OR/16/001 Previous work

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Dove, D, Bradwell, T*, Carter, G, Cotterill, C, Gafeira, J, Green, S, Krabbendam, M, Mellet, C, Stevenson, A, Stewart, H, Westhead, K.; INFOMAR: Scott, G, Guinan, J, Judge, M, Monteys, X.; MAREANO: Elvenes, S, Baeten, N, Dolan, M, Thorsnes, T, Bjarnadóttir, L, and Ottesen, D . 2016. Seabed geomorphology: a two-part classification system. British Geological Survey Internal Report, OR/16/001.
(* now at Stirling University)

Observations

While this is not intended to be an exhaustive review of all significant efforts to characterise seabed geomorphology, there is evidence that as soon as surveyors and geoscientists began acquiring data which yielded images/profiles of the seabed, they also began making inferences on the environmental origins and associations of the features observed. The first hydrographic surveys relied on lead-line surveys (17th century–mid-20th century) that provided a series of point observations of water depth, but carried significant navigational uncertainties. As an example, the early expeditions led by Wyville Thomson demonstrated the seabed was not a ‘barren waste’, but a diverse physiographic environment with equally diverse flora and fauna (Thomson, 1874[1]). This was followed by the unprecedented Challenger expedition, which over ~20 years and 50 reports laid the foundations for modern hydrography and oceanography.

The 1930s saw the development of single-beam echo sounders (SBES), providing the first acoustic images of the seabed, and with increasing volumes of data, provided greater scope for describing the nature of the seabed, e.g. examining the origin of submarine canyons (Daly, 1936[2]). In the 1950s and 1960s, increasing bathymetric observations enabled the first global-scale characterisation of seabed geomorphology: the production of physiographic charts, at first for individual ocean basins (e.g. Heezen et al., 1959[3]), and then ultimately for the global seabed (less than 73° latitude) (Heezen and Tharp, 1977[4]). This painstaking cartographic work conducted by Marie Tharp and Bruce Heezen demonstrated the great variation in seabed morphology/configuration, and through this, also made significant contributions to the contemporaneous plate-tectonic revolution.

The introduction of side-scan sonars (SSS) brought about a new element to seabed mapping, the ability to ‘sense’ significantly smaller features than was previously possible, even though SSS yield backscatter (intensity of acoustic return) data rather than bathymetry (depth) data. SSS data not only imaged features at higher resolution (10s cms), they provided imagery in a broader swath rather than along single 2D profiles. While initially intended for military applications, the technology had value to geoscientists, and opened up the capacity to image individual bedforms and better understand associated sedimentary and hydrodynamic processes (e.g. Belderson et al., 1972[5], Stride, 1982[6]).

With the increased usage and availability of SBES and SSS, there followed initiatives to classify sedimentary bedforms and compare them with fluvial analogues (e.g. Ashley, 1990[7]), much like this report describes an initiative to describe geomorphology using the increased availability of of multibeam echo sounder (MBES) data. From efforts such as this, it is clear that the classification of features does not simply represent a ‘stamp-collecting’ exercise, but through careful categorisation and compilation, yields important information on the variation of features (e.g. ‘bedform continuum’), and provides the ability to discriminate between the multiple environmental processes that constrain their development. SSS data are still of relevance today as not only do they typically provide higher-resolution data than hull-mounted MBES systems, they also give a ‘slant-range’ view of the seabed, which is of use for imaging upstanding features that may be less apparent in MBES data e.g. unexploded ordinance (UXO) and wreck mapping.

From the 1990s onward, survey-scale interpretations of geomorphology have primarily been based on MBES data (as well as Lidar data in the coastal zone), which provide a swath of co-registered bathymetry and backscatter data. As in the past, the resulting scientific progress has been distinctively data driven, both in terms of data quality and coverage. As described earlier, swath bathymetry data provide an extraordinary opportunity to image the seabed and understand the environmental processes that formed and actively govern that environment. Because of this, the last 20–30 years have seen a remarkable rush in academic and commercial work using these data to describe the seabed from the coastal environment (e.g. Finkl et al., 2005[8]; Gavrilov et al., 2005[9]; Moore et al., 2009[10]), to the continental shelves (e.g. Clarke et al., 1996[11]; Todd et al., 1999[12]; Kenyon and Cooper, 2005[13]; Dove et al., 2015[14]), continental slopes (e.g. Driscoll et al., 2000[15]; Mosher et al., 2010[16]; Dolan et al., 2008[17]), and deep-ocean basins (e.g. Edwards et al., 2001[18]; Hillier and Watts, 2007[19]).

Classification

The availability of high-quality swath bathymetry has led to a step-change in our understanding of numerous aspects of marine geology, and that is why it is somewhat surprising that there has been no significant attempt (to our knowledge) to structure the way we describe the geomorphology of the seabed. The application of geomorphic classification schemes in terrestrial geoscience is after all common place, including both manual and automated classification approaches (e.g. MacMillan and Shary, 2009[20]). One partial exception to this in the marine environment are the international standards documents produced by the International Hydrographic Organisation (IHO), including the ‘Standardization of undersea feature names’ (IHO and IOC, 2013[21]), and ‘Hydrographic ‘Dictionary’ (IHO, 1994). Apart from giving guidance on the appropriate way to name new features, the ‘feature names’ list includes a list of ‘generic terms’ and associated definitions. These ‘generic terms’ are relevant here and primarily fall into our ‘morphology’ (e.g. ‘ridge’) classification tree rather than our ‘geomorphology’ tree (e.g. ‘abyssal hill’). The ‘Hydrographic dictionary’ includes several further terms that relate to features in our ‘geomorphology’ classification (e.g. ‘sandwave’), though the greater than 7000 terms in this dictionary relate to all aspects of hydrography. Taken together, these IHO documents incorporate fewer geomorphological features than one might observe at seabed (and that we’ve attempted to include here), but the primary point of difference is that features are simply listed in alphabetical order rather than presented in a structure relevant to their origin or morphology. Nevertheless, we make a point of incorporating all relevant features listed in IHO standards, and adopt their glossary definitions where they cannot be improved upon. Within the field of benthic ecology, geomorphology is also indirectly addressed through ‘terrain attributes’ (e.g. slope, rugosity, etc), which are regularly incorporated into predictive models of the distribution of benthic habitats (e.g. Wilson et al., 2007[22]; Lecours et al., 2016[23]). While certainly of use, these terrain attributes only relate to aspects and/or derivatives of the morphology, and do not address the process-origin of features, their potential mobility/erodibility (i.e. geotechnical properties), or their association with the underlying geology.

Surprisingly, it is at the global scale where there have been systematic attempts (methodologically at least) to classify seabed geomorphology, which presumably relates to the availability of suitable data. Global bathymetry datasets are produced by extrapolating ship-borne bathymetric transects (broadly and irregularly spaced) according to satellite altimetry data (e.g. Smith and Sandwell, 1997[24], Becker et al., 2009[25]). These bathymetry models are of coarser km-scale resolution, but because they are continuous they enable a consistent logical mapping approach to be applied to the data. This has been undertaken for example to look at the global distribution of seamounts (e.g. Hillier and Watts, 2007[19]), and more broadly to assess global submarine geomorphology (Harris et al., 2014[26]) (Figure 3). It is this systematic approach that is required to identify, describe, and classify features at a broader range of scales, from characterising bedform/landform-scale features observed in high-resolution data (e.g. swath bathymetry), to continental-scale features observed in coarser bathymetric compilations. Considering the substantial attention paid to the classification of deposits and the distribution of sediment at seabed (e.g. Kostylev et al., 2001[27]; Diesing et al., 2012; Lark et al., 2015[28]), it seems timely (if not overdue) to develop a classification system that facilitates the consistent classification of seabed geomorphology. This is especially the case considering the increasing availability of high-resolution MBES data (e.g. (Figure 1)), as well as meso-scale regional bathymetry models (Figure 4) (based on compilations of SBES data and MBES data where available — e.g. EMODnet-Bathymetry) that support detailed geomorphic description and interpretation. It is also worth noting that this approach can apply below the seabed, where 3D seismic data depict buried surfaces recording environmental processes similar to those observed at seabed, providing further information on the architecture of the seabed and shallow sub-seabed (e.g. Andreassen et al., 2007[29]).

Figure 3    Geomorphic features map of the world’s oceans. Reproduced from Harris et al. (2014)[26].
Figure 4    Bathymetry offshore of the UK and surrounding areas (~150 m resolution). Source: European Marine Observation and Data Network (EMODnet). Colour scale clipped to emphasise relief inboard of the continental shelf break. Pink lines delineate national maritime boundaries.

Development of seabed geomorphology mapping within BGS

Between the late 1970s and early 1990s, the BGS undertook a systematic survey programme of geophysics and ground-truthing across the UK offshore (Fannin, 1989[30]), which resulted in the publication of a series of 1:250 000 scale maps (e.g. British Geological Survey, 1991[31]), regional reports (e.g. Cameron et al., 1992[32]), and associated peer-reviewed publications. Many of the 1:250k Quaternary and Seabed Sediment maps included inset maps of bedforms, more often than not focussed on mobile-sediment features (e.g. sediment waves). Through a number of commissioned projects, these disparate maps have been collated to produce a single layer of bedforms (e.g. Strategic Environmental Assessments (SEA) — www.gov.uk/guidance/offshore-energy-strategic-environmental-assessment-sea-an-overview-of-the-sea-process; Westhead et al., 2014[33]), but these maps are based primarily on SBES and SSS data and cannot match the detail provided by interpretation of swath bathymetry. Further to this, BGS geoscientists have produced numerous geomorphological maps through commissioned projects, for end-purposes including: offshore renewables, Oil & Gas platforms, cable/pipeline routes, and marine protected areas.

In 2005, BGS ran a pilot ‘Seabed Geology’ project to utilise all available data to produce maps of the seabed geology, including seabed sediments and geomorphology. Within the case study area, there was little swath bathymetry data available at the time, but detailed interpretations were made as high-resolution observations of bedforms on SSS data were extrapolated according to regional-scale bathymetric surfaces (50–100 m resolution) produced by re-gridded single-beam surveys. The intrinsic challenge with this approach was that the mapping effort was excessively time-consuming, and therefore prohibitively expensive. Nevertheless, a prototype geomorphology scheme was developed as part of the project. The classification system presented in this report reflects an evolution of that process, but is fundamentally changed as the previous scheme was primarily a catalogue of features that in some instances were defined by geomorphology (e.g. moraines), and in other instances by morphology (e.g. linear depressions). Separately, land survey geologists in BGS started work in the mid-2000s on compiling a glossary of glacial landforms, and these features are also incorporated here.

References

  1. ↑ THOMSON, C W. 1874. An Account of the General Results of the Dredging Cruises of H.M.SS. 'Porcupine' and 'Lightning' During the Summers of 1868, 1869, and 1870, Under the Scientific Direction of Dr Carpenter, J Gwyn Jeffreys, and Dr Wyville Thomson.
  2. ↑ DALY, R A. 1936. Origin of submarine canyons. American Journal of Science, (186), pp.401–420.
  3. ↑ HEEZEN, B C, THARP, M, and EWING M, 1959. The floors of the oceans I. The North Atlantic. Geological Society of America Special Papers; 65:1–26.
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  9. ↑ GAVRILOV, A N, DUNCAN, A J, MCCAULEY, R D, PARNUM, I M, PENROSE, J D, SIWABESSY, P J W, WOODS, A J, and TSENG, Y T. 2005, June. Characterization of the seafloor in Australia’s coastal zone using acoustic techniques. In Proceedings of the International Conference" Underwater Acoustic Measurements: Technologies & Results (pp.1075–1080).
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  13. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Kenyon 2005
  14. ↑ DOVE, D, AROSIO, R, FINLAYSON, A, BRADWELL, T, and HOWE, J A. 2015. Submarine glacial landforms record Late Pleistocene ice-sheet dynamics, Inner Hebrides, Scotland. Quaternary Science Reviews, 123, pp.76–90.
  15. ↑ DRISCOLL, N W, WEISSEL, J K, and GOFF, J A. 2000. Potential for large-scale submarine slope failure and tsunami generation along the US mid-Atlantic coast. Geology, 28(5), pp.407–410.
  16. ↑ MOSHER, D C, MOSCARDELLI, L, SHIPP, R C, CHAYTOR, J D, BAXTER, C D, LEE, H J, and URGELES, R. 2010. Submarine mass movements and their consequences (pp.1–8). Springer Netherlands.
  17. ↑ DOLAN, M F, GREHAN, A J, GUINAN, J C, and BROWN, C. 2008. Modelling the local distribution of cold-water corals in relation to bathymetric variables: Adding spatial context to deep-sea video data. Deep Sea Research Part I: Oceanographic Research Papers, 55(11), pp.1564–1579.
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  19. ↑ 19.0 19.1 HILLIER, J K, and WATTS, A B. 2007. Global distribution of seamounts from ship‐track bathymetry data. Geophysical Research Letters, 34(13).
  20. ↑ MACMILLAN, R A, and SHARY, P A. 2009. Landforms and landform elements in geomorphometry. Developments in soil science, 33, pp.227–254.
  21. ↑ INTERNATIONAL HYDROGRAPHIC ORGANIZATION (IHO) and Intergovernmental Oceanographic Commission (IOC), 2013, Standardization of undersea feature names—Guidelines, proposal form, terminology (Edition 4.1.0) Monaco, International Hydrographic Bureau, IHO–IOC Publication B–6.
  22. ↑ WILSON, M F, O’CONNELL, B, BROWN, C, GUINAN, J C, and GREHAN, A J. 2007. Multiscale terrain analysis of multibeam bathymetry data for habitat mapping on the continental slope. Marine Geodesy, 30(1–2), pp.3–35.
  23. ↑ LECOURS, V, DOLAN, M F, MICALLEF, A, and LUCIEER, V L. 2016. Characterising the ocean frontier: A review of marine geomorphometry. Hydrology and Earth System Sciences, 20.
  24. ↑ SMITH, W H, and SANDWELL, D T. 1997. Global sea floor topography from satellite altimetry and ship depth soundings. Science, 277(5334), pp.1956–1962.
  25. ↑ BECKER, J J, SANDWELL, D T, SMITH, W H F, BRAUD, J, BINDER, B, DEPNER, J, FABRE, D, FACTOR, J, INGALLS, S, KIM, S H, and LADNER, R. 2009. Global bathymetry and elevation data at 30 arc seconds resolution: SRTM30_PLUS. Marine Geodesy, 32(4), pp.355–371.
  26. ↑ 26.0 26.1 HARRIS, P T, MACMILLAN-LAWLER, M, RUPP, J, and BAKER, E K. 2014. Geomorphology of the oceans. Marine Geology, 352, pp.4–24.
  27. ↑ KOSTYLEV, V E, TODD, B J, FADER, G B, COURTNEY, R C, CAMERON, G D, and PICKRILL, R A. 2001. Benthic habitat mapping on the Scotian Shelf based on multibeam bathymetry, surficial geology and sea floor photographs. Marine Ecology Progress Series, 219, pp.121–137.
  28. ↑ LARK, R M, MARCHANT, B P, DOVE, D, GREEN, S L, STEWART, H, and DIESING, M. 2015. Combining observations with acoustic swath bathymetry and backscatter to map seabed sediment texture classes: The empirical best linear unbiased predictor. Sedimentary Geology, 328, pp.17–32.
  29. ↑ ANDREASSEN, K, ØDEGAARD, C M, and RAFAELSEN, B. 2007. Imprints of former ice streams, imaged and interpreted using industry three-dimensional seismic data from the south-western Barents Sea. Geological Society, London, Special Publications, 277(1), pp.151–169.
  30. ↑ FANNIN, N G T. 1989. Offshore investigations 1966–87. British Geological Survey Technical Report WB/89/2.
  31. ↑ BRITISH GEOLOGICAL SURVEY, 1991. Seabed Sediments — Spurn; 1:250 000 Series, British Geological Survey, Keyworth, Nottingham.
  32. ↑ CAMERON, T D J, CROSBY, A, BALSON, P S, JEFFREY, D H, LOTT, G K, BULAT, J, and HARRISON, D J. (1992). United Kingdom offshore regional report: the geology of the southern North Sea. HMSO for the British Geological Survey, London, 152p.
  33. ↑ WESTHEAD, R K, CAMPBELL, E, CARTER, G D O, DIAZ DOCE, D, GAFEIRA, J D L, GALES, J A, HOBBS, P R N, LONG, D, and MELLETT, C L. 2014. Geological constraints on development across the UK continental shelf: a study for the Crown Estate. British Geological Survey Commissioned Report, CR/14/050.
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