Duncan Tamsett

Sea-bed characterisation and classification from the power spectra of side-scan sonar data

This work is concerned with the automatic characterisation and classification of the sea-bed for side-scan sonar trace power-spectra.A parametric model of side-scan sonar trace power-spectra is developed from the equation for the magnitude frequency response of a Butterworth filter. The models parameters are optimised to give a least squares fit with observed spectra. Three of the optimised parameters are used to define features.The parametric optimisation approach to feature extraction is compared to a method described by Pace and Gao (1988) in which features are defined in terms of ratios of integrals over frequency intervals of observed (specially defined) power spectra.Scatter distributions in feature space are reduced to sets of numbers that define distribution fields and these constitute seabed characteristics. A classification exercise is undertaken to demonstrate the utility of the feature extraction methods. The discrimination between sea-bed types achieved by the feature extraction methods is succintly conveyed in class discrimination matrices.The merits of the spectral modelling approach to feature extraction are: (1) the features provide a meaningful description of the spectra from which they are extracted, (2) the features enjoy a degree of immunity to changes in noise in the signal from which they are extracted, and (3) the features provide excellent discriminants. The parametric fitting process, however, is slow. An important merit of the Pace and Gao (1988) approach is that feature extraction is rapid.

Comments on the development of rifts and transform faults during continental breakup; examples from the Gulf of Aden and northern Red Sea

Abstract A number of basins are observed to extend inland from the coasts on both sides of the Gulf of Aden. The basins are orientated at approximately right angles to the spreading direction and intersect the coasts at the meeting of sheared and rifted continental margins. They appear to be grabens, one wall of which is continuous with the half graben of the neighbouring rifted margin. It is suggested that these were once parts of a number of discrete rifts arranged en-echelon along a zone of lithospheric weakness during the early opening of the Gulf of Aden, which became redundant when transform faults formed. The proposed development of rifts and transform faults is similar to that of a spreading centre, transform fault, spreading centre pattern developed in the freezing wax model of Oldenburg and Brune (1975). The Gulf of Suez at the northern end of the Red Sea is interpreted in a similar way since it has a number of features in common with the basins in the continents adjacent to the Gulf of Aden.

Sampling trips for measuring discards in commercial fishing based on multilevel modelling of measurements in the North Sea from NE England

Abstract This study estimates the effects of variables (fishing, spatial, temporal) on discarding and proposes a scheme for measuring discarding. Data are used from fishing in the English North Sea for cod, haddock and whiting. Analysis of variance of discarding rates (numbers discarded/numbers caught) has provided estimates of components of variance associated with variables. Significant estimates of components of variance have emerged from data spanning 3 years. There is a large component of variance associated with trips (the lowest level at which data are available) reflecting an inherent noisiness in discarding rates. For haddock and whiting, there are significant components of variance associated with years . An effect of years on cod was not found for the years for which data are available. The variables: gear type , port of landing , fishing grounds and quarter , are crossed at a common level. Of these gear contributes the greatest variation in discarding rates. Port and grounds are also important contributors. Quarter has a small but significant effect. Multivariate analysis of variance has produced evidence for strong multilevel correlations in discarding rates between haddock and whiting. Discarding rates vary significantly with size of catch (trip) for cod and for haddock. Variables are reduced to strata , and analyses of variance for models incorporating strata carried out. An approach to sampling based on stratification with proportional allocation is discussed. This provides efficient sampling and ensures representative sampling in a problem for which there are relatively few observations, and several variables affecting the response variable.

Onboard sampling for measuring discards in commercial fishing based on multilevel modelling of measurements in the Irish Sea from NW England and N Wales

Abstract A method for onboard sampling of discards developed for the North Sea was found to be unsatisfactory in the Irish Sea. An alternative method for the Irish Sea provided data between August 1993 and September 1994 on discarding rates at the haul and within-haul levels, as well as the trip level (the lowest level for which data are available from the method used in the North Sea). The data are subjected to multilevel analysis of variance to measure components of variance associated with fishing and environmental parameters. Discarding rates at the haul level are affected by distance from the coast and duration of haul. Within-haul estimates of discarding rates are analysed for evidence that estimates from samples of catches for hauls are over-dispersed (relative to the dispersion associated with binomially distributed discarding rates). This has a bearing on the optimal size that samples of catches for hauls should be. No evidence for over-dispersion is found for the Irish Sea for which the data set with within-haul data is small. However, analysis of a data set from the English Channel for which there is a greater volume of data at the within-haul level, has indicated significant and sizeable over-dispersion. An approach is outlined for calculating the optimal size that samples of catches for hauls should be for estimating discarding rates as a function of year class, and taking account of over-dispersion. Optimal sample sizes are surprisingly small. This might render practicable the acquisition of samples of hauls by fishermen in the absence of an onboard technician for analysis by technicians post-trip.

Median valley tectonics: Air photographs of the Ghoubbet-Asal rift, Afar

Abstract The Ghoubbet-Asal rift in Afar is the western continuation of the Gulf of Aden seafloor spreading centre onto land. Air photographs were used to examine its small-scale tectonic structures. A central, relatively flat, zone about 5 km across is the main centre of volcanism. This is bounded by large-throw normal faults spaced at 1–2 km with scarps that face the spreading axis, which are responsible for the gross morphology of the lower walls of the median valley. Short, small-throw, antithetic faults are situated near the base of the main fault scarps in places. These are formed as the main fault blocks ascend the median valley walls, and make only a minor contribution to the morphology of the median valley. Rotation of the main fault blocks away from the spreading centre, as the crust ascends the valley walls, is the main process giving rise to the morphology of the transition from the median valley to the crestal mountains. Antithetic faults close to the bases of major synthetic faults seen on the air photographic data are uncommon observations on Gloria side-scan sonar data for marine slow-spreading centres. If present in greater numbers, however, the scarps of such features would lie in acoustic shadows cast by the major synthetic faults and thus be hidden from Gloria. Other interesting features on the air photographic data include a subsidiary rift that intersects a large-throw normal fault associated with the main rift, and a linear volcanic ridge which constitutes the main centre of volcanism within the spreading centre.

Sidescan Sonar Beam Function and Seabed Backscatter Functions From Trace Amplitude and Vehicle Roll Data

A sidescan sonars beam function (BF) (the sonars transmit-receive response versus inclination angle in the sonars frame of reference) can be extracted from an ensemble of contiguous sonar traces affected by sonar vehicle roll for a uniform seabed. The rotation of the sonars frame of reference relative to the seabeds frame of reference is employed to extract a number of discrete overlapping beam subfunctions each with angular extent corresponding to the extent of vehicle roll. These are then reconciled to form a composite sonar BF. A seabed backscatter function (SF) (the backscatter response of the seabed versus inclination angle in the seabeds frame of reference) may subsequently be inferred with respect to the BF from trace data, whether affected by roll or not. Independent sonar BF and seabed SFs allow the effect of sonar vehicle roll to be corrected when BF compensation is applied to images, and the effect of seabed slope to be corrected when SF compensation is applied. In an alternate approach to extracting a sonar BF, a seabed SF may be extracted first from the sonar traces affected by vehicle roll, by extracting a number of overlapping backscatter subfunctions, and reconciling these into a composite SF. The sonar BF may then be inferred from the trace data with respect to the SF. Seabed SFs inferred from trace data with respect to the BF may be used as a basis for seabed characterization.

A comparison of methods for onboard sampling of discards in commercial fishing

Two methods for onboard sampling of discards practised in UK waters are compared. The fishing gear was light trawl and the principal species caught were haddock and whiting. The quantities of fish discarded, as well as those for the market, were measured. The methods for onboard sampling of discards were carried out in parallel. Quantities discarded estimated using the two methods are compared against the measured quantities. No evidence for bias is found for either method. The results of other analyses of the data are presented, and the advantages and disadvantages of the methods discussed. A proposal is made for an approach to onboard sampling.

Synthetic Aperture Computation as the Head is Turned in Binaural Direction Finding

Binaural systems measure instantaneous time/level differences between acoustic signals received at the ears to determine angles λ between the auditory axis and directions to acoustic sources. An angle λ locates a source on a small circle of colatitude (a lamda circle) on a sphere symmetric about the auditory axis. As the head is turned while listening to a sound, acoustic energy over successive instantaneous lamda circles is integrated in a virtual/subconscious field of audition. The directions in azimuth and elevation to maxima in integrated acoustic energy, or to points of intersection of lamda circles, are the directions to acoustic sources. This process in a robotic system, or in nature in a neural implementation equivalent to it, delivers its solutions to the aurally informed worldview. The process is analogous to migration applied to seismic profiler data, and to that in synthetic aperture radar/sonar systems. A slanting auditory axis, e.g., possessed by species of owl, leads to the auditory axis sweeping the surface of a cone as the head is turned about a single axis. Thus, the plane in which the auditory axis turns continuously changes, enabling robustly unambiguous directions to acoustic sources to be determined.

Binaural Range Finding from Synthetic Aperture Computation as the Head is Turned

A solution to binaural direction finding described in Tamsett (Robotics 2017, 6(1), 3) is a synthetic aperture computation (SAC) performed as the head is turned while listening to a sound. A far-range approximation in that paper is relaxed in this one and the method extended for SAC as a function of range for estimating range to an acoustic source. An instantaneous angle λ (lambda) between the auditory axis and direction to an acoustic source locates the source on a small circle of colatitude (lambda circle) of a sphere symmetric about the auditory axis. As the head is turned, data over successive instantaneous lambda circles are integrated in a virtual field of audition from which the direction to an acoustic source can be inferred. Multiple sets of lambda circles generated as a function of range yield an optimal range at which the circles intersect to best focus at a point in a virtual three-dimensional field of audition, providing an estimate of range. A proof of concept is demonstrated using simulated experimental data. The method enables a binaural robot to estimate not only direction but also range to an acoustic source from sufficiently accurate measurements of arrival time/level differences at the antennae.