Hidden landscapes: the mapping of Ireland’s shelf geomorphology

From Hydro by Riccardo Arosio, Andrew Wheeler, Fabio Sacchetti, Aaron Lim

Mapping the seabed using high-res bathymetry data and semi-automated methods

The Marine Geoscience Research Group at University College Cork, under the aegis of the Irish Marine Institute, has published the first high-resolution geomorphological map of most of the Irish continental shelf: the Irish Shelf Seabed Geomorphology Map (ISSGM v2023).
This colossal mapping exercise took advantage of the vast INFOMAR multibeam echosounder dataset and used a protocol of semi-automated mapping techniques to accurately and rapidly extract seabed features.
The map is an important digital reference for policymakers, marine industries (e.g.
offshore renewables, fisheries and aquaculture) and future marine scientists.

For the past 25 years, Irish government-funded initiatives have carried out extensive seabed mapping campaigns, beginning with the Irish National Seabed Survey (INSS, 1999–2005) and continuing as the Integrated Mapping for the Sustainable Development of Ireland’s Marine Resource (INFOMAR) programme (2006–2026), funded by the Department of Environment, Climate and Communications.
To date, open source multibeam echosounder (MBES) data has been collected in ~91% of Ireland’s territorial waters (~880,000km2).
The INFOMAR datasets have served many purposes, such as UNCLOS claims, compliance with SOLAS regulations, shipwreck investigations and local spatial planning, to name a few.
But as the country increasingly looks towards the sea for its energy and resource needs, a holistic, accurate and detailed geomorphological map of the continental shelf, providing the users with easily accessible and understandable information about the nature and processes acting at the seabed, becomes indispensable.

To meet this challenge, the UCC group spent two years: (1) compiling and critically reviewing the scattered scientific knowledge of the geomorphology of the Irish continental shelf; (2) developing an effective and accurate mapping protocol to delineate and characterize landforms for a very large-scale dataset; (3) generating and adopting an internationally standardized classification system that can be easily compared to other maps; and (4) building an interactive GIS database to disseminate the map and scientific knowledge to stakeholders and the general public.
The final product is the Irish Shelf Seabed Geomorphology Map (ISSGM) version 2023 (Figure 1).

Figure 1: The Irish Shelf Seabed Geomorphological Map (ISSGM) version 2023 as it appears online, in Ireland’s Marine Atlas.

Manual or automated – a balancing act

Manually digitizing thousands of geomorphological features in approximately 110,000km2of up to 5m-resolution digital elevation model (DEM) bathymetry is not something that can be done even in a very long PhD project.
It should not be surprising then that the task of mapping the shelf geomorphology around Ireland made the employment of semi-automated techniques indispensable.
Machine learning (ML) is now commonly adopted in computer vision to rapidly classify images with outstanding results, identifying people or objects as trees, animals and so on.
ML techniques, as convolutional neural networks, have also been trialled on DEMs with the purpose of mapping geomorphology; however, the results are still unsatisfactory (Arosio et al., 2023a) and certainly insufficient for the level of detail that the UCC group and Marine Institute required.

Less sophisticated, but still very efficient, are the mathematical – or better geomorphometrical – operations on DEMs that allow a relatively rapid and consistent extraction of seabed features.
These methodologies cover the bulk of the work undertaken for this project, and they mostly belong to a type of technique called ‘residual relief’ separation.
The separation aims essentially to ‘peel’ away landforms of interest from the bathymetry surface using filtering techniques (Figure 2) similar to those used in image correction and noise removal.
More technically, the regional relief (i.e. the broadscale undulation of the terrain) is first approximated by a modified median filter using a circular focal neighbourhood tailored to the wavelength of the landforms of interest.
The filtered surface thus obtained is then subtracted from the original DEM to leave a ‘residual’ layer containing the landforms.
The residual relief raster can also be locally normalized to allow for amplitude variations in the features across the area.
In this way, provided the right filter thresholds are used, thousands of landforms can be rapidly delineated with minimal manual intervention.

In many places, the seabed is too complex for filtering to provide a satisfactory result.
MBES artefacts, palimpsests and heavily altered features hinder the identification of thresholds that can separate the ‘wheat from the chaff’, and in other cases the relevant features are so subtle or fragmented that it is impossible to isolate them using geomorphometry.
As a result, manual delineation or correction was, in many occasions, unavoidable.
Not only semantics: an international classification system

Maps of seabed geomorphology provide foundational information for a broad range of marine applications, and to be most effective, geomorphic characterization of the seabed requires standardized and interjurisdictional terminology that can be understood both regionally and internationally.
For this reason, the creation of the Irish map proceeded alongside an ongoing collaboration between geoscience agencies in the United Kingdom (BGS), Norway (NGU), Ireland (UCC and GSI) and Australia (GA).
The collaboration focused on developing a new standardized approach to meet this need, leading to the creation of the ‘MIM-GA two-steps classification system’ (Nanson et al., 2023).

Following this concept, seafloor geomorphology in the ISSGM is considered in two parts: (1) the shape (or morphology) of the seafloor and (2) the geomorphology of those shapes.
The first classification covers basic morphological definitions that describe seabed features by their shape (e.g. ridges, mounds, depressions, etc.).
Seabed morphology is used as the baseline for benthic habitat mapping and monitoring, linking seabed morphological classes with substrate composition to benthic and pelagic species distribution, and is an essential asset for marine spatial planning.
The second classification covers morphogenetic definitions that provide a geological interpretation of the features identified (e.g. drumlins, coral mounds, pockmarks, etc.).
Alternative interpretations of seafloor geomorphology can have considerable impacts on marine industries.
It is very important then to separate geometric classification of seafloor morphology from subsurface interpretations that can involve significantly more uncertainty.

The MIM-GA classification has attracted the attention of many institutions around the world and the GEBCO Sub-Committee on Undersea Feature Names (SCUFN).
More information can be found in Nanson et al. (2023).

Figure 2: An example of bedrock outcrops (in colour) ‘filtered out’ of a DEM (black and white bottom layer).

 

A map and a database

In its complete form, the ISSGM (v2023) includes 35 different landform units and four substrate types generally mapped at 20m/pixel resolution, with a few exceptions where a higher resolution (10m or 5m) was utilized if geological interest counterbalanced time, effort and hindrance caused by artefacts.
Together with the contents of the paper published with the ISSGM (Arosio et al., 2023b), the map illustrates the variety of landforms and processes active on the Irish continental shelf, including new and all previously mapped units.
Additionally, this work has revealed places where ground-truthing is lacking or completely absent but of potential geological interest, and identified landforms whose interpretation remains ambiguous, indicating avenues for potential future work.

Old questions and new mysteries

A description of all the findings would be too long for this article, but a quick ‘tour’ can give an idea of the breadth of information contained in the map.
The shelf can be nominally separated into four regions which, while they share a common shallow marine nature, show unique traits and geomorphological characteristics.
Starting from the north-west Irish shelf, we find a diverse association of glacial landforms including large recessional moraines (Figure 3A), iceberg plough marks and drumlins.
This is the Irish seabed region where glacigenic forms have been best and most extensively preserved, permitting detailed studies on the extent and retreat rate of the last ice sheet in the area.
The western Irish Sea seabed shows the most complex geomorphology, with at least half the area of the central and southern section of the region covered by large units and fields of dunes, mainly transverse and trochoidal (Figure 3B).
These unusually high and still puzzling trochoidal dunes are the features that have received most attention in past studies.
They are associated with linear or channel-like depressions, from which they obtain abundant scoured mobile sediment that permits their subsistence.
In the Celtic Sea, the most striking morphology is the palaeochannels, a dense network of buried to semi-buried fluvioglacial features that may be linked to the demise of the great ice sheet at the end of the last Ice Age (Figure 3C).
Further investigations in the shallow seismic stratigraphy are required to confirm the interpretations, which would improve the understanding of the distribution and structural control of Pleistocene palaeodrainage in the region.
Reaching finally the southwest, about 30% of the mapped seabed is covered either by bedrock outcrops or by bedrock only thinly covered by superficial sediment, making it the rockiest seabed region around the coast of Ireland.
The most extensive outcrop is the Waulsortian platform offshore north Kerry, whose massive limestone beds appear to be affected by relict karstic processes.
However, the most stunning bedrock structures crop out south of the mouth of the Shannon and north of Loop Head (Figure 3D).

Figure 3: Examples of landforms from the four nominal regions on the Irish shelf.
A) Recessional moraines forming a retreating pattern offshore Donegal.
B) Trochoidal dunes in the Irish Sea.
C) Sinuous palaeochannels winding on bedrock.
D) ‘Eye-shaped’ fold structures in bedrock offshore Loop Head.
Conclusion

As Ireland enters a new era of development for blue growth, offshore renewable energy and climate action, the Irish Shelf Seabed Geomorphological Map (ISSGM) version 2023 will hopefully be instrumental to many, providing a lucid and complete geomorphological database including a systematic review of previous research findings, easily accessible and ready to use.
The ISSGM is available online on the Irish Marine Atlas (https://atlas.marine.ie) under the Geology Theme.