A matched-filter approach to wave migration

Wave migration is a technique in which the reflectivity of the Earth is interpreted by extrapolating the fields measured on the surface into the ground. The motivation of this paper is to develop a generalized imaging algorithm based on a matched-filter that shows a mathematical connection between currently used migration techniques. The filter is determined by estimating the received signal when a specific test target exists in the ground. To keep the method general, a point scatterer is used as this target, while distributed objects are modeled without changing the filter characteristics by a collection of independent point scatterers. Also, the specific forms of the Green's functions, which describe wave propagation in the ground, are not included in the formation of this approach leaving more freedom in the implementation. When the filter is applied to measured data of a monostatic survey, the resulting method becomes a forward scattering problem in which these data become time-reversed current sources. Next, specific forward scattering techniques are applied to this matched-filter approach and the resulting methods are compared to traditional migration techniques. In doing so, we find that the general form of most migration techniques can be shown using a matched-filter, while the major differences lie in the actual interpretation of the wave propagation that is used to implement the filter. The similarities of the matched-filter-based approaches to traditional techniques are used to show a connection and general overview of wave migration. Finally, these methods are applied to data collected over pipes buried in sand.     

Array measurements of the bottom boundary layer and the internal wave field on the continental slope

Abstract

An array of current meters was placed on the continental slope and rise for two months in the autumn of 1970. The bottom boundary layer was penetrated on the slope. On the smallest array scale, of the order of 1 kilometer, the array functioned as a directional internal wave antenna. Moving shoreward, the current spectra show strong suppression of the inertial peak and strong enhancement of the semidiurnal tide. The measured wave number spectra show that the tidal energy is almost completely baroclinic, and probably being generated in the region where the slope becomes “critical” for the tidal period. If this area is typical of worldwide conditions, a substantial fraction of the dissipation of surface tides takes place on the continental slopes by conversion to baroclinic waves. The bottom boundary layer has been modeled by an extension of the work of Ellison (1956) to a sloping boundary in a fluid of positive stability. An equivalent constant eddy coefficient has the value 3 cm²/sec as determined from the measurements.     

A simple method to help determine landslide susceptibility from spaceborne InSAR data: the Montescaglioso case study

On December 3, 2013, a large complex landslide was triggered SW of the town of Montescaglioso (Southern Italy), causing the destruction of roads, commercial buildings and private dwellings, as well as several direct and indirect economic losses. A set of interferometric ground measurements acquired by the Cosmo-SkyMed satellite constellation and processed by means of the SqueeSAR algorithm was used to study the pre-event slope displacements in the entire Montescaglioso municipal area. Data span from January 30, 2012, to December 2, 2013, and show average line-of-sight velocities of 1–10 mm/year in the slope sector ultimately affected by the collapse. In retrospect, a time series analysis of the radar targets was performed in order to identify and characterize all the slope instabilities in proximity of the town. This was based on the setup of characteristic thresholds of displacement. The procedure permitted to locate several areas which recurrently exceeded these previously established thresholds, in consistency with the amount of precipitation. In particular, the major source of potential hazard in the area was indeed found where the December 3, 2013, landslide eventually occurred. The results of this quick data processing technique were validated through comparison with two independently developed landslide maps. This simple method, which is not supposed to diminish the importance of geomorphologic field surveys, could improve both the accuracy and the update rate of landslide susceptibility maps. Not relying on arbitrary or empirically derived approaches, it has the advantage of computing statistically based thresholds specific for each time series. By indicating the slope sectors in higher need of deeper in situ investigation, more support could be provided to administrative bodies for the processes of risk assessment and management.     

Comparison between in situ cone resistance and laboratory strength for overconsolidated north sea clays

Abstract

Cone resistance and laboratory strength measurements have been compared for stiff overconsolidated clays from five oil and gas fields in the North Sea. The clays considered are glacial in origin. The best agreement between cone resistance and laboratory strength is found by using an N_k factor equal to 17 in the formula

q_c = N_k . S_u + γZ.

The study shows that the cone gives highly reproducible results, whereas there is a considerable scatter in the laboratory strength determinations. When making use of CPTs for preliminary design, the authors recommend assuming an N_k of 15–20, depending on the type of problem under consideration.     

A vessel‐deployed passive postlarval collector to assess settlement of the American lobster Homarus americanus

Passive collectors are used widely in postlarval settlement and recruitment monitoring of spiny lobsters and crabs, but they have only been used in a limited way with clawed lobsters. For nearly two decades, diver‐based suction sampling has served to monitor spatial‐temporal patterns of American lobster (Homarus americanus) postlarval settlement and early juvenile abundance in shallow near‐shore nurseries. Collectors could reveal settlement patterns in zones beyond the practical limits of diving. In 2005, we launched a fisher‐scientist collaboration to evaluate the performance of passive collectors designed to extend the reach of sampling, and to be deployable from a vessel equipped with a standard pot‐hauler. Building on previous designs, our collectors comprised wire mesh trays lined with fine screening on the floor and walls and filled with cobble to simulate natural nursery habitat. Results indicate that no newly settled lobsters were lost during the retrieval process, and densities of young‐of‐year lobsters found in the collectors were similar to those in directly adjacent natural cobble habitat sampled by divers with suction samplers. The collectors also proved to be effective samplers of juvenile fish and crabs, suggesting a possibility for wider application. This success bodes well for expanded deployment of cobble collectors to broaden our understanding of the recruitment processes of lobster and other cobble‐dwelling fauna along the coast of New England, United States and Atlantic Canada.     

Indicators of abundance and spatial distribution of lobsters (Homarus americanus) from standard traps

Indicators of abundance for American lobster (Homarus americanus) based on 8 years of trap catch rates (catch‐per‐unit‐effort, CPUE) were evaluated. Volunteer harvesters recorded count, sex and size of lobsters captured in standard traps on a daily basis during the fishing season in coastal Nova Scotia, Canada. We examined the extent to which standardised CPUEs of prerecruits predict the future catches of legal sizes and explored spatial patterns in the abundance of lobsters of different size and reproductive status. The standardised CPUE of prerecruits was correlated with legal size catches in only one of five areas examined. This area had a strong signal of incoming recruitment. Improving the capacity of prerecruit CPUE for predicting legal size catches several years later most likely lies with model incorporation of variables associated with catchability. The spatial distribution of catch rates showed that the area with the highest historical landings per unit area also had the highest relative abundance of prerecruits. The spatial distribution data point to further areas of research related to recruitment processes in lobster in coastal Nova Scotia.     

A test of the ADV-based Reynolds flux method for in situ estimation of sediment settling velocity in a muddy estuary

Under conditions common in muddy coastal and estuarine environments, acoustic Doppler velocimeters (ADVs) can serve to estimate sediment settling velocity (w _s) by assuming a balance between upward turbulent Reynolds flux and downward gravitational settling. Advantages of this method include simple instrument deployment, lack of flow disturbance, and relative insensitivity to biofouling and water column stratification. Although this method is being used with increasing frequency in coastal and estuarine environments, to date it has received little direct ground truthing. This study compared in situ estimates of w _s inferred by a 5-MHz ADV to independent in situ observations from a high-definition video settling column over the course of a flood tide in the bottom boundary layer of the York River estuary, Virginia, USA. The ADV-based measurements were found to agree with those of the settling column when the current speed at about 40 cm above the bed was greater than about 20 cm/s. This corresponded to periods when the estimated magnitude of the settling term in the suspended sediment continuity equation was four or more times larger than the time rate of change of concentration. For ADV observations restricted to these conditions, ADV-based estimates of w _s (mean 0.48±0.04 mm/s) were highly consistent with those observed by the settling column (mean 0.45±0.02 mm/s). However, the ADV-based method for estimating w _s was sensitive to the prescribed concentration of the non-settling washload, C _wash. In an objective operational definition, C _wash can be set equal to the lowest suspended solids concentration observed around slack water.     

Asteroid (101955) 1999 RQ36: Spectroscopy from 0.4 to 2.4 μm and meteorite analogs

We present reflectance spectra from 0.4 to 2.4 μm of Asteroid (101955) 1999 RQ36, the target of the OSIRIS-REx spacecraft mission. The visible spectral data were obtained at the McDonald Observatory 2.1-m telescope with the ES2 spectrograph. The infrared spectral data were obtained at the NASA Infrared Telescope Facility using the SpeX instrument. The average visible spectrum is combined with the average near-infrared wavelength spectrum to form a composite spectrum. We use three methods to constrain the compositional information in the composite spectrum of Asteroid (101955) 1999 RQ36 (hereafter RQ36). First, we perform a least-squares search for meteorite spectral analogs using 15,000 spectra from the RELAB database. Three most likely meteorite analogs are proposed based on the least-squares search. Next, six spectral parameters are measured for RQ36 and their values are compared with the ranges in parameter values of the carbonaceous chondrite meteorite classes. A most likely meteorite analog group is proposed based on the depth of overlap in parameter values. The results of the least-squares search and the parametric comparisons point to CIs and/or CMs as the most likely meteorite analogs for RQ36, and COs and CHs as the least likely. RQ36 has a spectrally “blue” continuum slope that is also observed in carbonaceous chondrites containing magnetite. We speculate that RQ36 is composed of a “CM1”-like material. Finally, we compare RQ36 to other B-type asteroids measured by Clark et al. (Clark, B.E. et al. [2010]. J. Geophys. Res. 115, E06005). The results of this comparison are inconclusive. RQ36 is comparable to Themis spectral properties in terms of its albedo, visible spectrum, and near-infrared spectrum from 1.1 to 1.45 μm. However, RQ36 is more similar to Pallas in terms of its near-infrared spectrum from 1.6 to 2.3 μm. Thus it is possible that B-type asteroids form a spectral continuum and that RQ36 is a transitional object, spectrally intermediate between the two end-members. This is particularly interesting because Asteroid 24 Themis was recently discovered to have H₂O ice on the surface (Rivkin, A., Emery, J. [2010]. Nature 464, 1322–1323; Campins, H. et al. [2010a]. Nature 464, 1320–1321).