Mo Myce 06 Comparative Seismic Source Study for Buried Palaeolandscape Investigations in the Southern North Sea

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1 Mo Myce 06 Comparative Seismic Source Study for Buried Palaeolandscape Investigations in the Southern North Sea O.J. Zurita Hurtado* (Renard Centre of Marine Geology - Ghent University), T. Missiaen (Renard Centre of Marine Geology - Ghent University), M. De Clercq (Renard Centre of Marine Geology - Ghent University), W. Versteeg (Vlaams Instituut voor de Zee - VLIZ), P.P. Kruiver (Deltares) & M.P.E. De Kleine (Deltares) SUMMARY The southern North Sea has been an attractive region for human settlement throughout the late Quaternary. Unfortunately, to this date, little attention has been paid to the rich submerged archaeological potential in Belgium. Marine seismic techniques have been used for more than 20 years to investigate buried landscapes but in general the archaeological community often has to work with data acquired for other purposes, meaning they are not well adapted for archaeological studies. In this study, we aim to develop an efficient survey methodology to image buried palaeolandscapes for the purpose of archaeology, through the comparison of a wide range of high-resolution seismic source/receiver configurations. This should finally allow to accurately assess the geo-archaeological potential of the Quaternar

2 Introduction The southern North Sea has been attractive for human settlement throughout the late Quaternary. During periods of low sea level (up to 120 metres lower than today) large parts were dry land crossed by big rivers such as the Thames, Meuse and Rhine (Gibbard, 2007). These areas were attractive for animal and human communities and often crucial in the development of (pre)historic societies and civilisations. Although the signature of the European Convention on the Protection of the Archaeological Heritage (Malta-1992) boosted archaeological research on land, little attention has been paid to the rich submerged archaeological potential in Belgium so far. The main goal of this study is to develop an efficient survey methodology to image buried palaeolandscapes for the purpose of archaeology through the comparison of a wide range of highresolution seismic source/receiver configurations. This should finally allow an accurate assessment of the geo-archaeological potential of the Quaternary deposits in the Belgian part of the North Sea. Methodology Marine seismic techniques have been used for more than 20 years to investigate buried landscapes (Plets, 2007; Dix, 2008; Missiaen, 2010). Unfortunately, the archaeological community often has to work with data acquired for other purposes. Additionally, some archaeological contractors use only one seismic system as a straightforward solution for every problem they encounter. It is not our objective to come up with a list of good or bad techniques since the potential of each method does not only depend on the environmental setting but also on the type of geo-archaeological indicator (e.g. wood/stone structures, organic deposits, river valleys,...). The resulting methodology is intended to be a guide for all those involved in commercial activities at sea (aggregate industry, wind farm developers,...) as well as the international scientific community. It will allow significant cost reduction during archaeological evaluation phases of commercial activities and help avoiding costly damage and/or loss of valuable time during the operational phase of the works. The current focus of the project is on measurement (i.e. survey) methodology, whereas the interpretation of palaeolandscapes and geo-archaeological potential will be performed in a later stage. Test area and geological background Different geological environments will be studied in this project. Our first focus was on buried river valleys as these are known hot-spots for prehistoric life. A test area was therefore chosen in the socalled Ostend Valley, offshore the city of Ostend (Belgium) (Figure 1). This funnel-shaped buried valley structure started out as a small river valley during the Saalian (Mathys, 2009). With rising sealevel during the Eemian, the river valley evolved into an open estuary, where coastal and tidal forces reshaped it into the funnel-shaped valley we know today. During the Weichselian ice age, sea-level dropped, producing further fluvial incisions, wind activity increased and earlier fluvial sediments were accumulated in the form of sand ridges which gradually damned the valley, thereby redirecting the river to the north. During the subsequent Holocene sea-level rise the valley got buried beneath tidal and marine sediments, including several large sandbanks. Data Acquisition In October 2013 more than 120 kilometres of 2D high-resolution seismic reflection data were acquired at the Ostend Valley (Figure 1). Six different seismic sources were used during the survey: (1) SIG sparker, (2) RCMG Centipede sparker, (3) IKB Seistec boomer, (4) Applied Acoustic AA300 boomer, (5) Edgetech X-Star chirp, and (6) Innomar SES-2000 parametric echosounder. Each source has a particular frequency range output resulting in high- or low-resolution images with a lowor high-penetration into the subsurface (Table 1). The same Test Line was used for all sources (Figure 1) in order to allow optimal comparison.

3 Figure 1 Left: Overview map of Belgium and the offshore survey location (Google Earth), Right: Isobath map of the Top-Palaeogene unconformity (based on Mathys, 2009) focusing on the Ostend Valley (flanked by two higher platforms) and location of the Survey Test Line (in red). When applicable, two different receiver configurations were used to record the data; (1) a single channel streamer (traditional sub-bottom profiling) and (2) a multichannel streamer (24 channels, 3.125m channel spacing). Both streamers were towed simultaneously behind the vessel. Seismic Source Frequency Vertical Penetration Number of Range (Hz) resolution (m) (m)* Receivers 1 SIG sparker > 0.75 < / 24 2 Centipede sparker > 0.40 < 50 1 / 24 3 Seistec boomer > 0.25 < 20 1 / 24 4 AA 300 boomer > 0.35 < 50 1 / 24 5 Edgetech X-Star chirp > 0.20 < Innomar SES > 0.15 < 20 1 Table 1. Overview of the different seismic sources used during the survey. All data were acquired in similar weather conditions. * Penetration values are valid for sandy sea bottom only. Sparker and boomer data were processed using RadExPro, chirp and PES data with dedicated software. Data processing for the single channel data consisted of: predictive deconvolution, band pass filtering, swell correction, burst noise removal, spatial filtering and spherical divergence correction. The multichannel data processing consisted of: CMP binning, swell correction, band pass filtering, trace balancing, velocity analysis every 250 CDP s, normal move-out correction, external mute, CDP stack, burst noise removal, spatial filtering, spherical divergence correction. Results The main Test Line crossed the Ostend Valley perpendicularly at one of its deepest sections. At this particular location the valley is covered by a large tidal sandbank. This setting represents a big challenge as the short wavelength sound waves are quickly absorbed in the heterogeneous coarse sandy sediments, producing a decrease in penetration depth (Van Lancker, 2009). The resulting seismic profiles are summarized in Figure 2. It can be observed that the SIG sparker (1) produces images with fairly good resolution in the shallow sections but deeper reflectors are blurred or indistinguishable. The Centipede sparker (2) shows the best resolution and penetration depth of all tested sources throughout the whole section. The Seistec boomer signal (3), recorded with the streamer and not with its internal receiver, is relatively coarse and as a consequence obscures very shallow reflections and produces thick multiples. On the other hand it shows good penetration depth

4 and good resolution in the deeper sections. Data acquired with the AA300 boomer (4) shows lower resolution and poorer penetration depth than the other sources. Profiles collected with the X-Star chirp (5) and the PES (6) show very low penetration (less than a few m) but very high resolution in the shallow most section. It is this depth range that is relevant for shallow archaeology that can potentially get disturbed by commercial activities at sea. In general the PES signal gave a better image of the internal structure of the tidal sandbank and the shallow deposits than the X-Star chirp. 2 km 50ms 35m Figure 2 Geological interpretation of the Ostend Valley at the survey location. Single channel sections obtained with 1) SIG sparker, 2) Centipede sparker, 3) Seistec boomer, 4) AA300 boomer, 5) X-Star chirp, 6) Innomar SES Same dimensions for each section. The bottom of the valley, as well as some sediment infills and incisions, are clearly visible on images 2 and 3. The valley s upper boundary is often masked by multiple reflections and can only be inferred from image 2. So-called Post Stack demultiple techniques proved ineffective. From the lower frequency sources tested, only the Centipede sparker was capable of imaging shallow sediments below the sandbank. Multichannel data was processed for all relevant sources. For comparison purposes, we processed the data independently using 12, 18 and 24 channels. Figure 3 shows results of the Centipede sparker for the full streamer length. The multichannel stack section clearly shows a better signal to noise ratio than single channel data. Long period multiples have been considerably attenuated, mostly thanks to detailed structural velocity analysis. Resolution of the deeper parts of the valley is significantly higher in the stack profiles and its internal structure and lateral limits can now be clearly identified. On the other hand, resolution in the shallow parts of the profile is inferior. Stacking with 24 or 18 channels had negligible effects on the data, and reducing the number of channels to 12 resulted in a small decrease in data quality. Further reduction to 6 channels seriously deteriorated the image.

5 2 km 50ms 35m Figure 3 Comparison between single channel (top) and stacked (bottom) data (Centipede sparker). Conclusions and recommendations Marine seismic data are an ideal tool to study buried landscapes, such as prehistoric river valleys, as they provide fast, cost-effective and non-destructive high-resolution information of the sub-seafloor. The choice of source/receiver configuration will depend on survey objectives and the intrinsic tradeoff between resolution and penetration. When the target is deeper than a few tens of metres, and in combination with a shallow water column, it is recommended to favour multichannel acquisition over traditional (single channel) sub-bottom profiling. For a similar channel interval, using 18 channels streamers will be sufficient. For our study area we recommend using the Centipede sparker in combination with the Parametric Echosounder in order to get the highest resolution information of both shallow and deep events. Future survey work will focus on additional sources and different geological environments. References Plets, R. [2007] The Acoustic Imaging, Reconstruction and Characterization of Buried Archaeological Material. PhD Thesis, University of Southampton, 190. Dix, J. [2008] High resolution sonar for the archaeological investigation of marine aggregate deposits. York: Archaeology Data Service (doi: / ) Mathys, M. [2009] The Quaternary geological evolution of the Belgian Continental Shelf, southern North Sea, Geology and Soil Science. PhD Thesis, Ghent University, 382. Van Lancker, V., Dufour, I., Mathys, M., Versteeg, W. and De Batist, M. [2009] Towards a highresolution 3D-analysis of sand-bank architecture on the Belgian Continental Shelf. Final Report. Belgian Science Policy, Brussels Annexes. Missiaen, T. [2010] The potential of seismic imaging in marine archaeological site investigations. Relicta (Vlaams Instituut voor het Onroerend Erfgoed), 6,