The dynamic time-warping approach to comparison of magnetic-susceptibility logs and application to Lower Devonian calciturbidites (Prague Synform, Bohemian Massif)

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Abstract

The effectiveness of alignment of magnetic susceptibility (MS) stratigraphic sections by means of the dynamic time warping (DTW) approach was tested with positive results. The DTW is a robust and effective tool for the alignment of the sequential MS data which is particularly sensitive to MS-log patterns, their details, successions, distortion or absence from logs. Moreover, the DTW performance proved to be appropriate for sections which involve a great deal of irregularly spread compacted or thick-bedded intervals and variously positioned and sized gaps. In view of the fact that the lateral changes in short- to medium-term sedimentation rates and local variations in diagenetic compaction are not exceptional but rather common features of the compared sedimentary sections, the DTW becomes a significant tool for stratigraphic comparison of sections with good potential of becoming an alternative to cyclostratigraphic correlation methods. Conditions governing the formation of Lower Devonian (Pragian) carbonate clinoforms obviously appear to be this case, and testing of the DTW on this type of MS sections suggests that the computationally robust results remain approximately the same, even if the importance of high values (peaks) is nonlinearly decreased or increased. In such cases the DTW outperforms the distortion-based vector quantisation (VQ), continuous density hidden Markov models (CDHMM), wavelet transformation (WT) or cross-correlation (CC) based approaches. The near absence of artificially introduced preconditions or predefined elements is also an advantage. The alignment of the sections using the DTW gives us warning about distorted intervals, thus reducing the risk of assembling quite different types of rhythmic patterns or cycles together. The overall ambition of this paper is to introduce the DTW alignment technique as an effective tool for MS log correlation. Extensive testing of the requirements for input data and constraints according to maximum differences between the compared stratigraphic segments are reserved for other studies. For the Pragian of the Barrandian area (specifically, calciturbidite-dominated clinoforms), the DTW-based interconnection between the relevant points of two sections has a resolution which is close to sampling distances (several cm) and is about two orders of magnitude higher compared to that which can be achieved by the bio-, chemo- or lithostratigraphic tools. The first results confirmed the presence of numerous visible and hidden gaps and condensations in the sections.

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A propos de : Jindrich HLADIL

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A propos de : Ladislav SLAVIK