Evaluación de Resultados

Sidescan sonar surveys of Lochs Lee, Callater and Muick were undertaken using a Klein Hydroscan E401, high resolution, short range, system operating at 400kHz. This comprises a towed transducer fish, electromechanical tow cable and split trace graphic recorder, producing a sepia coloured sonograph on wet, electrosensitive paper. Survey

speeds were maintained as near constant as possible to 3 knots (1.5ms-1). Due to adverse weather conditions at the time of the Loch Lee and Loch Muick surveys and logistical restrictions on Loch Callater, dead reckoning was the only possible means of position fixing. Event marks were made on the sonographs, and noted on the survey record, when the towed fish passed landmarks on the shores and subaqueous topographical features recognised on the new bathymetrical charts.

The sidescan sonar surveys aimed to:-

i) Identify topographic and surface sediment type variations within the three water bodies

ii) Complement echosounder (Chapter 2), sub bottom profiling (Chapter 3), coring (Chapter 6 ) and surface sediment (Chapter 5) data.

4.4 LOCH LEE

4.4.1 Sidescan sonar survey

Loch Lee was surveyed during September 1989. A total of four circuits were made (Figure 4.1), covering a total traverse length of approximately 18km.

4.4.2 Sonograph description and interpretation

To enable easy location of the features recognised, descriptions of the sonographs are presented using the zones identified in Chapter 2 (Figure 2.3). A non-genetic interpretation of the sonographs in each zone is given with the description of the sonographs. A genetic interpretation is given in Chapter 7).

4.4.2.1 Zone 1 Southwestern Shallows

The floor of the southwestern shallows is characterised by low backscatter levels interrupted by large, dark reflectors (Plate 4.2). These reflectors show a distinct linear distribution along the boundary between Zones 1 and 2. The absence of an acoustic

shadow behind the reflectors is thought to be due to the lack of contrast possible with the low backscatter level of the surrounding loch floor. It is believed that this sonograph configuration represents aquatic plants growing in fine sands or silts. During earlier surveys (see Chapter 2, section 2.4.2.1) the existence of weed growth in this area was observed.

4.4.2.2 Zone 2 Central Trough

The nearshore slopes of Zone 2 are characterised by alternate dark and light shore- parallel banding extending a maximum of approximately 45m from the shore (Plate 4.3). This configuration is interpreted as shore-parallel terraces, the dark bands representing the highly reflective back ('riser') of the terrace and the lower intensity areas the flat ('tread'). A maximum of five terraces are identified. High intensity point reflectors (boulders) are recognised lying on the terrace flats, particularly on the most shore proximal terraces. Occasionally acoustic shadow zones occur behind the boulders, suggesting that they protrude a significant distance above the bed. However, this is an unusual occurrence in Loch Lee, therefore it is inferred that most boulders have either a slab-like form, or are partially buried in finer grained sediment. The extremely steep slope angles close to the mouths of the streams on the northern slopes are represented by a very high reflection amplitude (Plate 4.4).

Immediately below the shoreline terraces an area of very low, uniform backscatter occurs (see for example Plate 4.3) at this position throughout Loch Lee. This area frequently mns parallel to the base of the terraces extending up to 20m further into the loch, but on the northern slopes a more irregular distribution occurs (Plate 4.5). Lobes of this low backscatter reflector extend from a promontory into areas of medium backscatter. Very occasionally high intensity boulder reflectors are recognised within this low intensity area confirming that this is not an acoustic shadow zone. It is suggested that this acoustic signature represents fine grained sediment such as silt or mud. This hypothesis is accepted on the basis of evidence from sediment sampling (section 5.3.3).

The remainder of the basin within Zone 2 is composed of a predominantly uniform, medium intensity backscatter reflector. The sediment in this area is fine grained; sediment sampling confirms that the loch floor in this area is covered by mud, sandy mud, silt and sandy silt. An exception to this pattern occurs close to the mouth of the Burn of Duchrey on the northern slopes of Zone 2 (Plate 4.4). Approximately 60m from the loch shore there occurs an area of mottled darker reflectors preceding a lighter area. The dark area is composed of two distinct types of reflection targets. First, those with acoustic shadow zones; this type has a lateral spread of 20m downslope, and secondly irregularly shaped areas of slightly lower reflectivity. The lighter area is approximately 10-15m wide and contains no tonal variations. Thus this dark-light area of sonograph is interpreted as being a slope facing the sonar transducer, composed of boulders with intervening hollows, casting an acoustic shadow onto the slope behind.

Hollows have also been identified in a broad swathe 50-60m wide across the floor of the loch in Zone 2. They are first recognised close to the mouth of the unnamed stream (Figure 4.1) and extend to the northeast over a distance of approximately 600m. High reflectivity boulder signatures occur widely scattered around the base of the shore-parallel terraces. At one location on the southern slopes (Plate 4.6), the high density signature characteristic of a boulder can be recognised below the shore-parallel terrace sequence with a linear, low reflection intensity area running upslope behind it. It is suggested that the low intensity area is a linear topographic low.

4.4.2.3 Zone 3 Northeastern Shallows

Zone 3 is characterised by a much 'rougher' acoustic texture. Shore-parallel terraces extend a maximum of 75m from the shore on both sides of the loch (Plate 4.7). The flats of the terraces are covered by numerous boulders with associated acoustic shadows. The maximum width of the terraces occurs where promontories extend into the loch. The loch floor below the terraces shows a range in backscatter between low and medium levels. Medium backscatter areas, probably composed of fine sand or silt, are located adjoining the

promontories and have a scattering of boulders over the surface. Within the medium backscatter area adjoining the most northwesterly promontory (Plate 4.7), high intensity reflectors face the transducer with a distinct area of acoustic shadow lying behind them. This is interpreted as representing part of the bedrock spur which forms the promontory, exposed through finer grained sediments as a rocky outcrop.

Low backscatter levels occur throughout the remainder of Zone 3, between the promontories and over the central part of the loch floor. This acoustic texture is believed to represent fine silts or muds. Where this texture lies adjacent to the shore-parallel terraces and medium backscatter areas, intermittently distributed boulders occur. On the southern side of the loch the backscatter increases to an intermediate, medium to low level (Plate 4.8) and a distinct mound, marked by a uniform high reflectivity, rises above the loch floor. This mound, also apparent on the bathymetric chart (Figure 2.2), is not composed of individually distinguishable boulders, and does not cast an identifiable acoustic shadow. An interpretation of the genetic processes of this and all other features is given in Chapter 7.

4.5 LOCH CALLATER