is an area of incipient thrusting (the protothrust zone) where protothrusts sole out into a protodecollement. Synthetic seismogram modeling of the reverse-polarity reflection from the protodecollement shows a 100-m-thick layer with a slightly lower velocity relative to the sediments above it. Above the protodecollement, velocity continues to increase landward. We suggest that in this area the diffusive flow of pore water out of the sediment is augmented above the protodecollement by fault-focused flow. Below the protodecollement a reversal in velocity may be due to an increase in porosity resulting from overpressuring of pore fluid trapped by reduction of the permeability of the sediment above the protodecollement. Farther landward, where thrusting has formed a fault-bend fold, velocity values are lower in the accreted section of sediments relative to the velocity at a comparable subbottom depth in the protothrust zone. The decrease in velocity is a result of microfracturing of the highly consolidated sediments accompanying uplift and folding and reflects the increasing role of fracturing and faulting in the control of dewatering of the sediments.

Models and maps: predicting the distribution of corals and other benthic macro-invertebrates in shelf habitats

Presence-absence data of benthic macro-invertebrates and associated habitat (i.e., sediment type and depth) were collected using a towed camera sled in selected areas along the coast off southern California. Using this information, we developed generalized linear models (GLMs) to predict the probability of occurrence of five commonly observed taxa (cup corals, hydroids, short and tall sea pens, and brittle stars in the sediment) within the Santa Barbara Channel (SBC). We employed a utility-based approach to validate the models and to identify optimal cutoff thresholds for the GLM predictions, given a relative cost of false negatives and false positives. We present optimal cutoff values under a range of cost ratios. Out-of-sample predictive accuracy, assuming equal costs of false negatives and false positives, ranged from 75% to 89%. Estimated area under the characteristic curve (AUC) in our models ranged from 0.76 for brittle stars in sediment to 0.91 for cup corals. An AUC value above 0.7 is an acceptable level of performance, between 0.8 and 0.9 is excellent, and above 0.9 is outstanding. We developed predictive maps of probability of occurrence using our validated models and a seafloor character map based on fine-scale (2–5 m) data. Cup corals and hydroids had high predicted probabilities of occurrence in areas of hard substrata, while short and tall sea pens were predicted to occur in parts of the SBC that had unconsolidated and mixed sediment. Our model predicted that brittle stars would occur throughout the entire channel on various bottom types. The combination of high-resolution seafloor character maps and predictive models of invertebrate distributions will aid managers in identifying habitats of particular concern and areas with vulnerable, deep-sea corals. Our models will be useful for marine spatial planning and ecosystem-based management, as well as for assessing the effectiveness of essential fish habitat closures and other marine protected areas.

Recent Deformation along the Offshore Malibu Coast, Dume, and Related Faults West of Point Dume, Southern California

Offshore faults west of Point Dume, southern California, are part of an important regional fault system that extends for about 200 km, from near the city of Los Angeles westward along the south flank of the Santa Monica Mountains and through the northern Channel Islands. This boundary fault system separates the western Transverse Ranges, on the north, from the California Continental Borderland, on the south. Previous research showed that the fault system includes many active fault strands; consequently, the entire system is considered a serious potential earthquake hazard to nearby Los Angeles. We present an integrated analysis of multichannel seismic- and high-resolution seismic-reflection data and multibeam-bathymetric information to focus on the central part of the fault system that lies west of Point Dume. We show that some of the main offshore faults have cumulative displacements of 3–5 km, and many faults are currently active because they deform the seafloor or very shallow sediment layers. The main offshore fault is the Dume fault, a large north-dipping reverse fault. In the eastern part of the study area, this fault offsets the seafloor, showing Holocene displacement. Onshore, the Malibu Coast fault dips steeply north, is active, and shows left-oblique slip. The probable offshore extension of this fault is a large fault that dips steeply in its upper part but flattens at depth. High-resolution seismic data show that this fault deforms shallow sediment making up the Hueneme fan complex, indicating Holocene activity. A structure near Sycamore knoll strikes transversely to the main faults and could be important to the analysis of the regional earthquake hazard because the structure might form a boundary between earthquake-rupture segments.

Habitats and Benthos of an Evolving Fjord, Glacier Bay, Alaska

Publisher Summary Glacier Bay National Park and Preserve is located in southeastern Alaska. It is a fjord system that bifurcates into two main northern tributaries: the West Arm and Muir Inlet, also known as the East Arm. This chapter focuses on the main southern bay and Muir Inlet, the 41-km long, 1–4-km wide fjord. Historic deglaciation of the region is well documented, resulting in a dynamic estuarine environment with vigorous tidal currents and rapid sedimentation. These factors lead to seafloor instability and constantly change benthic environments. The diverse settings in the estuary generate productive food webs; recreational, commercial, and subsistence fisheries; large populations of marine mammals and seabirds; and vessel-based tourism. Benthic habitat mapping has been completed for Glacier Bay (in 2005) and Muir Inlet (in 2010) following multibeam sonar surveys in these areas. Though different classification schemes were used, the classes are based on geomorphologic features, substrate classes, and depth zones, which make the results comparable. There is greater diversity of habitat (and epifauna) in Glacier Bay as compared to Muir Inlet. Substrate classes ranging from sand to coarser sediments make up the majority of habitat in Glacier Bay, whereas mud is the dominant substrate in Muir Inlet. These results suggest that Muir Inlet and other inlets to the north of Glacier Bay are very efficient traps of fine glacial sediment and should be considered distinct biotopes for ecological management purposes.

Bathymetry, acoustic backscatter, and seafloor character of Farallon Escarpment and Rittenburg Bank, northern California

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