ALBERT

Notes: The prevalence of infection of the West Australian dhufish Glaucosoma hebraicum off the lower west coast of Australia by Philometra lateolabracis was greater among females than males, which contrasts with the situation with Pagrus auratus in waters off New Zealand. Live P. lateolabracis were represented solely by females and were found only in the gonads of fish of mature size (L50 at first maturity = c. 300 mm) caught between December and April and thus during the spawning period of dhufish. Since the gonads were largest during that period, they, and particularly ovaries, would provide an abundant food source in the form of blood. The prevalence of the parasite (live + dead individuals) increased with host body size to reach a maximum of c. 80% in the 700–799 mm length class of females and c. 50% in the largest length class of males. During the spawning period of G hebraicum, P. lateolabracis developed from small, non-gravid females to large gravid females, representing an increase in mass of more than 200 times. The maximum length of P. lateolabracis (470 mm) in G hebraicum is the greatest yet recorded for this species. Gonadosomatic indices provided no evidence that infection by P. lateolabracis leads to a conspicuous atrophy of the ovaries, which contrasts with the situation with the gonads of some of the teleosts infected by Philometra species, and presumably reflects, in part, the small volume of the ovary occupied by P. lateolabracis (c. 3%) and the low intensity of infection (mean = 2.0, maximum = 7). Although the presence of live P. lateolabracis did not stimulate an obvious tissue response by the ovaries during the spawning period of G. hebraicum, both the dead and any live parasites that remain after spawning become encapsulated in fibrous tissue.

Notes: Deep seismic reflection profiles show that the lower part of the continental crust often contains many strong subhorizontal reflections. Based on forward modelling, these reflections are interpreted as a sequence of alternating high- and low-velocity layers. A waveform inversion provides an optimum model by minimizing the misfit between observed data and synthetic seismograms. However, it is very difficult to invert for the whole data set, partly because of the large computation time needed in modelling and partly because the high amplitude reflected energy in the upper crust dominates the misfit function.To overcome these difficulties we propose a three-step strategy for inverting the deeper part of seismic reflection data over a narrow aperture in which the velocity structure can be assumed to be 1-D. First, both source wavelet and recorded wavefield are propagated in the plane-wave domain downwards to a particular depth for a given 1-D medium. Because the source-receiver distances are short for deep reflections it is assumed that these reflections are at normal incidence. It is also assumed that the near-surface velocity is known or has been estimated using other inversion schemes. The downward propagation algorithm takes account of nearsurface effects, such as multiple reflections and attenuation. Second, starting from a known background-velocity model, a non-linear iterative waveform inversion is applied to the propagated data set. The method is based on minimizing the difference, sample by sample, between observed and calculated wavefields in a least-squares sense. The forward modelling, which consists of calculating the data from the model, uses a reflectivity method to compute the synthetic seismogram in the plane-wave domain. The model perturbation is calculated with the help of the conjugate gradients method. The non-linear inversion gives the short wavelengths of impedance contrast, which is a function of density and velocity, in the lower crust.Third, an interpretational step is tried in order to recover the medium wavelengths of velocity. The algorithm is tested on synthetic data and then applied to a part of the WAM data set, south-west of England. A good fit to the real data with the optimum model predicts that the velocity contrast of the layers in the lower crust varies from 0.2 km s-1 to 0.5 km s-1 and the thickness from at least 0.075 km to 1 km.

Notes: As part of an intensive study of a small area of oceanic lithosphere, the British Institutions Reflection Profiling Syndicate (BIRPS) acquired closely spaced deepseismic-reflection profiles over the Early Cretaceous crust of the Cape Verde abyssal plain off West Africa. The survey consisted of profiles spaced at 4 km arranged into strike lines parallel to the old sea-floor spreading axis (‘isochron’ profiles) and orthogonal dip lines oriented in the original direction of spreading (‘flow’ profiles). A large-capacity, well-tuned airgun source and very quiet shooting conditions ensured a high signal-to-noise ratio for deep reflection. Devising a strategy for mitigating contamination from ‘wrap-around’ multiples arriving from previous shots enabled us to use the minimum possible shot-point interval (50 m) allowed for collecting long (18 s) records. Data processing was oriented towards a medium with low root-mean-square velocity, steeply dipping structure, and pervasive low apparent velocity noise from diffraction at the top of the igneous crust. The contrast between the isochron and flow profiles is striking. Isochron profiles are typically highly reflective throughout the igneous crust, consisting of bright, bidirectionally dipping reflection sets that extend in places from the top of the igneous basement down to the interpreted Moho reflection. These reflections do not offset intracrustal or top-basement structure and thus are not interpreted as faults: an igneous intrusive origin seems more likely. Flow profiles are more sparsely reflective but show individual steeply dipping reflections best developed in the upper igneous crust, continuing down in places to the Moho. Dipping reflections on the flow profiles are interpreted as major normal faults since they are clearly associated with offsets of the top of the basement as well as truncation of horizontal reflections within the igneous crust. The dominant dip of these reflections is to the west towards the spreading ridge axis. Reflections from the vicinity of the Moho, while well developed in some places, are not particularly prominent across the survey area. Moho reflections appear to show a different structural relation to crustal features on the isochron and flow profiles: on isochron profiles, dipping reflections occasionally flatten out into, and may merge with, the Moho reflection; on flow profiles, as dipping crustal reflections approach the Moho reflection, they are usually abruptly cut off by it without extending deeper. This survey shows how oceanic crustal structure can vary rapidly over relatively small areas, provides convincing evidence that a structurally complex fabric dominates oceanic igneous crust, and gives a conclusive observation of faults that penetrate the entire oceanic crust.

Notes: We have developed statistical models of upper and middle crustal seismic velocity heterogeneity based on geologic maps and petrophysical data from the Lewisian gneiss complex, Scotland. the level of heterogeneity we have measured is relevant to seismic exploration of the crystalline crust using conventional reflection and refraction techniques. We digitized two 1:10560 geologic maps of Laxfordian (Archean) age Lewisian rocks on a 26.8m grid. Both maps are believed to be representative of the upper and middle crust in north-western Scotland, and both are believed to provide cross-sectional views of parts of the crust. the digital maps were characterized by the statistics of their lithologic populations and by their 2-D spatial autocorrelation functions. Different lithologies were assigned seismic velocities appropriate for the mid-crust using petrophysical data. Three lithologies are dominant: silicic gneisses (Vp∼6.2 km s−1), mafic amphibolites (Vp— 6.75 kms−1), and intermediate composition schists (Vp—6.5kms−1). Both maps have self-affine spatial fabrics.The first map covers the core of a syncline. Its autocorrelation function defines a medium with a fractal dimension of 2.78, a horizontal characteristic length of about 244m and a vertical correlation of about 133m (aspect ratio is 2:1). It has an essentially trimodal velocity (lithology) population consisting of 37 per cent silicicgneiss, 43 per cent mafic amphibolites, and 20 per cent schists. This map is representative of 30-40 per cent of Laxfordian rocks. the second map is a plan view which can be rotated 90° about an axis perpendicular to strike to give a cross-section. This map is characterized by a fractal dimension of 2.55, with a horizontal correlation length of about 111m and a vertical correlation of about 38m (aspect ratio 3:1). It has a nearly bimodal population consisting of 77 per cent silicic gneisses and 22 per cent mafic amphibotites. It is representative of 60-70 per cent of Laxfordian rocks.Lastly we examine the seismic response of an upper to middle crust based on our statistical models using acoustic and elastic 2-D finite-difference synthetic seismograms. Short-offset shot records demonstrate that a Lewisian upper crust produces scattered waves which significantly disrupt signals reflecting from deeper levels. Measurements of transmission scattering Q, and coda decay rates confirm that seismic scattering in Lewisian-type crust is strong. the migrated CMP response of a Lewisian crustal model shows the characteristic ‘salt and pepper’ pattern often observed in the upper crust, and described, incorrectly, as ‘transparent'. We suggest that ‘translucent’ is a more appropriate descriptor.