A short length of channel on Pico Partido volcano, Lanzarote, provides us the opportunity to examine the dynamics of lava flowing in a channel that extends over a sudden break in slope. The 1-2-m-wide, 0.5-2-m-deep channel was built during the 1730-1736 eruptions on Lanzarote and exhibits a sinuous, well-formed channel over a steep (11°slope) 100-m-long proximal section. Over-flow units comprising smooth pahoehoe sheet flow, as well as evidence on the inner channel walls for multiple (at least 11) flow levels, attest to unsteady flow in the channel. In addition, superelevation is apparent at each of the six bends along the proximal channel section. Superelevation results from banking of the lava as it moves around the bend thus causing preferential construction of the outer bank. As a result, the channel profile at each bend is asymmetric with an outer bank that is higher than the inner bank. Analysis of superelevation indicates flow velocities of~8 m s -1 . Our analysis of the superelevation features is based on an inertia-gravity balance, which we show is appropriate, even though the down-channel flow is in laminar flow. We use a viscosity-gravity balance model, together with the velocities calculated from superelevation, to obtain viscosities in the range 25-60 Pa s (assuming that the lava behaved as a Newtonian liquid). Estimated volume fluxes are in the range 7-12 m 3 s -1 . An apparent down-flow increase in derived volume flux may have resulted from variable supply or bulking up of the flow due to vesiculation. Where the channel moves over a sharp break in slope and onto slopes of~6°, the channel becomes less well defined and widens considerably. At the break of slope, an elongate ridge extends across the channel. We speculate that this ridge was formed as a result of a reduction in velocity immediately below the break of slope to allow deposition of entrained material or accretion of lava to the channel bed as a result of a change in flow regime or depth.

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A 15 kHz cross-hole seismic tomography survey was completed between two boreholes that cross a wellknown subvertical fracture called the "Room 209 fracture" on the 240 Level of the Atomic Energy of Canada Limited Underground Research Laboratory. This survey measured in situ P-and S-wave velocity and amplitude relations across the fracture between the two boreholes. A velocity anisotropy of about 1.5% (peak-to-peak) exists in the rock mass for both P and S waves. Consistently faster velocities were observed for rays oriented roughly parallel to the Room 209 fracture. This anisotropy is consistent with geological and geotechnical models of microcrack populations for the region. Tomographic velocity images for both the P-and S-waves show a distinct change across the fracture. Higher velocities occur east of the fracture, whereas lower velocities occur to the west. The velocity differences indicate minor changes in the rock structure from one side of the fracture to the other which correlate with observed differences in alteration and fracture frequency. There is a slight reduction in velocity at the fracture itself. A study of seismic amplitudes was also completed. A weak anisotropy also exists in the amplitude data, with higher amplitudes for rays that parallel the fracture direction. The P-wave attenuation ranges between 0.03 and 0.7 dB/m across the survey panel.The region of the fracture shows a subtle increase in the P-wave attenuation of about 0.04 dB/m above the local background. These measurements provide a useful demonstration of the sensitivity of the technique. The results correlate well with geological models and simple physical models for seismic wave propagation in sparsely fractured granite.Résumé : Une étude de tomographie sismique 15 kHz de trou à trou a été complétée entre deux trous de forage traversant la fracture subverticale connue sous l'appellation «fracture de la chambre 209» du niveau 240 du Laboratoire souterrain de recherches, de l'Énergie atomique du Canada Limitée. Cette étude a permis de mesurer in situ les relations entre les vitesses et les amplitudes des ondes P et S de chaque côté de la fracture entre les deux trous de forage. On observe dans la masse rocheuse une anisotropie des vitesses des ondes P et S de l'ordre de 1,5% (pic-à-pic). Les rayons avec orientation grossièrement parallèle possèdent des vitesses qui sont régulièrement plus rapides. Cette anisotropie est compatible avec les modèles géologique et géotechnique représentant la densité des microfissures dans la région. Les images de la tomographie révèlent qu'il se produit en traversant la fracture un changement distinct des vitesses des ondes P et S. Les vitesses les plus rapides apparaissent à l'est de la fracture, tandis que les vitesses les plus lentes se manifestent du côté ouest. Les différences de vitesse sont dues à des changements mineurs dans la structure de la roche d'un côté à l'autre de la fracture, elles reflètent les différences observées dans l'altération et la fréquence des fractures. Il se produit dans la...