- Email: [email protected]
Lake sonar surveys and the search for sub-fossil wood
Dendrochronologia 30 (2012) 61–65
Contents lists available at SciVerse ScienceDirect
Dendrochronologia journal homepage: www.elsevier.de/dendro
Technical note
Lake sonar surveys and the search for sub-fossil wood Rob Wilson ∗ , C. Richard Bates School of Geography and Geosciences, University of St Andrews, United Kingdom
a r t i c l e
i n f o
Article history: Received 17 January 2011 Accepted 3 May 2011 Keywords: Sonar survey Sub-fossil wood Dendrochronology
a b s t r a c t Following the successful utilisation of lake preserved sub-fossil woody material to extend living Scots pine chronologies in Scandinavia, ongoing research in the Scottish Highlands aims to build a similar multimillennial long climatically sensitive pine chronology. This paper details explorative research testing the use of sonar methods to facilitate the search for sub-fossil material in lake environments. Although the method clearly identifies elongate anomalies that are consistent with submerged tree stems in water depths >1.5 m, it does not allow the identification of sub-fossil wood remnants in shallow water (<1.0 m) or heavily vegetated bays. Therefore, for the successful survey of lakes, a combination of traditional and sonar methods must be applied. Ongoing research will now explore the utilisation of these methods to more remote locations where boat access is not possible. © 2011 Istituto Italiano di Dendrocronologia. Published by Elsevier GmbH. All rights reserved.
Introduction Over the last 20 years, the utilisation of tree-rings as indicators of past climate came of age and dendroclimatogy is now one of the dominant palaeoclimate archives for studying high resolution late Holocene climate in the mid-to-high latitudes (Jones et al., 2009). Hemispheric temperature reconstructions of the last 1000 years, mainly dominated by tree-ring (TR) data, have been a particular focus in the recent IPCC report (2007) but estimates of the period prior to 1300 still remain poorly constrained (D’Arrigo et al., 2006) due to a paucity of millennial long proxy records. In fact, the quality of many of the local/regional millennial long TR based temperature sensitive chronologies is also less robust during the medieval period due to the decreasing number of TR series back in time (D’Arrigo et al., 2006; Esper and Frank, 2009). There is therefore an urgent need to sample new and update old millennial length temperature sensitive TR archives around the northern hemisphere. It is a major challenge for dendroclimatologists to build long climatically sensitive TR chronologies. Within Europe, living trees rarely attain ages >250–300 years in age (Eckstein, 1982) and extension of chronologies can only be made through the use of historical TR material (Wilson et al., 2004) or sub-fossil material. The use of historical material (e.g. beams in buildings), however, is a challenge with respect to the identification of the original provenance of the wood and in only a few rare examples, has such material
∗ Corresponding author. E-mail address: [email protected] (R. Wilson).
be used for dendroclimatic reconstruction (Richter and Eckstein, 1990; D’Arrigo and Jacoby, 1991; Brázdil et al., 2002; St. George and Nielsen, 2002; Wilson and Topham, 2004; Wilson et al., 2005; Büntgen et al., 2005, 2006) although one approach to overcome the location issue is to average over a very large region (Büntgen et al., 2011). The use of sub-fossil material from lakes, however, has proven to be a very successful strategy for extending living chronologies (Eronen et al., 2002; Grudd et al., 2002; Linderholm and Gunnarson, 2005; Grudd, 2008; Gunnarson, 2008). Not only is the original location of the trees known (i.e. at or near the lake shore), but also the anoxic environment of lake sediments and lower water column can often preserve woody material for many thousands of years. Wilson et al. (2011) recently detailed the potential of using sub-fossil Scots pine (Pinus sylvestris) material in the Cairngorm region of the Scottish Highlands to develop a multi-millennial long chronology. Radiocarbon dating of preserved pine stems from two lakes (Loch an Eilein and Loch Gamnha – Fig. 1) indicated sub-fossil material covering the last ∼8000 years. They concluded that a millennial length pine chronology from the NW Cairngorm region of Scotland is a feasible and realistic objective and that given time, a near Holocene length chronology may also be possible using preserved material extracted from lakes. However, for such chronologies to be developed, a substantial amount of subfossil pine material will be required from multiple lakes throughout the Highlands. To date, surveys of small to medium sized lakes (100–1000 m across) have been undertaken using what can be termed as “traditional Scandinavian methods”; (1) visual inspection from the shore, (2) snorkelling close to the shorelines and (3) “feeling” in the loose sediments with feet. Although this has been
1125-7865/$ – see front matter © 2011 Istituto Italiano di Dendrocronologia. Published by Elsevier GmbH. All rights reserved. doi:10.1016/j.dendro.2011.05.001
62
R. Wilson, C.R. Bates / Dendrochronologia 30 (2012) 61–65
Fig. 1. Map showing the location of Loch an Eilein (study lake – Fig. 2) with respect to other lakes where sub-fossil woody material has been found using traditional survey methods. Also shown is the present woodland cover and location of living pine sites sampled for dendrochronological analyses in the region.
an effective method for some lakes (Fig. 1), it is time consuming and has failed in some Scottish lakes due to the dark peaty opaque nature of the water which allows little light penetration, effectively making the visual identification of sub-fossil material impossible from the lake shore or even snorkelling. In this short paper, we present proof-of-concept results of using sonar survey methods to help overcome these difficulties in the search for sub-fossil material by specifically targeting one of the lakes where sub-fossil material has already been identified and sampled. Methods Very high resolution sonar techniques such as sidescan sonar and multibeam sonar are becoming standard survey methods in marine geological, archaeological and biological investigations where there is a requirement to map features on the seafloor at resolutions down to centimetres (Bates and Moore, 2002). The sidescan sonar produces a detailed sonar amplitude picture of the seafloor where acoustic reflection strength is mapped to show different features that are upstanding above the seafloor (Jones, 1999). Multibeam sonar and bathymetric sidescan sonar or swath sonar have been developed over the last 15 years to not only produce a detailed amplitude picture but also to map the location of
features on the seafloor while producing a digital terrain model (DTM – Hughes Clarke et al., 1996). Both techniques have application for mapping inland lakes and reservoirs but both usually necessitate large boat access due to the logistical requirements of power and navigation aids. In this study, we utilised a new system that has been developed based on a customised shallow-draft catamaran for shallow water survey. The catamaran, a Zeego Sports Boat, was fitted with a Topcon Hiper differential GPS, SEA SwathPlus 468 kHz bathymetric sonar and TSS DMS205 motion reference unit. The hardware is controlled by a dedicated IP57 waterproof computer running Hypack Max (Coastal Inc) navigation software and Swath (SEA ltd) sonar acquisition software. The system is navigated along parallel survey lines and acquires sonar data in a swath that extends beneath and to each side of the boat for a distance of approximately 30 m. The swath width is controlled by the water depth with greater swath widths possible in deeper water. Sonar data can be acquired in water as shallow as 1 m. However, the data is generally more robust in water depths of 1.5 m or greater. For this proof-of-concept study, we targeted Loch an Eilein in the north-west Cairngorms (Fig. 1) as it has good vehicular and boat access from the north shore and previous “traditional” surveys have identified large amounts of pine sub-fossil material in some sheltered bays (Wilson et al., 2011). 12 survey lines were acquired at approximately 40 m line spacing with the addition of two sets
R. Wilson, C.R. Bates / Dendrochronologia 30 (2012) 61–65
63
Fig. 2. Bathymetric survey results of Loch an Eilein. Boxes indicate locations of Figs. 3 and 4, respectively. Ovals denote locations of heavily vegetated shallow bays.
of parallel lines to the shore at approximately 40 and 60 m from the shore. Individual sonar lines were processed for bathymetry using SwathPlus Grid Processor (SEA) and Fledermaus (IVS Inc) and mosaiced into a final DTM for integration with the surrounding land topography using ArcGIS. The amplitude data that shows a pseudopicture of features on the lake floor was processed using SonarMAP (Chesapeake Ltd). The resulting complete coverage picture of the lake floor was overlaid on the bathymetric map at a 2 m resolution using ArcGIS. For areas of specific interest, for example near to the shoreline where it was anticipated that wood material might exist at water depths that would facilitate easy recovery, further high resolution maps at 0.25 m resolution were produced for overlay on the bathymetry. Results The survey of Loch an Eilein was completed in 5 h with 100% sonar overlap at an average of 40 m line separation. The bathymetric derived DTM (Fig. 2) has a maximum water depth of 22 m with four distinct sub-basins separated by shallow (2–4 m depth) ridges. At this time, it is not known if these ridges are related to the geological bedrock or may reflect a moraine sequence through the area. The amplitude overlay to the lake shows three types of lake floor, namely soft mud, exposed rock and sandy-mud with boulders. The two shallow bays to the east and south west are extensively populated with reeds and other aquatic plants (Fig. 2). Along the shoreline a number of discrete bathymetric and amplitude anomalies were mapped that are consistent with the size and shape of submerged wood. The anomalies, however, were best imaged in water depths of 1.5 m or greater. Examples are shown in Figs. 3 and 4 for the areas around the northern bay and along the southern shore. Both of these examples show elongate anomalies of 2–3 m in length and less than 0.5 m in width that are consistent with submerged tree stems. As both the amplitude and bathymetry
maps were acquired using the differential RTK GPS these anomalies were positioned to an accuracy of less than 20 cm. Although no sampling has been made at the northern end of the lake, much of the material so far extracted from the lake has been taken from the southern shoreline (Wilson et al., 2011)–coincident with the anomalies highlighted in the sonar images Discussion and conclusion Sonar survey appears to be an effective method for identifying preserved sub-fossil material and could be an especially useful method for surveying lakes where the water is too peaty and opaque for traditional surveying methods. This is very much the case in many smaller higher elevation lakes in the Scottish Highlands. Another advantage of this approach is speed. The survey of Loch an Eilein, a moderately large (1 km2 ) lake, was relatively quick, but allowed precise positioning of relict woody material which will allow later strategic fieldwork to target sub-fossil sample rich areas without relying on time consuming traditional surveying methods. It should also be emphasised that during the survey the sonar method shows real-time preliminary results, indicating possible sample rich areas, allowing further scans to obtain additional sonar data, thus ensuring the highest resolution information possible. The main weakness of the approach is that sonar techniques require a minimum working water depth of at least 1.0–1.5 m. In very shallow lakes this might not be achievable and there are obvious limitations searching for sub-fossil material closer to the shore. In addition, the sonar data is compromised in areas where there is significant vegetation in the water column as the sonar signal can often not penetrate through dense plant material (Fig. 2). It is in such shallow water, vegetated areas (i.e. sheltered bays) where a substantial amount of sub-fossil material has been found in Loch an Eilein (Wilson et al., 2011) although it should be emphasised that traditional methods can adequately survey these regions. Finally, the sonar method is also limited in that it cannot penetrate into the
64
R. Wilson, C.R. Bates / Dendrochronologia 30 (2012) 61–65
Fig. 3. (A) Bathymetric and side scan sonar results for the northern end of Loch an Eilein. (B) The upper right box shows a 3D view looking towards the northeast where the light grey elongate features (highlighted with ovals) represent the sub-fossil wood material.
Fig. 4. (A) As Fig. 3 but for southern end of Loch an Eilein. (B) The upper left box shows a zoomed in 3D view looking towards the southwest where the light grey elongate features (highlighted with an oval) represent the fallen wood material at the edge of the loch.
R. Wilson, C.R. Bates / Dendrochronologia 30 (2012) 61–65
sediment and only identifies material resting on or sticking out of lake sediment, therefore biasing identified material to more recent times as older material may be buried. The high frequency of the sonar means that penetration of the sediments on the lake floor is not possible. When wood material becomes buried, a lower frequency sonar system or a different technique is required to detect the buried wood. However, with a lower frequency sonar two problems are encountered. The resolution of features is also lowered and it is likely that this would result in the wood not being individually recognised. In addition, lower frequency systems require greater power to drive them and this would require a larger vessel, something that is rarely possible on small upland lakes. The results from this proof-of-concept study clearly show the potential of using sonar surveys to facilitate the search for subfossil woody material in lakes. Although the method will prove useful in the dark opaque waters of highland lakes and is relatively quick, traditional survey techniques will still be needed for heavily vegetated areas and regions of water depth <1 m. The combination of both approaches, however, will allow a complete survey of the lake bottom and shorelines and the sonar results will facilitate the identification of sample rich regions to minimise the amount of traditional surveying to be undertaken. Future reconnaissance work will explore alternative methods using remotely operated vehicles for deploying the sonar in very shallow water and the use of ground penetrating radar systems floated/towed behind the shallow-draft boat to allow some penetration into the sediments. GPR has been used successfully in fresh-water lakes for mapping bathymetry and also for obtaining sub-bottom records in engineering studies (Haeni, 1996). For the smaller higher elevation lakes, where there is no vehicular access, the new generation of survey platforms using remotely controlled boats and autonomous underwater vehicles for survey could prove useful as they can be configured for bathymetric survey and for sub-bottom surveying while also being of smaller size allowing easier deployment to remote locations. Acknowledgments This work was co-funded through the EU (Millennium – 017008-2) and Leverhulme Trust (RELIC – F/00 268/BG). We would like to thank Melanie Chocholek for help with fieldwork and Rothiemurchus Estates and Scottish Natural Heritage for allowing the survey to be undertaken on Loch an Eilein. References Bates, C.R., Moore, C., 2002. Acoustical methods for marine habitat surveys. HydroInternational 6 (1), 47–49. Brázdil, R., Stepánková, P., Kyncl, T., Kyncl, J., 2002. Fir tree-ring reconstruction of March–July precipitation in southern Moravia (Czech Republic) A.D. 1376–1996. Climate Research 20, 223–239. Büntgen, U., Esper, J., Frank, D.C., Nicolussi, K., Schmidhalter, M., 2005. A 1052year tree-ring proxy for Alpine summer temperatures. Climate Dynamics 25, 141–153.
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Büntgen, U., Frank, D.C., Nievergelt, D., Esper, J., 2006. Summer temperature variations in the European Alps, A.D. 755–2004. Journal of Climate 19, 5606–5623. Büntgen, U., Tegel, W., Nicolussi, K., McCormick, M., Frank, D., Trouet, V., Kaplan, J.O., Herzig, F., Heussner, K.U., Wanner, H., Luterbacher, J., Esper, J., 2011. 2500 years of European climate variability and human susceptibility. Science 331 (6017), 578–582, doi:10.1126/science.1197175. D’Arrigo, R.D., Jacoby, G.C., 1991. A 1000-year record of winter precipitation from northwestern new Mexico USA: a reconstruction from tree-rings and its relation to El Nino and the southern Oscillation. The Holocene 1 (2), 95–101. D’Arrigo, R., Wilson, R., Jacoby, G., 2006. On the long-term context for late 20th century warming. Journal of Geophysical Research 111, D03103, doi:10.1029/2005JD006352. Eckstein, D., 1982. Europe. In: Hughes, M.K., Kelly, P.M., Pilcher, J.R., LaMarche, V.C. (Eds.), Climate from Tree-Rings. Cambridge University Press, Cambridge, pp. 142–148. Eronen, M., Zetterberg, P., Briffa, K., Lindholm, M., Meriläinen, J., Timonen, T., 2002. The supra-long Scots pine tree-ring record for Finnish Lapland: part 1, chronology construction and initial inferences. The Holocene 12, 673–680. Esper, J., Frank, D.C., 2009. The IPCC on a heterogeneous Medieval Warm Period. Climatic Change 94, 267–273. Grudd, H., Briffa, K.R., Karlén, W., Bartholin, T.S., Jones, P.D., Kromer, B., 2002. A 7400-year tree-ring chronology in northern Swedish Lapland: natural climatic variability expressed on annual to millennial timescales. The Holocene 12, 657–665. Grudd, H., 2008. Torneträsk tree-ring width and density AD 500–2004: a test of climatic sensitivity and a new 1500-year reconstruction of north Fennoscandian summers. Climate Dynamics 31, 843–857, doi:10.1007/s00382-007-0358-362. Gunnarson, B.E., 2008. Temporal distribution pattern of subfossil wood in central Sweden: perspective on Holocene humidity fluctuations. The Holocene 18, 569–577. Haeni, P., 1996. Use of ground-penetrating radar and continuous seismic-reflection profiling on surface-water bodies in environmental and engineering studies. Journal of Environmental Engineering Geophysics 1, 1–27. Hughes Clarke, J.E., Mayer, L.A., Wells, D.E., 1996. Shallow-water imaging multibeam sonars: a new tool for investigating seafloor processes in the coastal zone and on the continental shelf. Marine Geophysical Researches 18, 607–629. IPCC, 2007. Climate change 2007: the physical science basis. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., Miller, H.L. (Eds.), Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge/New York, NY. Jones, E.J.W., 1999. Marine Geophysics. J. Wiley & Sons Ltd, 474 pp. Jones, P.D., Briffa, K.R., Osborn, T.J., Lough, J.M., van Ommen, T.D., Vinther, B.M., Luterbacher, J., Wahl, E.R., Zwiers, F.W., Mann, M.E., Schmidt, G.A., Ammann, C.M., Buckley, B.M., Cobb, K.M., Esper, J., Goosse, H., Graham, N., Jansen, E., Kiefer, T., Kull, C., Küttel, M., Mosley-Thompson, E., Overpeck, J.T., Riedwyl, N., Schulz, M., Tudhope, A.W., Villalba, R., Wanner, H., Wolff, E., Xoplaki, E., 2009. Highresolution paleoclimatology of the last millennium: a review of current status and future prospects. The Holocene 19, 3–49. Linderholm, H., Gunnarson, B., 2005. Summer temperature variability in central Scandinavian in the last 3600 years. Geografiska Annaler A 87 (1), 231–241. Richter, K., Eckstein, D., 1990. A proxy summer rainfall record for southeast Spain derived from living and historic pine trees. Dendrochronologia 8, 76–82. St. George, S., Nielsen, E., 2002. Hydroclimatic change in southern Manitoba since A.D. 1409 inferred from tree rings. Quaternary Research 58 (2), 103–111. Wilson, R.J.S., Esper, J., Luckman, B.H., 2004. Utilising historical tree-ring data for dendroclimatology: a case study from the Bavarian forest, Germany. Dendrochronologia 21 (2), 53–68. Wilson, R.J.S., Topham, J., 2004. Violins and climate. Theoretical and Applied Climatology 77, 9–24. Wilson, R.J.S, Luckman, B.H., Esper, J., 2005. A 500-year dendroclimatic reconstruction of spring/summer precipitation from the lower Bavarian forest region, Germany. International Journal of Climatology 25, 611–630. Wilson, R., Loader, N., Rydval, M., Paton, H., Frith, A., Mills, C., Crone, A., Edwards, C., Larsson, L., Gunnarson, B., 2011. Reconstructing Holocene climate from tree rings—the potential for a long chronology from the Scottish Highlands. The Holocene, doi:10.1177/0959683611405237.
Contents lists available at SciVerse ScienceDirect
Dendrochronologia journal homepage: www.elsevier.de/dendro
Technical note
Lake sonar surveys and the search for sub-fossil wood Rob Wilson ∗ , C. Richard Bates School of Geography and Geosciences, University of St Andrews, United Kingdom
a r t i c l e
i n f o
Article history: Received 17 January 2011 Accepted 3 May 2011 Keywords: Sonar survey Sub-fossil wood Dendrochronology
a b s t r a c t Following the successful utilisation of lake preserved sub-fossil woody material to extend living Scots pine chronologies in Scandinavia, ongoing research in the Scottish Highlands aims to build a similar multimillennial long climatically sensitive pine chronology. This paper details explorative research testing the use of sonar methods to facilitate the search for sub-fossil material in lake environments. Although the method clearly identifies elongate anomalies that are consistent with submerged tree stems in water depths >1.5 m, it does not allow the identification of sub-fossil wood remnants in shallow water (<1.0 m) or heavily vegetated bays. Therefore, for the successful survey of lakes, a combination of traditional and sonar methods must be applied. Ongoing research will now explore the utilisation of these methods to more remote locations where boat access is not possible. © 2011 Istituto Italiano di Dendrocronologia. Published by Elsevier GmbH. All rights reserved.
Introduction Over the last 20 years, the utilisation of tree-rings as indicators of past climate came of age and dendroclimatogy is now one of the dominant palaeoclimate archives for studying high resolution late Holocene climate in the mid-to-high latitudes (Jones et al., 2009). Hemispheric temperature reconstructions of the last 1000 years, mainly dominated by tree-ring (TR) data, have been a particular focus in the recent IPCC report (2007) but estimates of the period prior to 1300 still remain poorly constrained (D’Arrigo et al., 2006) due to a paucity of millennial long proxy records. In fact, the quality of many of the local/regional millennial long TR based temperature sensitive chronologies is also less robust during the medieval period due to the decreasing number of TR series back in time (D’Arrigo et al., 2006; Esper and Frank, 2009). There is therefore an urgent need to sample new and update old millennial length temperature sensitive TR archives around the northern hemisphere. It is a major challenge for dendroclimatologists to build long climatically sensitive TR chronologies. Within Europe, living trees rarely attain ages >250–300 years in age (Eckstein, 1982) and extension of chronologies can only be made through the use of historical TR material (Wilson et al., 2004) or sub-fossil material. The use of historical material (e.g. beams in buildings), however, is a challenge with respect to the identification of the original provenance of the wood and in only a few rare examples, has such material
∗ Corresponding author. E-mail address: [email protected] (R. Wilson).
be used for dendroclimatic reconstruction (Richter and Eckstein, 1990; D’Arrigo and Jacoby, 1991; Brázdil et al., 2002; St. George and Nielsen, 2002; Wilson and Topham, 2004; Wilson et al., 2005; Büntgen et al., 2005, 2006) although one approach to overcome the location issue is to average over a very large region (Büntgen et al., 2011). The use of sub-fossil material from lakes, however, has proven to be a very successful strategy for extending living chronologies (Eronen et al., 2002; Grudd et al., 2002; Linderholm and Gunnarson, 2005; Grudd, 2008; Gunnarson, 2008). Not only is the original location of the trees known (i.e. at or near the lake shore), but also the anoxic environment of lake sediments and lower water column can often preserve woody material for many thousands of years. Wilson et al. (2011) recently detailed the potential of using sub-fossil Scots pine (Pinus sylvestris) material in the Cairngorm region of the Scottish Highlands to develop a multi-millennial long chronology. Radiocarbon dating of preserved pine stems from two lakes (Loch an Eilein and Loch Gamnha – Fig. 1) indicated sub-fossil material covering the last ∼8000 years. They concluded that a millennial length pine chronology from the NW Cairngorm region of Scotland is a feasible and realistic objective and that given time, a near Holocene length chronology may also be possible using preserved material extracted from lakes. However, for such chronologies to be developed, a substantial amount of subfossil pine material will be required from multiple lakes throughout the Highlands. To date, surveys of small to medium sized lakes (100–1000 m across) have been undertaken using what can be termed as “traditional Scandinavian methods”; (1) visual inspection from the shore, (2) snorkelling close to the shorelines and (3) “feeling” in the loose sediments with feet. Although this has been
1125-7865/$ – see front matter © 2011 Istituto Italiano di Dendrocronologia. Published by Elsevier GmbH. All rights reserved. doi:10.1016/j.dendro.2011.05.001
62
R. Wilson, C.R. Bates / Dendrochronologia 30 (2012) 61–65
Fig. 1. Map showing the location of Loch an Eilein (study lake – Fig. 2) with respect to other lakes where sub-fossil woody material has been found using traditional survey methods. Also shown is the present woodland cover and location of living pine sites sampled for dendrochronological analyses in the region.
an effective method for some lakes (Fig. 1), it is time consuming and has failed in some Scottish lakes due to the dark peaty opaque nature of the water which allows little light penetration, effectively making the visual identification of sub-fossil material impossible from the lake shore or even snorkelling. In this short paper, we present proof-of-concept results of using sonar survey methods to help overcome these difficulties in the search for sub-fossil material by specifically targeting one of the lakes where sub-fossil material has already been identified and sampled. Methods Very high resolution sonar techniques such as sidescan sonar and multibeam sonar are becoming standard survey methods in marine geological, archaeological and biological investigations where there is a requirement to map features on the seafloor at resolutions down to centimetres (Bates and Moore, 2002). The sidescan sonar produces a detailed sonar amplitude picture of the seafloor where acoustic reflection strength is mapped to show different features that are upstanding above the seafloor (Jones, 1999). Multibeam sonar and bathymetric sidescan sonar or swath sonar have been developed over the last 15 years to not only produce a detailed amplitude picture but also to map the location of
features on the seafloor while producing a digital terrain model (DTM – Hughes Clarke et al., 1996). Both techniques have application for mapping inland lakes and reservoirs but both usually necessitate large boat access due to the logistical requirements of power and navigation aids. In this study, we utilised a new system that has been developed based on a customised shallow-draft catamaran for shallow water survey. The catamaran, a Zeego Sports Boat, was fitted with a Topcon Hiper differential GPS, SEA SwathPlus 468 kHz bathymetric sonar and TSS DMS205 motion reference unit. The hardware is controlled by a dedicated IP57 waterproof computer running Hypack Max (Coastal Inc) navigation software and Swath (SEA ltd) sonar acquisition software. The system is navigated along parallel survey lines and acquires sonar data in a swath that extends beneath and to each side of the boat for a distance of approximately 30 m. The swath width is controlled by the water depth with greater swath widths possible in deeper water. Sonar data can be acquired in water as shallow as 1 m. However, the data is generally more robust in water depths of 1.5 m or greater. For this proof-of-concept study, we targeted Loch an Eilein in the north-west Cairngorms (Fig. 1) as it has good vehicular and boat access from the north shore and previous “traditional” surveys have identified large amounts of pine sub-fossil material in some sheltered bays (Wilson et al., 2011). 12 survey lines were acquired at approximately 40 m line spacing with the addition of two sets
R. Wilson, C.R. Bates / Dendrochronologia 30 (2012) 61–65
63
Fig. 2. Bathymetric survey results of Loch an Eilein. Boxes indicate locations of Figs. 3 and 4, respectively. Ovals denote locations of heavily vegetated shallow bays.
of parallel lines to the shore at approximately 40 and 60 m from the shore. Individual sonar lines were processed for bathymetry using SwathPlus Grid Processor (SEA) and Fledermaus (IVS Inc) and mosaiced into a final DTM for integration with the surrounding land topography using ArcGIS. The amplitude data that shows a pseudopicture of features on the lake floor was processed using SonarMAP (Chesapeake Ltd). The resulting complete coverage picture of the lake floor was overlaid on the bathymetric map at a 2 m resolution using ArcGIS. For areas of specific interest, for example near to the shoreline where it was anticipated that wood material might exist at water depths that would facilitate easy recovery, further high resolution maps at 0.25 m resolution were produced for overlay on the bathymetry. Results The survey of Loch an Eilein was completed in 5 h with 100% sonar overlap at an average of 40 m line separation. The bathymetric derived DTM (Fig. 2) has a maximum water depth of 22 m with four distinct sub-basins separated by shallow (2–4 m depth) ridges. At this time, it is not known if these ridges are related to the geological bedrock or may reflect a moraine sequence through the area. The amplitude overlay to the lake shows three types of lake floor, namely soft mud, exposed rock and sandy-mud with boulders. The two shallow bays to the east and south west are extensively populated with reeds and other aquatic plants (Fig. 2). Along the shoreline a number of discrete bathymetric and amplitude anomalies were mapped that are consistent with the size and shape of submerged wood. The anomalies, however, were best imaged in water depths of 1.5 m or greater. Examples are shown in Figs. 3 and 4 for the areas around the northern bay and along the southern shore. Both of these examples show elongate anomalies of 2–3 m in length and less than 0.5 m in width that are consistent with submerged tree stems. As both the amplitude and bathymetry
maps were acquired using the differential RTK GPS these anomalies were positioned to an accuracy of less than 20 cm. Although no sampling has been made at the northern end of the lake, much of the material so far extracted from the lake has been taken from the southern shoreline (Wilson et al., 2011)–coincident with the anomalies highlighted in the sonar images Discussion and conclusion Sonar survey appears to be an effective method for identifying preserved sub-fossil material and could be an especially useful method for surveying lakes where the water is too peaty and opaque for traditional surveying methods. This is very much the case in many smaller higher elevation lakes in the Scottish Highlands. Another advantage of this approach is speed. The survey of Loch an Eilein, a moderately large (1 km2 ) lake, was relatively quick, but allowed precise positioning of relict woody material which will allow later strategic fieldwork to target sub-fossil sample rich areas without relying on time consuming traditional surveying methods. It should also be emphasised that during the survey the sonar method shows real-time preliminary results, indicating possible sample rich areas, allowing further scans to obtain additional sonar data, thus ensuring the highest resolution information possible. The main weakness of the approach is that sonar techniques require a minimum working water depth of at least 1.0–1.5 m. In very shallow lakes this might not be achievable and there are obvious limitations searching for sub-fossil material closer to the shore. In addition, the sonar data is compromised in areas where there is significant vegetation in the water column as the sonar signal can often not penetrate through dense plant material (Fig. 2). It is in such shallow water, vegetated areas (i.e. sheltered bays) where a substantial amount of sub-fossil material has been found in Loch an Eilein (Wilson et al., 2011) although it should be emphasised that traditional methods can adequately survey these regions. Finally, the sonar method is also limited in that it cannot penetrate into the
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R. Wilson, C.R. Bates / Dendrochronologia 30 (2012) 61–65
Fig. 3. (A) Bathymetric and side scan sonar results for the northern end of Loch an Eilein. (B) The upper right box shows a 3D view looking towards the northeast where the light grey elongate features (highlighted with ovals) represent the sub-fossil wood material.
Fig. 4. (A) As Fig. 3 but for southern end of Loch an Eilein. (B) The upper left box shows a zoomed in 3D view looking towards the southwest where the light grey elongate features (highlighted with an oval) represent the fallen wood material at the edge of the loch.
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sediment and only identifies material resting on or sticking out of lake sediment, therefore biasing identified material to more recent times as older material may be buried. The high frequency of the sonar means that penetration of the sediments on the lake floor is not possible. When wood material becomes buried, a lower frequency sonar system or a different technique is required to detect the buried wood. However, with a lower frequency sonar two problems are encountered. The resolution of features is also lowered and it is likely that this would result in the wood not being individually recognised. In addition, lower frequency systems require greater power to drive them and this would require a larger vessel, something that is rarely possible on small upland lakes. The results from this proof-of-concept study clearly show the potential of using sonar surveys to facilitate the search for subfossil woody material in lakes. Although the method will prove useful in the dark opaque waters of highland lakes and is relatively quick, traditional survey techniques will still be needed for heavily vegetated areas and regions of water depth <1 m. The combination of both approaches, however, will allow a complete survey of the lake bottom and shorelines and the sonar results will facilitate the identification of sample rich regions to minimise the amount of traditional surveying to be undertaken. Future reconnaissance work will explore alternative methods using remotely operated vehicles for deploying the sonar in very shallow water and the use of ground penetrating radar systems floated/towed behind the shallow-draft boat to allow some penetration into the sediments. GPR has been used successfully in fresh-water lakes for mapping bathymetry and also for obtaining sub-bottom records in engineering studies (Haeni, 1996). For the smaller higher elevation lakes, where there is no vehicular access, the new generation of survey platforms using remotely controlled boats and autonomous underwater vehicles for survey could prove useful as they can be configured for bathymetric survey and for sub-bottom surveying while also being of smaller size allowing easier deployment to remote locations. Acknowledgments This work was co-funded through the EU (Millennium – 017008-2) and Leverhulme Trust (RELIC – F/00 268/BG). We would like to thank Melanie Chocholek for help with fieldwork and Rothiemurchus Estates and Scottish Natural Heritage for allowing the survey to be undertaken on Loch an Eilein. References Bates, C.R., Moore, C., 2002. Acoustical methods for marine habitat surveys. HydroInternational 6 (1), 47–49. Brázdil, R., Stepánková, P., Kyncl, T., Kyncl, J., 2002. Fir tree-ring reconstruction of March–July precipitation in southern Moravia (Czech Republic) A.D. 1376–1996. Climate Research 20, 223–239. Büntgen, U., Esper, J., Frank, D.C., Nicolussi, K., Schmidhalter, M., 2005. A 1052year tree-ring proxy for Alpine summer temperatures. Climate Dynamics 25, 141–153.
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