Case Study – Kangaroo Island Cable Route Survey
Hydrographic and Topographic Surveying – Kangaroo Island Cable Route Survey
SA Power Networks engaged PHS to conduct a hydrographic and topographic survey for a proposed replacement of an undersea cable across the Backstairs Passage from Fishery Beach, Cape Jervis to Cuttlefish Bay, Kangaroo Island.
A 200m corridor, as shown in Figure 1, was surveyed. The area extended from Fishery Beach Transition Station, extending offshore to Cuttlefish Bay Transition Station on Kangaroo Island. Water depths ranged from 0-60m.
This survey involved three phases of operation:
- Offshore multibeam and magnetometer survey onboard the M/V ‘Destiny’.
- Nearshore singlebeam survey using a remotely controlled ‘Z-Boat 1800’.
- Onshore topographic survey for the entry and exit points of the proposed cable route.
Each phase was surveyed separately using different methodologies, and the three datasets were later combined to produce one continuous dataset for the entire route.
Figure 1: Proposed Kangaroo Island Cable Route
Offshore Multibeam Bathymetry and Backscatter Survey
PHS mobilised a high-resolution wideband R2 Sonic 2024 multibeam echosounder to obtain accurate water depths and backscatter seafloor imagery. The below extract (Figure 2) presents a sample of the bathymetry data collected along the proposed route. The different colours indicate varying depth and contours are presented every 5m.
Figure 2: Multibeam Bathymetry Data Extract
While bathymetry data is calculated from the time it takes for the acoustic energy to return to the sonar, backscatter data is calculated from the intensity of acoustic energy. As different bottom types scatter the acoustic energy differently, this information can be used to determine the seafloor’s physical properties. For example, a softer bottom such as mud will return a weaker signal than a harder bottom such as rock.
Figure 3 presents an extract of the multibeam backscatter data. The darker areas of the mosaic represent a weaker acoustic signal return, and the lighter areas represent a stronger acoustic signal return.
Figure 3: MBES Backscatter – Cuttlefish Bay
The multibeam backscatter data provides a clear image of the seabed allowing the existing 1993 and 1965 subsea cables to be clearly detected as per Figure 4.
Figure 4: MBES Backscatter Extract – Observed Existing Cables
Offshore Magnetometer Survey
A Geometrics G-882 marine magnetometer was mobilised to identify any ferrous/magnetic anomalies within the survey limit. All magnetic anomalies observed were attributed to the existing 1965 and 1993 cables. An example magnetometer extract showing an anomaly observed at the 1993 cable is presented below in Figure 5.
Figure 5: Magnetometer Data Extract – Magnetic anomaly at the existing 1993 cable
Nearshore Z-Boat Survey
For the shallow near shore sections of the survey, a remote survey vessel ‘Z-Boat 1800’ (Figure 6) was deployed, mobilised with a singlebeam echosounder to obtain accurate water depths.
Figure 6: Z-Boat 1800 deployment
The advantage of using a Z-Boat is that it is able to access areas which would not be able to be safely accessed using the M/V ‘Destiny, ensuring a complete continuous data set as the cable route transitions from land to shore.
Figure 7: Combined Topographic, Nearshore and Offshore Datasets at Cuttlefish Bay
Onshore Topographic Survey
For the onshore sections of the proposed route at Cuttlefish Bay and Fishery Beach, a topographic survey was conducted using a Trimble R8 RTK System.
The RTK rover was set at a known height and points were collected in a grid pattern across the Fishery Beach and Cuttlefish Bay sites.
Using the collected XYZ points, the contours of the area and an outline of the existing infrastructure was able to be plotted. An extract of the topographic survey data is presented in Figure 9.
Figure 8: Topographic Survey Data Extract at Fishery Beach
Horizontal and Vertical Control
One GNSS base station was set up near the Fishery Beach Transition Station. It was used throughout the project to supply RTK corrections to the M/V ‘Destiny’ for the offshore survey, Z boat 1800 for the nearshore survey, and Trimble R8 RTK Rover for the topographic survey. This base station was coordinated using two existing permanent survey benchmarks.
Figure 9: GNSS Base Station at Fishery Beach