Conclusion on grain size distribution based on back- back-scatter measurementsback-scatter measurements

For a probable conclusion on sediment type distribution, the local backscatter intensities were inspected more closely. The angular responses could not be used due to the influence of seafloor topography on the angular responses.

Examining morphologically relatively flat areas of the dataset, it can be recog-nized that local variations in backscatter strength are present. To link back-scatter strengths to grain sizes, the sediment sampling locations are examined

6.3. Influences on backscatter responses more closely. They are situated on mounds or valleys, and their backscatter re-sponse is comparatively low to their surrounding (Fig. 6.7).

Figure 6.7: Perspective view of sediment sampling locations in Fledermaus (VE = 6). The line of sight is oriented towards north for all images.

The backscatter values at these stations differ slightly (Tab. 6.1). Station SO213-14 shows the highest backscatter response (-27 dB), which is probably caused by the nodule abundance. A nodule-bearing seafloor results in higher seafloor rough-ness and therefore yields a higher backscatter response than sandy or silty seabed (Scanlon et al., 1992). At the other two sites calcareous ooze was determined as predominant sediment with a grain size in the order of fine sand to coarse silt.

Such finer sediment has a lower roughness and the acoustic impulse penetrates deeper into the seabed where it is stronger attenuated. Therefore, the acquired backscatter values are lower (-33 dB and -34 dB) than at SO213-14, which cor-responds to the general assumption that coarser sediments show stronger back-scatter intensities.

Station Sediment type Backscatter strength

SO213-14 gravel/clay -27 dB

SO213-15 very fine sand/coarse silt -33 dB

SO213-17 fine sand/very fine sand -34 dB

Table 6.1: Backscatter strengths of sediment sampling locations.

In Fig. 6.8 the Parasound echograms at the sampling locations are depicted.

During the acquisition of these echograms, the ship’s speed was reduced until the sampling location was reached. Therefore the mayor part of the data shows the ensonification of one particular seafloor sector.

6.3. Influences on backscatter responses

The echogram of SO213-14 shows the strongest reflection and smallest seabed penetration of the signal in comparison to the other locations. This corresponds to the observation of highest backscatter response compared to the other stations and can be explained by the presence of manganese nodules. The hardest reflector is not found at the surface, which may be due to side effects. Side effects occur if the horizontal resolution of the Parasound system is lower than the dimension of seafloor features. As a result, a larger number of seafloor features are ensonified simultaneously and their responses are overlain in the echogram. This effect can also be observed above the surface of SO213-14, where a weak and wavy sediment layer of a neighboring topographic elevation can be distinguished.

Figure 6.8: Parasound echograms of sediment sampling locations. Refer to App. B, Fig. B.3 for a larger representation.

At SO213-15 and SO213-17 the penetration of the signal into the seafloor is greater (about 20 m) than at SO213-14 (about 10 m). Site SO213-17 shows stratified sediment layers, whereas at SO213-15 the different layers are not recog-nizable and side effects can be observed. SO213-15 shows a hard surface reflector, whereas the upper sediment layer at SO213-17 implicates a greater water content

6.3. Influences on backscatter responses due to its transparent appearance in the echogram. A higher water content in the seabed surface results in a lower impedance contrast at the water-sediment boundary and therefore a weaker backscattering signal. This higher water con-tent may be the reason why the backscatter value at SO213-17 is lower than at SO213-15 even though the grain size is larger.

Parasound data was collected during backscatter and bathymetric data acquisi-tion along the profiles. However, the data could not be used for further informa-tion on sediment distribuinforma-tion throughout the area due to side effects caused by strong topography variations. The sub-bottom profiler has a larger beam angle (4^◦) than the EM 120 (2^◦) and therefore a lower horizontal resolution. This way, adjacent seafloor features influence the collected data which is visible as side ef-fect in the echogram like pointed out before.

The remote detection and investigation of manganese nodules by sonar systems is of large interesting for a future resource exploitation. Yet, problems in de-tection arise as manganese nodules vary strongly in sizes and seafloor coverage and therefore make a prediction of their general backscatter response difficult.

Nodule-bearing seafloor generally yields a higher backscatter response (Scanlon et al.,1992). Different backscatter surveys with towed sidescan sonars with vari-ous frequencies discovered that it is possible to estimate the percentage of seafloor covered by nodules when a frequency of 30 kHz and higher is used. Conclusions on the nodule size are possible using a frequency of 9 kHz and to lesser extent at 15 kHz (Weydert, 1990). The mapping of nodules is crucial at low frequencies, as the visibility of nodules depends on the contrast to the surrounding sediment and therefore on the frequency. As the wavelength of the EM 120 (about 16 cm) is larger than the size of the nodules (few centimeters), they should not be visible in the backscatter data (Mitchell, 1993). However, the nodules in the investiga-tion area show a large density and are therefore combined into larger acoustic targets which also can be detected in low-frequency data (Chakraborty et al., 2004; Scanlon et al., 1992).

The obtained values for nodule-bearing seabed (-27 dB) and silt-to-sandy se-diments (-33 dB) can be applied throughout the survey area (for flat seafloor topography) to determine the boundary of nodule abundance. When inspecting the linear depressions in the folded part in the north of the dataset (area A), they show similar backscatter values (ld1: -26 dB, ld2: -27 dB, ld3: -25 dB) as at SO213-14. Therefore a similar density of manganese nodule as at SO213-17 can be assumed at these locations. Furthermore, as the backscatter values generally do not decrease below -30 dB in area A, it can be concluded that manganese nodules spread all over this part. This assumption is further strengthened by the presence of large water depths (below 4,000 m) which are greater than the assumed CCD. The only exception regarding the higher backscatter level is the fault (f1), which is displayed in the angle-invariant data with a value of -31 dB for its deeper part in the east. This backscatter response is considerably lower than the average of that part, but higher than at SO213-15 and SO214-17. A possible explanation for this local decrease in intensity could be that the man-ganese abundance is lower in this fault than in the other part of that area.

6.3. Influences on backscatter responses

Figure 6.9: Mosaic generated in FMGT showing angle-invariant data with a different color palette than gray scale to enhance backscatter strength variations.

The color palette ranges from dark red (high backscatter strengths) to light blue (low backscatter strengths). The sediment sampling locations are marked by white circles. (Mercator projection, standard parallel: 39^◦ S) [App. A, Fig.

A.15]

Area B is characterized by average backscatter values between -35 dB and -29 dB.

Higher intensities can be correlated to topographic variations, which is further analyzed in Sec. 6.3.2. When investigating the distribution of low backscatter strength throughout that region, it can be observed that the intensity of the backscattered signal decreases towards the peak of the Guafo Ridge (Fig. 6.9).

This would initially indicate a decrease in grain size towards the south (De Falco et al., 2010). This assumption does not correlate with the measured grain sizes at the ground-truthing locations SO213-15 and SO213-17. Different explanations are possible for this contradiction: First, the measured grain sizes do not rep-resent the overall sediment distribution (i.e., the coarser grain size at SO213-17 is just a local variation). Second, it could be possible that the seabed in the south has a greater water content in the upper sediment layer and therefore lower backscatter intensities. As the second possibility corresponds with the observa-tion made in conjuncobserva-tion with the Parasound data, this seems to be the more presumable explanation.

6.3. Influences on backscatter responses

The locally increased water content in the upper sediment layer might be ex-plained by currents. In the area of investigation the currents generally flow from the south towards the north with a distraction by the Coriolis force in western direction. A southern seafloor current might be diffracted by the morphological elevation of the Guafo Ridge. Parts of the current could ascend with the Guafo Ridge and result in turbulences on the northern side of the ridge. The current’s speed is presumably correlated to the distance to the ridge. As the current de-creases in speed, carried sediment particles are accumulated. This might explain the concluded sediment distribution as coarser material is precipitated further south and finer material in the north in conjunction with the decrease in the current’s speed.

Area C is characterized by high backscatter responses (about -20 dB). The pres-ence of manganese nodules can be excluded as possible cause for the high back-scatter response because the water depth lies above the CCD. As the southern slope of the Guafo Ridge is very steep, sediments cannot accumulate easily and slides are enforced. Basement outcrops in this part are very likely and would ex-plain the high backscatter response as a result of a high impedance contrast. In Fig. 6.5-g part of the southern scarp is depicted. Local backscatter minima can be distinguished, which might be caused by the erosion of sediments and dismantling of the scarp resulting in sedimentary wedges (w) with low backscatter response.

The assumption of basement outcrop as source for high backscatter strengths is corroborated when examining the data recorded by the individual profiles for the southern scarp. High backscatter intensities were collected by each profile and are independent of the recording direction. An instrumental cause can therefore be neglected.

As the EM 120 was not calibrated, absolute backscatter values were not deter-mined, and can therefore not directly be matched with other backscatter mea-surements of different investigations. However, the relative values between two sediment types can be compared to examples in literature: The difference in average backscatter intensities between area C (basement) and area B (sandy sediment) lies around 15 dB. Similar observations were made by Keeton et al.

(1996) (13 kHz) where a backscatter contrast of 10 dB was reported between this two facies for uncalibrated data.

6.3.2 Influence of seafloor topography on backscatter