Due to the geometry of the side scan system it is not possible to accurately calculate the depth from water surface to target, for objects at a distance from the traverse line. Consequently, where depths to targets have been quoted they are taken from comparison with the bathymetric chart and as such should only be considered as estimates.
5.3. LOCH AINORT: SONOGRAPH ANALYSIS.
To preserve the side scan traces in near perfect condition, so as to enable any other interested workers to verify the statements made here, features from all the sonographs were traced onto transparencies and annotated. The original traces have been stored in an opaque sealed bag to prevent fading and loss of definition. The traverses taken from Loch Ainort have been broken down into a total of 66 individual fixes corresponding to Magellan readings (See Section 5.1.). These fixes will be continually referred to in the text (they in turn can be located on Figure 5.1a.) in order to accurately locate any of the features described. To facilitate analysis, the sonographs from each survey line were divided into sections covering one, two or three of these fixes. Such a division produced
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A3 size, or smaller, segments of trace from which to work. Analysis took the form of identifying individual positive and negative targets (inclusive of their shadow zones) on the transparencies, as well as plotting their location on isometric diagrams at a scale of
1:10,560. In addition, the traces were divided into zones of equal tonal quality following the relative tonal scale shown in Table 5.1.. This Table also shows the potential grain size equivalent for each tonal zone based on the scale created by Belderson et al. (1972). However, the descriptions given below will retain the tonal terminology with the genetic interpretation being presented in Section 8.2..
Tonal Colouration
Material Grain Size
Very Dark Bedrock Dark Dark/Medium Medium/Dark Gravels Medium Medium/Light Light/Medium Sands Light Light/Very Light i Very Light Silts/Clays
Table 5.1.: The categories of tonal colouration and the approximated corresponding material characteristics, employed during sonograph interpretation (Adapted from Belderson
etal., 1972).
In total 25 km of lines were shot in Loch Ainort, along five discrete survey lines (See Figure 5.1a.). Traverse 1 runs north-eastwards parallel to the south-eastern shoreline, starting from the head of the loch approximately 180 m offshore and finishing within the open waters at the mouth of the loch, 510 m west of Scalpay. Traverse 2 starts in these open waters, travels westwards across the mouth of the loch towards Maol Ban, and then turns sharply to the south-west to travel parallel to the north-western shore. Traverse 2 continues parallel to this shore but veers to 530 m offshore in order to avoid the cages of the Marine Harvest fish farm. This traverse finishes at the head of the loch. Traverse 3 follows a central route along the axis of Loch Ainort, turning due north 425 m from the coast of Scalpay to run parallel to its
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western shore. On reaching Rubha Reireag, Traverse 3 turns through 90° to travel due west across the unnamed strait between Scalpay and the mainland, finishing 100 m from Maol na Gainmich. The end point of Traverse 3 also represents the start point of Traverse 4 which travels due south across the mouth of Luib na Moil. Traverse 4 passes the point of Maol Ban, 80 m offshore, and then follows a south-easterly course across the mouth of Loch Ainort finishing in the prominent embayment south-west of Rubh' an Aird Dhuirche. Finally, Traverse 5 effectively repeats part of Traverse 1 but is in the opposite direction. It starts 425 m from the mouth of the loch and runs south-westwards parallel to the shore, finishing in the shallow waters at the head of the loch.
For the purposes of describing the side scan sonographs, Loch Ainort has been divided into five broad geographical areas (See Figure 5.15.) following the descriptive divisions presented in Section 4.3.. Due to the detail of the geomorphic features identified from the sonographs the “Inner Loch” area (See Section 4.3.1.1.) has been further sub-divided into two shore sections and an axial zone (See Sections 5.3.1. to 5.3.3.). However, due to the nature of many of the identified targets these sub-divisions do not represent static boundaries and many features are described in more than one section. The loch is characterised by a series of linear positive targets or ridges and point reflector clusters or boulder clusters. Therefore for ease of identification the former have been numbered and prefixed with AR it being an acronym for Ainort Ridge, whilst the latter are prefixed with BC an acronym for Boulder Cluster.
For reference the location of morphological features described in Sections 5.3.1. to 5.3.5. is given in both OSGB36 co-ordinates and in relation to the minute fixes recorded directly on the side scan sonographs.
5.3.1. NORTH-WESTERN SHORE.
Confirming the information obtained from the bathymetric charts (See Section 4.3.), the side scan sonographs show that the main body of the loch has an asymmetrical cross section, with its axis offset towards the north-western shore. The south-easterly facing slopes (average angles of 12°) of this shore act as good reflectors for the incoming acoustic pulse. As a result
Chapter 5, Side Scan Sonar
Figure 5.15. Geographical sub-divisions used for the interpretation of the side scan sonographs acquired from Loch Ainort.
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these slopes are reproduced on those sonographs, in close proximity to the shore (between fixes 2.6 [156710 829700] and 2.8[ 156480 829500]), as “medium” to “dark” tonal areas. The continued sharp drop off from these slopes towards the centre of the loch fails to provide strong reflectors to the south-east. As a result no bottom profile is recorded between fixes 2.10 [156080 828910] and 2.16 [154870 827880], the trace being dominated by an area of no reflection. In this region, the outer portions of the sonograph do show reflectors present to the south-east, in the form of uniform areas of “medium” tonal shades which grade into this “blank” zone. Consequently, the traces from the deepest parts of this north-western channel fail to provide further significant information on the nature of these slopes.
The side scan record between fixes 2.6 [156710 829700] and 2.10 [156080 828910] is close enough to the shore, and in sufficiently shallow water, to provide some detail of the outer section of the north-western margin of Loch Ainort. At the outer edge of the trace between fix 2.7[156660 829670] and 2.8 [156480 829500] three shore-parallel benches are
*
clearly visible at shallow depths. The central bench line represents the most prominent and continuous reflector of the three (See Figure 5.16.). The upper bench is also well defined but lacks continuity, whilst the lowermost bench is extremely disrupted and poorly represented (See Figure 5.16.). In addition, there is a fourth gentler break in slope running parallel to the shoreline between 120 and 130 m from the shore. This inclined surface extends for almost the entire length of the western margin of the loch. The offshore portion of this slope is characterised by clusters of weak “scallop” shaped features (See Figure 5.16.), with raised (3.7 m) frontal/lower segments.
A further 80 m offshore between fixes 2.8 [156480 829500] and 2.9 [156300 829310] a linear positive target (AR22) is identified (See Figure 5.16.). This ridge has a crestal orientation of 015°, and an axial length of 255 m. An associated topographical high is present on the bottom profile with a crestal relief of between 1 and 1.4 m. The ridge has a slightly asymmetrical profile with its easterly facing limb having a true d range of 0° - 16° compared to an average dip of 12° for the westerly facing limb (See Section 5.2.3.). The entire ridge is encompassed by a “very dark” homogeneous tonal area. These “very dark” tonal surroundings do, however, result in the partial masking of the ridge, making identification of what is a complex target pattern difficult. The ridge is interpreted as being a single continuous
Chapter 5. Side Scan Sonar
Figure 5.16. Sonograph showing shore parallel benches, weak “scallop” shaped features and