Layer-bound systems of polygonal faults are found in sequences of very fine-grained sediments that have typically undergone passive subsidence and burial. In the absence of tectonic extension, the heave of the faults must be complemented by horizontal compaction of the sediments. Density inversion, syneresis and low coefficients of friction on fault planes have all been proposed as causal mechanisms for the development of polygonal fault systems, but most sequences that contain polygonal faults are not underlain by sediments of lower density and there is a lack of evidence to support the idea that syneresis is responsible. Laboratory measurements of clay properties and a recent field test based on well data strongly suggest that low coefficients of residual friction in fine-grained sediments are key to the growth of faults that eventually develop into polygonal systems. However, coefficients of residual friction apply to faults only after initial slip has taken place, so some other mechanism must be responsible for the initial nucleation of the faults. Various speculative suggestions have been made, but there is no evidence that nucleation of those faults that evolve into polygonal systems differs fundamentally from the processes involved in the nucleation of other faults in soft sediments.

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Extensive polygonal networks of normal faults have reportedly been identified within layerbound sequences in about 28 sedimentary basins worldwide. The gentle regional dips, passive tectonic settings and geometry of the fault networks have led to the conclusion that faulting must have resulted from gravity-driven mechanical compaction. The faulted sequences comprise very fine-grained sediments, with lithofacies that range from smectitic claystones to almost pure chalks. In most, if not all, cases it is clear that volumetric contraction has occurred with horizontal contraction of the sediments complementing the heave of the faults. One explanation which has previously been offered is that the fine-grained sediments have shrunk due to syneresis, a process that involves spontaneous contraction of the solid network with expulsion of the pore fluid. However, syneresis is an implausible mechanism because it does not explain the observed lithological variation in the sediments concerned, why the initiation of faulting occurs in the depth range 100-1000 m, and why faulting continues for millions of years. A much simpler explanation is that shear failure inevitably results from one-dimensional compaction if the coefficient of friction is sufficiently low; and there is some evidence from laboratory measurements that the coefficient of friction is likely to be exceptionally low in these fine-grained sediments. Qualitatively, low coefficients of friction also explain why these compaction faults preferentially dip towards the basin margin where the regional dip of the bedding is greater than 1˚. Furthermore, they help to explain the origin of a polygonal fault system in the Eromanga Basin, South Australia, where the situation is complicated by the presence of a low velocity, ductile layer at the base of the faulted sequence.

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Traveltime tomography is an appropriate method for estimating seismic velocity structure from arrival times. However, tomography fails to resolve discontinuities in the velocities. Wave‐equation techniques provide images using the full wave field that complement the results of traveltime tomography. We use the velocity estimates from tomography as a reference model for a numerical propagation of the time reversed data. These “backpropagated” wave fields are used to provide images of the discontinuities in the velocity field. The combined use of traveltime tomography and wave‐equation imaging is particularly suitable for forming high‐resolution geologic images from multiple‐source/multiple‐receiver data acquired in borehole‐to‐borehole seismic surveying. In the context of crosshole imaging, an effective implementation of wave‐equation imaging is obtained by transforming the data and the algorithms into the frequency domain. This transformation allows the use of efficient frequency‐domain numerical propagation methods. Experiments with computer‐generated data demonstrate the quality of the images that can be obtained from only a single frequency component of the data. Images of both compressional [Formula: see text] and shear wave [Formula: see text] velocity anomalies can be obtained by applying acoustic wave‐equation imaging in two passes. An imaging technique derived from the full elastic wave‐equation method yields superior images of both anomalies in a single pass. To demonstrate the combined use of traveltime tomography and wave‐equation imaging, a scale model experiment was carried out to simulate a crosshole seismic survey in the presence of strong velocity contrasts. Following the application of traveltime tomography, wave‐equation methods were used to form images from single frequency components of the data. The images were further enhanced by summing the results from several frequency components. The elastic wave‐equation method provided slightly better images of the [Formula: see text] discontinuities than the acoustic wave‐equation method. Errors in picking shear‐wave arrivals and uncertainties in the source radiation pattern prevented us from obtaining satisfactory images of the [Formula: see text] discontinuities.