We used the seismic physical modelling approach to study the effect of fracture thickness or aperture on P-wave attenuation, using a laboratory scale model of two horizontal layers. The first layer is isotropic while the second layer has six fractured blocks, each consisting of thin penny-shaped chips of 3？mm fixed diameter and same thickness to simulate a set of aligned vertical fractures. The thickness of the chips varies according to the blocks while the fracture density remains the same in each block. 2D reflection data were acquired with the physical model submerged in a water tank in a direction perpendicular to the fracture strikes using the pulse and transmission method. The induced attenuation was estimated from the preprocessed CMP gathers using the QVO method, which is an extension of the classical spectral ratio method of attenuation measurement from seismic data. The results of our analysis show a direct relationship between attenuation and the fracture thickness or aperture. The induced attenuation increases systematically with fracture thickness, implying more scattering of the wave energy in the direction of increasing aperture. This information may be useful to differentiate the effect caused by thin microcracks from that of large open fractures. 1. Introduction Fractures open at depth tend to be aligned normal to the direction of minimum in situ stress acting on them [1], giving rise to seismic anisotropy. Over the years, seismic anisotropy has been increasingly used as a potential tool to characterize natural fractured hydrocarbon reservoirs (e.g., [2–5]). The equivalent medium theories of seismic wave propagation provide the basis of using seismic anisotropy to detect fractures from seismic data. One of such theories is the Hudson’s theory [6, 7] which provides a link between fracture density and measured azimuthal anisotropy. However, the theory fails to account for the issues of fracture scale lengths. For instance, many small cracks or a few large cracks within the same volume of material can result in the same fracture density. Furthermore, equal number of cracks with the same diameter but with varying thicknesses or apertures within the same volume of material can give rise to the same fracture density. In reservoir rocks, it is possible to have aligned fractures of the same density but at different scales and consequently, the investigation of the effects of the fracture scale lengths and thicknesses or apertures on seismic wave response may be of great interest in fractured reservoir characterization. An adequate understanding
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