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OSL Dating and GPR Mapping of Palaeotsunami Inundation: A 4000-Year History of Indian Ocean Tsunamis as recorded in Sri Lanka

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Abstract

To evaluate and mitigate tsunami hazard, as long as possible records of inundations and dates of past events are needed. Coastal sediments deposited by tsunamis (tsunamites) can potentially provide this information. However, of the three key elements needed for reconstruction of palaeotsunamis (identification of sediments, dating and finding the inundation distance) the latter remains the most difficult. The existing methods for estimating the extent of a palaeotsunami inundation rely on extensive excavation, which is not always possible. Here, by analysing tsunamites from Sri Lanka identified using sedimentological and paleontological characteristics, we show that their internal dielectric properties differ significantly from surrounding sediments. The significant difference in the value of dielectric constant of the otherwise almost indistinguishable sediments is due to higher water content of tsunamites. The contrasts were found to be sharp and not to erode over thousands of years; they cause sizeable electromagnetic wave reflections from tsunamite sediments, which permit the use of ground-penetrating radar (GPR) to trace their extent and morphology. In this study of the 2004 Boxing Day Indian Ocean tsunami, we use GPR in two locations in Sri Lanka to trace four identified major palaeotsunami deposits for at least 400 m inland (investigation inland was constrained by inaccessible security zones). The subsurface extent of tsunamites (not available without extensive excavation) provides a good proxy for inundation. The deposits were dated using the established method of optically stimulated luminescence (OSL). This dating, partly corroborated by available historical records and independent studies, contributes to the global picture of tsunami hazard in the Indian Ocean. The proposed method of combined GPR/OSL-based reconstruction of palaeotsunami deposits enables estimates of inundation, recurrence and, therefore, tsunami hazard for any sandy coast with identifiable tsunamite deposits. The method could be also used for anchoring and synchronizing chronologies of ancient civilisations adjacent to the ocean shores.

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Acknowledgments

The authors are grateful to the anonymous referees for the thorough reading of the manuscript and helpful comments.

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Authors and Affiliations

  1. Institute for the Environment, Physical Sciences and Applied Mathematics (EPSAM), Keele University, Keele, ST5 5BG, UK

    Ranjith Premasiri, Peter Styles, Victor Shrira & Nigel Cassidy

  2. School of Archaeology, University of Oxford, South Parks Road, Oxford, OX1 3PS, UK

    Jean-Luc Schwenninger

  3. Department of Earth Resources Engineering, University of Moratuwa, Moratuwa, Sri Lanka

    Ranjith Premasiri

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Appendix: GPR Processing Sequence

Appendix: GPR Processing Sequence

The following processing sequence was applied to the Sri Lankan GPR data:

  1. 1.

    Zero adjust (static shift)—during a GPR survey, the first waveform to arrive at the receiver is the air wave., which is delayed due to propagation in the connecting cables. We, therefore, need associate zero-time with zero-depth, so that any time offset due to instrument recording is removed before interpretation of the radar image. This is done individually for each radar section but in this survey was typically about 7 ns.

  2. 2.

    Background removal—background noise is present due to ringing in the antennae, producing a coherent banding effect across the radar section. We sum all the amplitudes of simultaneous reflections along a profile and divide by the number of traces to give a composite signal, which is an average of all background noise which is then subtracted from the data.

  3. 3.

    Gain—gain compensates for amplitude variations in the GPR image as early signal arrival times have greater amplitude than later arrival times because of geometric spreading and intrinsic attenuation. Time-variable gain functions are to equalize the amplitudes of the recorded signals constant along the trace. We have used an inverse amplitude decay gain, which compensates for both geometric spreading and attenuation simultaneously in conjunction with automatic gain control, which balances the signal amplitudes and emphasizes more subtle features.

  4. 4.

    Frequency filtering—GPR data are collected with source and receiver antennae of specified dominant frequency, however, the recorded signals include a band of frequencies wider than this specific frequency. Frequency filtering removes undesirable higher and/or lower frequencies to produce a more interpretable GPR image. This is called band-pass filtering and in this survey a pass band was selected interactively for each section but typically this covered a range of 0.1–3.0 GHz.

  5. 5.

    Deconvolution—the recorded radar signal is the interaction between the layered earth and its reflecting horizons which depend on electromagnetic contrast (which is what we require) and the radar source function, which is generated by reflection at each interface and adds to the final recorded signal. Deconvolution is the inverse filtering operation that attempts to remove the effects of the source wavelet to better interpret GPR profiles as images of the earth structure. There many different deconvolution methodologies but the one, which we have used, is known as sparse deconvolution (i.e. the Earth has mostly zero reflectivity with a few ‘sparse’ strong reflectors) with a Blackman–Harris (a sum of weighted shifted sinc functions) window.

  6. 6.

    Median filter—this enhances the continuity of reflector signals by replacing each value by the 5 × 5 median of the values around it.

  7. 7.

    Depth conversion—the radar sections are converted from time to depth using the velocities determined experimentally in the laboratory and the depth correlation established in the field.

  8. 8.

    Colour scale—finally, an appropriate colour scale and set of brightness values is chosen to produce a good interpretable radar section. This is a subjective process but can make significant improvements in clarity.

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Premasiri, R., Styles, P., Shrira, V. et al. OSL Dating and GPR Mapping of Palaeotsunami Inundation: A 4000-Year History of Indian Ocean Tsunamis as recorded in Sri Lanka. Pure Appl. Geophys. 172, 3357–3384 (2015). https://doi.org/10.1007/s00024-015-1128-4

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