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Discussion |
1 3 Finedon Hall, Finedon NN9 5NG, UK (e-mail: davidjoanjames@compuserve.com)
2 Fault Analysis Group, Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland (e-mail: fault@fag.ucd.ie)
3 Fault Analysic Group, Liverpool University Marine Laboratory, Port Erin Isle of Man, IM9 6JA, UK
4 Institute of Geological and Nuclear Sciences, PO Box 30-368, Lower Hutt, New Zealand
5 Badley Earth Sciences Ltd, North Beck House, North Beck Lane, Hundleby, Lincs PE23 5NB, UK
Scientific editing by Richard England.
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D. M. D. James writes: The paper is much to be welcomed as bringing new and stimulating ideas to the ongoing debate concerning the origin of polygonal faults (
Cartwright & Lonergan 1996; Cartwright & Dewhurst 1998). However, if the proposed mechanism of blind faulting above sites of incipient diapirism is correct, then demonstration is required of significant thickness variation of the causative mobile layer and of a density inversion across its upper boundary. The following remarks aim largely to seek geophysical reassurance on these aspects, and as such are also relevant to many of the increasing number of Journal papers that utilize seismic and well data from the oil and gas industry.
The candidate mobile layer is stated to be a shale, fortuitously preserved at its depositional thickness of 35 m in the only control well apparently available. It is critical for the authors interpretation (fig. 8b) that both top and bottom of this shale are mappable seismically and the character of the sonic log (fig. 1c) gives some encouragement that this may indeed be possible; ideally via an exact tie to the well based on a well shoot, an acoustic impedance profile from well logs and knowledge of the phase and polarity of the seismic. No such tie is mentioned but assuming that the position of the shale is approximately as shown in fig. 3a, rather than its accompanying rather obvious seismic mistie in fig. 3b, the distinctive and rather tram line character at this level makes it
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