EGM310 Rory Quinn

! [1]

Introduction to EGM310 URS practical sessions – weeks 5 to 8

The practical sessions in weeks 5 to 8 are designed to provide you with the skills necessary to complete the associated URS assignment. These practical classes draw on the skills you developed in weeks 1 to 4.

The practical sessions in weeks 5 and 6 are designed to equip you with the skills and confidence to interpret and render bathymetric and backscatter data derived from multi-beam echo-sounder surveys. During these sessions, you will also build the skeleton of the ArcGIS project you will use as the basis for the assignment.

The practical session in week 7 is designed to complete the ArcGIS project by adding databases and developing the rules by which you will select and digitize a cable route. The week 8 session is a ‘clinic’ in which you are free to discuss any issues you have with the processing and interpretation of the data prior to completing your assignment, due on Thursday week 9 at 4PM.

EGM310: URS Practical 1 – interpreting and rendering high-resolution bathymetric data

Aim of practical

To gain confidence in interpreting high-resolution bathymetric data derived from multi-beam echo-sounder surveys and to start building your ArcGIS project for the underwater remote sensing assignment.

Learning outcomes

By the end of this practical you should:

• have developed the basic skills to interpret high-resolution bathymetric data for geological, geomorphological and archaeological applications;

• know how to generate shaded relief bathymetric models.

The JIBS data

To address the need for high-resolution bathymetric data off the north coast of Ireland, the Joint Irish Bathymetric Survey (JIBS) was instigated as a partnership between the Maritime and Coastguard Agency (MCA) and the Marine Institute (MI), funded under the INTERREG IIIA Programme (€2,133,508).

The JIBS project commenced on 10 April 2007 and was completed in September 2008, providing:

• full-coverage multi-beam bathymetry data within the 3 nautical mile coastal strip from Fanad Head (Co. Donegal) to Rathlin Island (Co. Antrim), and

• ground-truthed, geo-coded backscatter data for the same area.

You will use these bathymetric and backscatter data for the URS assignment.

EGM310 Rory Quinn

! [2]

EXERCISE 1: Interpreting high-resolution bathymetric data

The interpretation of MBES bathymetric data can be approached from many different perspectives. In this practical session, we will concentrate on the interpretation of these data from geological, geomorphological, archaeological and anthropogenic perspectives. Therefore, the interpretative skills you learn in this practical are directly applicable to the assignment. Work through each section of this in turn, recording your answers for future reference. Ensure that you can satisfactorily answer each question before moving on to the next example. If you have any queries or do not fully understand any of the terminology or concepts involved, please address questions to myself or to one of the postgrad demonstrators.

Geological interpretation

The geological interpretation of MBES bathymetric data concentrates on the recognition, mapping and interpretation of bedrock geology. Bedrock (solid hard rock) is usually easily differentiated from unconsolidated (soft loose) sediment by obvious textural differences. In engineering scenarios (e.g. pipeline or cable installations on the seafloor), it is best to avoid bedrock as the engineering works needed to bury cable in rock is logistically more complicated and time-consuming and therefore more expensive. Therefore, an ability to differentiate bedrock from sediment is important.

1. In this example of MBES bathymetric data, can you differentiate bedrock geology from unconsolidated sediments? Describe the textural differences between the bedrock geology and the unconsolidated sediments in words.

2. This is the same portion of the seabed imaged above, but this time the data is viewed in 3-dimensions. Examine the image and note the additional ease in differentiating bedrock from sediment by imaging the clear break in slope at the bedrock-sediment contact.!!

EGM310 Rory Quinn

! [3]

3. Tectonic features are caused by deformation of the Earth’s crust. Deformation describes processes such as faulting, shearing, folding, and compression or extension between rocks. The signature of this tectonic deformation in rocks can be mapped using MBES data. In the example: [a] Identify and signatures you think might be linked to tectonic activity. [b] Given that these data are orientated so that north is 'up', in what orientation are the strongest tectonic fabrics? [c] Can you think of any dominant structural trend in Ireland and the UK that these tectonic fabrics are aligned to? Hint: look at a map of Ireland and the UK on Google maps – do you see any trend in the landscape that mimics this trend?!!

4. The MBES image below is viewed from offshore the Giant's Causeway on the Antrim coast. Can you identify any evidence for lava flows in these data that

EGM310 Rory Quinn

! [4]

might pre-date (i.e. be older than) the Giant's Causeway? If you are unsure, it might help to look at an aerial photo of the Giant’s Causeway to prompt you…!

5. This MBES data example shows a type of volcanic feature termed ‘pillow lavas’ associated with rapid cooling of lava as it is extruded on to the surface of the earth.

After working through these five examples, you should now be able to differentiate between bedrock and soft sediment on MBES data – this will help you a lot in the completion of your assignment.

EGM310 Rory Quinn

! [5]

Geomorphological interpretation

Sediments are routinely and extensively reworked and redistributed, especially in shallow water environments, by tidal and wave energy (Blondell and Murton, 1997). The geomorphological interpretation of MBES bathymetric data concentrates on the study of sedimentary bedforms and the processes that shape them. Marine geomorphological mapping is most commonly directed towards the mapping of sedimentary signatures such as ripples, megaripples, sheets etc. formed in response to hydrodynamic processes. In engineering scenarios (e.g. pipeline or cable installations on the seafloor), it is best to avoid areas of the seafloor where bedform signatures indicate the seabed is mobile. Mobile beds can cause damage to engineering installations and hence are avoided where possible.

The orientation, dimensions and shape of bedforms can be used to infer local energy conditions, giving a proxy for the strength of the local hydrodynamic regime. One such classification scheme is given by Belderson et al. (1982), demonstrating the succession of bedforms encountered on continental shelves, made by tidal currents, with the corresponding mean spring peak near-surface tidal currents (Figure 1). This classification assumes that the dominant grain size for the material falls within the sand class. Sediments on the shelf off the north coast of Ireland comprise sand with minor amounts of gravel (Cooper et al., 2002) and, as such, comparison of the bedforms observed on the multibeam data with the block diagrams of Belderson et al. (1982) can give a first estimation of the flow regime. The only areas for which the flow regime cannot be determined from multibeam data alone are areas where the seabed is entirely flat, as this could indicate a very low tidal flow, too low to transport sediment, or a very high flow, removing all sediment and creating an erosional surface.

Using the Belderson et al. (1982) classification scheme (Figure 1), classify the following four examples of bedforms from the JIBS MBES data under the headings:

• Energy conditions (low, medium or high), • Mean spring near surface tidal current (cms-1), and • Bedform classification (e.g. small sand wave, large sand wave, sand

ribbon etc.).

Note, that for some examples, multiple bedform types may be present. Please classify all types in each example.

EGM310 Rory Quinn

! [6]

Figure 1: Block diagram of the main flow regime bedforms made by tidal currents on the continental shelf, with the corresponding mean spring peak near-surface tidal currents in cms-1 (modified from Belderson et al., 1982).

EGM310 Rory Quinn

! [7]

EGM310 Rory Quinn

! [8]

Archaeological interpretation

Interpretation of sonar data from an archaeological perspective tends to concentrate on the recognition and identification of sonar 'anomalies', i.e. on appreciable differences between a constant or smoothly varying background and a strong or 'anomalous' sonar signature.

In many reconnaissance mapping surveys (for example the JIBS survey), the surveyor often identifies many anomalies. Sometimes, larger object such as shipwreck are easy to identify, whilst smaller objects are difficult to interpret. Also, some large objects produce relatively small anomalies if a large portion of the structure is buried. Therefore, it helps to have a basis for assigning probability levels to targets:

• Natural targets tend to possess irregular shapes ('there are no straight lines in nature'). Shadows associated with natural features often demonstrate poor structure.

• Conversely, man-made objects are often regular or angular and have sharp shadows. The interpreter must be careful however, as often man-made object can be buried (or semi-buried) and therefore the associated anomaly may be masked by the overlying sediment. This is particularly true in the case of wreck-mounds.

• The context of an anomaly is important. Anomalies sometimes seem out of place in context with the surrounding seafloor. For example, a small angular anomaly on a planar sand seafloor may be deemed of higher interest than a small angular anomaly on a rock-strewn platform. Again, caution must be taken in interpretation. It is not always correct to discount the angular anomaly on the rock-strewn platform. Experience (and luck) come into play here.

1. The SS Lochgarry foundered off the east coast of Rathlin Island (within the study area for your assignment) in 1942, with the loss of 23 lives. On the basis of

EGM310 Rory Quinn

! [9]

the discussion above, can you identify the wreck site of SS Lochgarry in these MBES data?

It may help to zoom in.

2. Scour occurs at the seafloor when sediment is eroded from an area in response to forcing by waves and currents. Commonly, scour is initiated by either the migration or change in morphology of bedforms or by the intentional (e.g. coastal engineering) or accidental (e.g. shipwreck) introduction of an object to the seafloor. Marine structures (intentional or accidental) are vulnerable to erosion due to scouring by waves and tidal currents, and scour processes can ultimately lead to the complete failure and collapse of structures on the seafloor. Scour signatures are widely reported in marine sciences, and their development and importance in short- and long-term site evolution in coastal engineering and seabed development, glacial and geomorphological research, mine burial and detection, biology and archaeology are commonly described in the scientific literature (Quinn, 2006). Hint: You should think about this in relation to your module assignment.

Examine the image of SS Richard Montgomery below and identify the scour features formed at the site in response to tidal flow. SS Richard Montgomery was an American Liberty ship built during World War II, one of the 2,710 used to carry cargo during the war. She was wrecked off the Nore in the Thames Estuary in 1944 with around 1,500 tons of explosives on board, which continue to be a hazard to the area.

EGM310 Rory Quinn

! [10]

3. Browse the images on the ADUS web-site (http://www.adus-uk.com/wreck-images) to see some very high resolution sonar images of wreck sites taken with a dual-head RESON MBES.

Interpretation of anthropgenic features

EGM310 Rory Quinn

! [11]

[a] Can you interpret evidence of anthropogenic (human) influence in this image taken from the Juniper Hill area between Portstewart and Portrush? [b] If so, what makes the anthropogenic signature stand out from the background natural signature in these data?

You should have now developed some basic skills and confidence that allows you to interpret the bathymetric data for your assignment.

References

Belderson, R.H., Johnson, M.A. and Kenyon, N.H., 1982. Bedforms. In: A.H. Stride (Editor), Offshore tidal sands. Chapman and Hall, (London, New York), pp. 27-57.! Blondel, and Murton, B.J., 1997, Handbook of Seafloor Sonar Imagery, PRAXIS-Wiley & Sons, 314 pp.

Quinn, R., 2006, The role of scour in shipwreck site formation processes and the preservation of wreck-associated scour signatures in the sedimentary record. Journal of Archaeological Science, 33 (10): 1419-1432.

EGM310 Rory Quinn

! [12]

EXERCISE 2: Downloading and extracting the data files for the assignment

The data for the URS assignment is uploaded to Blackboard as a zip file, named urs_assignment_data.zip.

1. Create a new folder on your external hard drive (HD) where you will store the data for the practical classes and assignment. I suggest you name this folder with a name that makes sense – e.g. egm310_urs indicating the module code and module the component.

2. Download the file urs_assignment_data.zip to the newly created folder on your external hard drive (HD).

3. Extract the contents of the zip file to the folder. You should have 15 individual files in the folder. The files beginning with the word bathymetry are the JIBS bathymetric data, the files beginning with the words NI_bs are the JIBS backscatter data and the files beginning with UKHO list the known shipwrecks in the study area, derived from the UK Hydrographic Office database.

You now have all of the data required to complete the assignment over the next four weeks. To complete the project, you will need to derive a series of additional products from these datasets.

Over the next four weeks, you will use these data in the practical sessions and to complete the assignment. You therefore need to bring the HD with you to all sessions.

EGM310 Rory Quinn

! [13]

EXERCISE 3: Loading and rendering the MBES bathymetric data for the URS assignment

1. Start ArcMap. If an initial dialog box appears, choose to start using ArcMap with a new empty map.

2. Before you do anything else, save this empty map space as an MXD file. It is important that you save this file to the same folder on the HD that you created in Exercise 2. This MXD file will become the basis for your assignment. Give it a sensible filename such as URS_EGM310.mxd - a filename that makes sense when you glance at it.

3. Now add the three files bathymetry_1m_NI_Clip.img, NI_BS_mosaic_all_v1_Clip.img and UKHO_2009.shp to your project by selecting the Add Data icon on the menu bar. The layer bathymetry_1m_NI_Clip.img is 1m-resolution MBES bathymetry extracted from the JIBS survey between the north coast of Ireland and Rathlin Island. The layer NI_BS_mosaic_all_v1_Clip.img is the corresponding backscatter data at 1m resolution and the UKHO_2009.shp layer is the shipwreck database of know shipwrecks in the area as identified by the UK Hydrographic Office. One of the limitations of this database is that it does not contain information on wrecks that pre-date the 17th century.

4. For the remainder of this practical session, we will concentrate on the bathymetric and shipwreck data. Therefore, turn off the backscatter layer by deselecting it in the Table of Contents frame.

5. By default the raster layer representing the bathymetry appear in grey scale. To aid interpretation of these data, you will now change the colour scale of the bathymetric data. Right click bathymetry_1m_NI_Clip > Properties > Symbology. At the left side, click on stretched. On the right side change the Colour Ramp to a rainbow colour-scale (cold colours representing deep values, warm colours representing shallow values). Can you see how using colour palettes to render the bathymetry can aid interpretation?

6. We will now artificially illuminate the offshore landscape (i.e. create a virtual sun) to further aid interpretation. To illuminate the bathymetry, go to ArcToolbox (red toolbox icon in the menu at the top of the screen). Select Spatial Analyst Tools > Surface > Hillshade. When the dialogue box opens, choose bathymetry_1m_NI_Clip.img as the input raster and leave the Azimuth and Altitude at the default values. An azimuth value of 315 with an altitude value of 45 means you are going to illuminate the offshore landscape with a sun placed at 315° (in the northwest) at an angle of 45° off the horizon. Select OK – it will take a few minutes to generate the hillshade surface. Once created, it should look like the screen capture below.

EGM310 Rory Quinn

! [14]

7. Now, we want to combine both of these renderings of the offshore landscape into one final rendering of the offshore landscape. To do this, we put the hillshade surface behind the colour-coded bathymetry and make the colour-coded bathymetry semi-transparent to allow the illumination to show through. First of all, drag the illumination layer below the colour bathymetry layer in the Table of Contents. Now double-click the colour-coded bathymetry layer, go to the Display tab and change Transparency to 40%. The map should now look something like this:

8. You should now have an artificially-illuminated digital elevation model (DEM) representing offshore bathymetry for your assignment. This map should become the main decision-making tool for your cable route.

9. Now, we are going to look at the shipwreck database and use it to try and identify shipwreck sites from the MBES data. The first step is to activate the shipwreck database UKHO_2009.shp in the Table of Contents if it is not already active. You can change the symbols used to plot these data if you wish by double clicking the layer and changing the symbol type, size and colour.

10. Now, get familiar with these data – the only way to do this is to engage with the data. Start by answering these basic questions:

• Where are the deepest areas in your study area? Where are the shallow areas?

• Using the UKHO data, zoom in to specific shipwreck sites and see if you can image and identify these sites in the MBES data. Is there good correlation between the UKHO database and the features imaged in the MBES data?

• Can you identify sedimentary bedforms in the MBES data? If so, can you classify them according to the Belderson scheme used in Exercise 1?