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In August of 1985 the crustal structure underlying the Kenya rift valley was investigated by long range explosion seismology. The experiment (KRISP85) consisted of two seismic lines in the central sector of the rift, one along the axis (140 km) and the other across it (50 km). Interpretation of the data, including time-term analysis and ray tracing has yielded the following information. The thickness of rift infill varies from about 6 km below Lake Naivasha to about 2 km and 1.5 km below Lake Magadi and Lake Bogoria respectively. The underlying material has a P-wave velocity of 6.05 +/- 0.03 km/s which suggests the rift is underlain by Precambrian metamorphic basement. A localised high velocity zone identified to the east of Nakuru may be associated with basic intrusive material. The P-wave velocity increases discontinuously to 6.45 +/- 0.05 km/s at a depth of 12.5 +/- 1-0 km. This depth is similar to that inferred for the brittle-ductile transition zone from a study of local seismicity in the Lake Bogoria region. A high P-wave velocity layer (7.1 +/- 0.15 km/s) occurs at 22 +/- 2 km depth which might be associated with a sill-like basic intrusion in the lower crust. An upper mantle velocity of 7.5 +/- 0.2 km/s (unreversed) is reached at a depth of 34.0 +/- 2.0 km. This implies that only moderate crustal attenuation has occurred beneath the central sector of the rift. No evidence was obtained for the existence of an "axial intrusion" reaching to shallow levels below the rift and causing crustal separation as suggested by previous studies. Relative residuals determined for 46 teleseismic events recorded by a 15 station, small aperture seismic array in the vicinity of Lake Bogoria indicate considerable lateral heterogeneity in the upper crust. An Aki inversion of the relative residuals has revealed the existence of two distinct low velocity zones which may be associated with magma chambers.

Moveout velocities sampled frequently along seismic horizons on a selection of seismic lines are used to derive interval velocities in an 'inversion' algorithm developed from work published by Hubral. This inversion is based on zero-offset raytrace modelling in a simplistic local ground model. The 'Hubral algorithm' is incorporated into a database which allows spatial smoothing of velocities. The spatial consistency of derived interval velocities can then be assessed by reference to mis-ties at line intersections, while interval velocities from well data can be used to check their validity. These principles have been used to derive interval velocities both from real data and from 'synthetic' data generated by common mid-point raytracing over schematic ground models. The latter study reveals that the procedure performs well if the local subsurface sampled by the CMP gather conforms approximately to the simplistic ground model assumed by the Hubral algorithm. The method is unsuitable in areas of faulting and interval velocity heterogeneity, and may yield spurious results over fold axes. Application of the procedure to real data indicates that it is generally desirable to smooth both moveout velocities before inversion and interval velocities after inversion. Comparison with well information shows that interval velocities derived by the Hubral algorithm are consistently higher than those measured from calibrated velocity logs. This observation is disturbing, since the derived interval velocities require a correction if they are to be used for depth conversions, but the discrepancy cannot be explained by ray theoretical considerations. No advantage appears to be gained by the 'layer-by-layer' mode of inversion over the 'direct' inversion, despite the greater potential for error propagation anticipated in the latter. Further work on different data sets is required to justify general use of the layer-by-layer mode of inversion.

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This study presents a detailed investigation into the microseismic response of a rockmass being excavated in a high stress environment. AECL's Underground Research Laboratory, Manitoba, provided a unique opportunity to non-invasively monitor a tunnel excavation with a 3D microseismic array. The 46 m long cylindrical Mine-by tunnel was excavated at 420 m depth predominantly using a non-blasting method, therefore the damage zone and crack initiation is primarily due to the effect of stress redistribution and concentration following each excavation increment. Both manual and automated source parameter processing techniques are contrasted and used on the data set of over 20000 microseismic events of magnitude, MW =-1.5 to - 4.5. The relatively homogeneous and unfractured nature of the rockmass allowed the validity of fundamental spectral models to be tested. The seismicity is found to extend 1.0 m into the roof and floor regions of the tunnel and 0.8 m ahead of the tunnel face. Spatial and temporal trends in the source parameters are used to compare the seismic response of the two rock types present along the tunnel, namely granite and granodiorite. Seismicity starts earlier and occurs over a shorter time interval in the granite. Additionally, a late second phase of seismicity is found to occur in the granodiorite with these events having a similar magnitude to those at the excavation face. These trends, due to petrofabric and geotechnical differences in the two rock types, are important for safety reasons and mine design. The excellent sensor focal sphere coverage enabled the production of well constrained source mechanism solutions using both first motions and moment tensor analysis methods, and allowed source types to be contrasted with spectral parameters. Seismicity ahead of the tunnel face is predominantly deviatoric and it is concluded that this source type is resulting from movement on face parallel tensile cracks that formed early during the tunnel excavation