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Victoria University Antarctic Research Expedition Science and Logistics Reports 2004-05: VUWAE 49

Transducer levelling

Transducer levelling

This procedure was successful and required two days this season. The following procedure was carried out:
  • Drilling through the sea ice foot (approx 5-6 m thick) with a 200 mm diameter auger directly above the position where the transducer exits the submerged rock.
  • Melting out ice in the 12 m long transducer casing using a mini hot water drill built for the purpose.
  • Temporarily installing lighting and a submarine video camera in the tide crack approximately 3-4 m from the transducer position to locate the transducer under the sea ice foot.
  • Locate the levelling probe on the transducer and level back to the Tide Gauge Bench Mark (CRTG BM1).
  • Levelling Result: Transducer is 8.016 m below CRTG BM1.

The fast sea ice near shore to the tide gauge has remained in place for at least the last 3 years and probably has prevented flushing of ice from under the ice foot that would normally occur when the near shore ice and ice foot breaks away in late summer (Jan-Feb). A previous event affecting the tide gauge on 20 February 1997 has also shown that by late in the summer warming has occurred in the surrounding rock so that ice in the transducer casing can melt completely and potentially would allow the transducer to move from its location. A film of ice has grown on the submerged rock surfaces in the sub ice foot cavity over the last 3 years, obscuring the transducer but the ice is permeable so has not affected the tidal measurements. However the calibration measurements have indicated a nonlinear progressive shallowing of the transducer. Video observation this season showed that the transducer had physically moved up the casing and this page 2 probably occurred progressively as the last three years calibration results (shown below) would indicate.

CAPE ROBERTS TIDE GAUGE VALIDATION RESULTS
(Derived from LINZ calibration measurements and data processing)
Height of reference mark above Tide Gauge Zero
CAPE ROBERTS (Cape Roberts TGBM1 (B93M))
2000 – 2001 8.553m
2001 – 2002 8.506m
2002 – 2003 8.332m
2002 – 2004 8.196m

The extended period of multi year fast ice near shore to the tide gauge has had an adverse effect on the tide gauge data. The transducer position should remain fixed for reliable long-term measurements but this is not guaranteed at present where the transducer is held in the cage by gravity and relies on ice in the casing locking it into position. A more reliable construction would be to replace the existing plastic transducer casing and locating cage with a new casing and J slot locking system that locates and locks the transducer. The techniques and equipment now developed to level the existing transducer would to used also to replace the transducer casing.