-Col. R.A.M.C. ABOUT 18 months ago I was able to present to this Section with my colleague, Colonel Lister, certain clinical observations on the disturbances of vision produced by gunshot injuries of the visual cortex and of the optic radiations. From these we drew certain conclusions on the cortical representation of the retina, and particularly on the segmental correspondence of different areas of the retina with separate zones of the visual cortex. Our chief conclusions were:1. The upper half of each retina is represented in the dorsal, and the lower in the ventral part of each visual area.2. The centre for macular or central vision lies in the posterior extremities of the visual areas, probably in the margins and the lateral surfaces of the occipital poles. The macular region has not a bilateral representation.3. The centre for vision subserved by the periphery of the retinae is probably situated in the anterior ends of the visual areas, and the serial concentric zones of the retina from the macula to the periphery 'Read in the Section of Ophthalmology of the Royal Society of Medicine, December 12, 1917.

The Shear‐Wave Experiment at Atomic Energy of Canada Limited's Underground Research Laboratory was probably the first controlled‐source shear‐wave survey in a mine environment. Taking place in conjunction with the excavation of the Mine‐by test tunnel at 420 m depth, the shear‐wave experiment was designed to measure the in situ anisotropy of the rockmass and to use shear waves to observe excavation effects using the greatest variety of raypath directions of any in situ shear‐wave survey to date. Inversion of the shear‐wave polarizations shows that the anisotropy of the in situ rockmass is consistent with hexagonal symmetry with an approximate fabric orientation of strike 023° and dip 35°. The in situ anisotropy is probably due to microcracks with orientations governed by the in situ stress field and to mineral alignment within the weak gneissic layering. However, there is no unique interpretation as to the cause of the in situ anisotropy as the fabric orientation agrees approximately with both the orientation expected from extensive‐dilatancy anisotropy and that of the gneissic layering. Eight raypaths with shear waves propagating wholly or almost wholly through granodiorite, rather than granite, do not show the expected shear‐wave splitting and indicate a lower in situ anisotropy, which may be due to the finer grain size and/or the absence of gneissic layering within the granodiorite. These results suggest that shear waves may be used to determine crack and mineral orientations and for remote monitoring of a rockmass. This has potential applications in mining and waste monitoring.

In 2001, a wide-azimuth 2C/3D OBS survey was acquired over an offshore carbonate field in the Middle East, and processed with techniques specifically designed to preserve and extract the azimuthal effects in the data. The results showed significant azimuthal anisotropy in amplitude, with apparent azimuthal variation in the P-wave AVO slope of as much as 100% at selected target horizons. The azimuth of the most positive slope was generally NW/SE, in agreement with the regional tectonic trend. The magnitude of the anisotropy varied markedly, with patches of strong anisotropy, with a granularity of 10's to 100's of metres, and a regional trend on the 2km scale. Vertical variation of these patches supports the hypothesis that the effects are due to subsurface anisotropy, rather than acquisition artifacts. Introduction A well-established technique used on surface reflection seismic data is Amplitude-Variation-with-Offset (AVO). AVO uses the amplitude of seismic reflection at a given horizon, as a function of increasing source-receiver offset distances, to infer lithological and fluid properties at that horizon. AVO analysis may also examine raypaths of varying source-receiver azimuths, in what is known as Amplitude- Variation-with-Offset-and-Azimuth (AVOA). With additional assumptions, AVOA allows the determination of fracture strike direction and fracture density. Such information may be interpreted and integrated with reservoir models to infer the localized stress field, tensor permeabilities, and fluid-flow directions. In contrast to methods (such as seismic coherency) that determine large-scale faults, AVOA analysis determines media properties much smaller than the seismic wavelength. These can be key to understanding a reservoir. AVOA was first reported by Lynn and Thomsen1 for land data, and by Mallick and Frazer2 and Lefeuvre3 for marine data, with theory first reported by Thomsen4. For conventional 3D marine seismic surveying techniques, the narrow distribution of source-receiver azimuths precludes detailed azimuthal studies on P-wave data. Only recently, through the use of Ocean Bottom Seismic (OBS) surveys, has marine data possessed azimuthal distribution appropriate for AVOA analysis. A modern 2C/3D OBS seismic survey over a carbonate field in the Middle East was acquired in 2001 with a full distribution of offsets and azimuths. In addition to the conventional processing flow, designed for proper imaging of the field, special techniques were applied to preserve and extract the azimuthal information for physical characterizationof the subsurface. The results yield the only small-scale picture of the fracturing pattern that is possible away from well control, and are therefore likely be extremely important in future reservoir management. Theory AVOA exists because fractures and other small features in a formation cause seismic properties (such as reflectivity and velocity) to vary with azimuth, in what is known as azimuthal anisotropy. By contrast, polar anisotropy is due to a fabric or pattern in a rockmass such that the elastic properties vary with polar angle (the angle from the vertical) only; this is the simplest form of anisotropy. However, when the horizontal azimuths are not all equivalent, as (for example) in the case of vertical parallel fractures, seismic properties will vary with azimuth (compass direction). In a simple case, the equations which govern the seismic waves are simply those of polar anisotropy, but with the polar axis horizontal, instead of vertical. In a more realistic case (i.e. the case of shales or thin-bedded sequences with a set of vertical fractures), the equations are those of orthorhombic symmetry (cf Tsvankin5). Azimuthal anisotropy is typically attributed to fractures, cracks, and microcracks, aligned by tectonic paleostress and current stress in the reservoir (Crampin6), although deposition history and style may be influential (Sayers7). As anisotropy is controlled by sub-wavelength properties of the rockmass, an understanding of azimuthal anisotropy is a window to the patterns of fractures and cracks pervasive within a formation.

The ability to analyse shear‐wave anisotropy in a mine environment is greatly aided by using multiple source orientations of a reproducible, impulsive shear‐wave source. The analysis of what is probably the first controlled source shear‐wave experiment in a mine environment demonstrates clearly that shear‐wave polarizations and time delays between split shear‐wave arrivals are reliably measured because of the use of multiple source orientations rather than a single shear‐wave source. Reliability is further aided by modelling the shear‐wave source radiation pattern, which allows for the unequivocal discrimination between seismic raypaths where shear‐wave splitting did and did not occur. The analysis also demonstrates the great importance of high reproducibility of the seismic source for the use of shear waves in time‐lapse surveys to monitor changes in a rockmass.

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