Bedrock target analysis for ROV rockdrill sampling and existing sample stratigraphic and mineralogical verification (BeTar_drill)
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Date
2021-03
Authors
Strachan, Ruaihri
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University College Cork
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Abstract
The dynamic seascapes found in continental margin settings generally host many vertical structures, such as fault escarpments and submarine canyon systems. This terrain is exemplified on the Irish Atlantic Margin (IAM) by the Porcupine Bank Canyon (PBC). Bedrock has been found here in the past, however, surveying in these areas continues to be a difficult task due to issues regarding access and sampling conditions. As a result, a lack of both knowledge and detail generally exists for offshore outcrop, which is in stark contrast to typical terrestrial geological data. Modern advances in marine robotics and seafloor imaging techniques has allowed for renewed offshore scientific expeditions in deep marine settings, being guided by high-resolution topographical data (gathered through multibeam echosounding) and Remotely Operated Vehicles (ROVs). Here, updated high-resolution bathymetric data, coupled with bedrock samples that have been drilled in-situ and a series of video transects spread out across the PBC have been used to develop methods for offshore geological ‘fieldwork’, through marine bedrock mapping and sample characterisation, whilst also using footage to recreate the seafloor through Structur-from-Motion (SfM) photogrammetry, in order to produce accurately scaled, lifelike, geological outcrop models.
The steps presented within this thesis with regards to bedrock mapping tie in with marine geomorphometry and habitat suitability modelling techniques. This attempts to quantitatively describe the surface of the seafloor, and identify key terrain attributes for which bedrock has previously been encountered and proven to occur. Further terrain data can then be reclassified based on the interquartile ranges (IQRs) for these characteristic sections, identifying potential occurrence of outcrop in non-surveyed areas. This study applies these methods across more than 2000 km2 of canyon terrain,
and have yielded a 60% accuracy for bedrock predictions. These geological maps, coupled with high-definition video footage and physical rock samples, have allowed for a reappraisal of the bedrock encountered within the PBC. Bedrock samples indicate an underlying basement rock of lower greenschist facies metamorphosed igneous rocks.
17 detailed bedrock models have been constructed through the ROV footage also, and through this process, a robust method for applying SfM photogrammetry to offshore deep marine video imagery has been outlined. This has been successful across different surveys which were conducted in the PBC in recent years. The outputs from these models can further be used to record structural detail on bedrock exposed in these complex terrains, aid in distinguishing between lithologies, and also aid in planning future sampling opportunities (through rotary drilling or similar systems).
The methods outlined herein aim to enhance future geological studies in the Irish offshore, whilst also providing a potential tool for ecological studies which aim to target bedrock surrogates for investigation. Through large scale mapping programs, both in Ireland and abroad, public and institutional, a massive opportunity exists to build on from this PBC ‘proof of concept’ study, bridging the void which generally persists when it comes to knowledge on deep marine environments.
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Keywords
Marine geological mapping , Submarine canyon , Bathymetry , Spatial analysis , Continental margin , Bedrock drilling , Photogrammetry
Citation
Strachan, R. 2021. Bedrock target analysis for ROV rockdrill sampling and existing sample stratigraphic and mineralogical verification (BeTar_drill). MRes Thesis, University College Cork.