Introduction:

A seismic refraction survey was conducted on sea ice near Butter Point from November 26th to December 3rd, 1980. The aim was to provide data on sediment thickness for possible further drilling and to investigate the cause of a gravity anomaly reported by Sisson (1980). He suggested that the gravity anomaly could be attributed to a basement fault, downthrown to the northeast.

Instrumentation:

A 12-channel SIE/RS44 refraction seismograph was used. The seismograph was operated by the second author in a Snow-Trac at temperatures of approximately −10°C. The only adaption required for the low temperatures was the replacement of several large electrolytic capacitors in the RS44 recorder.

The 12 vertical geophones were spaced at 29.95m intervals as determined by the geophone cable. They were frozen into holes chipped in the sea ice, which was covered in most placed by 100-200mm of snow. Noice levels were extremely low, even during "blowing snow" conditions.

The explosives used were 1.1kg cartridges of AN60 and 1.6kg cartidges of AN95. They were suspended on detonating cord down 150mm diameter holes drilled through the 2m thick ice by means of a "one man" gasoline powered auger. Plain No. 6 caps and safety fuse were used by the first author to fire the charge. Except for one day, when an assistant was available, the survey was carried out by two people (the authors) using a Snow-Trac, a Snow-Tric snowmobile, and a sledge for transport.

Details of charge size and depth are tabulated in Appendix IA. At the depths used we did get bubble pulses which sometimes obscured later arrivals. However, bringing the charge closer to the surface of the ice to reduce bubble pulses destroyed the shot holes, which we needed to reuse in order to save time.

A shot instant detector (described in Appendix IF) switched on a tone about six seconds before each shot, and terminated it at the shot instant. The tone was transmitted by radio to the recorder and marked the shot instant on the records.

Positioning of Survey:

Two reversed lines were shot, using four shot points (SP.I to SP.IV) shown in Fig. 1 and Fig. 2. The four shot points were surveyed by Messrs. C. Fink and G. Neale of the N.Z. Lands and Survey Department. Line 2 crosses the positions of the proposed fault at SP.III. Using SP.III both halves of Line 2 were reversed.

The spreads were positioned using a metre wheel and lined up on Trig Herb. Their exact offset distances and angles with respect to the line from the shot point to the spread were then determined using the water wave arrival page 11

Figure 2: Interpretations of Line 1 (A) and Line 2 (B).

page 12 times to determine the distance from each shot point, which together with the surveyed distances between the shot points enabled us to solve the resultant triangle and, assuming the spread was lined up on Trig Herb, to calculate the angle between spread and shot directions.

The water wave velocity was determined using shots fired from one shot point to another, since those distances were known to within a few metres. The mean velocity was 1443m.s⁻¹ over 6 results, which ranged from 1441.2 - 1446.1m.s⁻¹.

Results and Analysis:

Appendix IB lists the spread offset distances and the apparent velocities and intercepts of the refraction arrivals. The velocities have been corrected for angle between spread and shot directions.

The time distance graph for line 1 (Appendix IC) shows a single refraction arrival of high apparent velocity typical of basement rock. The interpretation by the first author using a novel ray tracing method (Appendix IG) and a velocity of 5655m.s is shown in Fig. 2A.

Refractions above the basement were not recorded from line 1, but depth sounding data nearby (Ward - this report) require a low velocity layer between the sea floor and basement. The interpretation in Fig. 2A assumes for the layer a velocity of 2600 m.s, the same velocity determined for the sediment layer at SP.III on line 2.

The time distance graph for line 2 (Appendix ID) indicates one sedimentary layer over the basement at the West end and three sedimentary layers at the East end. Guided by the least squares line segments for each spread, straight time distance lines satisfying the theoretical requirements for a plane-layer structure were drawn on (Appendix ID). The arrivals to the East from SP.III are very poor and the 2207m. s⁻¹ line has been drawn in using known bathymetry and the time interrupt of the 2322m.s⁻¹ line.

The adopted lines for the sedimentary layers were interpreted in terms of dipping plane layers (Fig. 2B), using the formulation of Mota (1954). Then the basement arrivals were interpreted by the first author using the ray tracing method (Appendix IG). An average sedimentary velocity of 2600m.s⁻¹ has been assumed on the West half of the line.

The 3547m.s⁻¹ layer would appear to be cut off as you move towards the west since the 3681m.s⁻¹ refraction was very weak and the arrivals on spreads 6 and 7 from SP.II (3398m.s⁻¹ and 3146m.s⁻¹ arrivals) are believed to be basement refractions and their low apparent velocity is not consistent with an intermediate velocity layer with as high a velocity as 3547m.s⁻¹, on such a shallow dip. This cut-off is consistent with but not indicated by our arrivals from that layer. The alternative is that the interface between the 2730 and 3547m.s layers has undulationsin it so as to increase its dip to the east below and west of spreads 6 and 7. There is no arrival from SP.IV on spread 6 supporting this, but if it were the case, the basement positions shown between 3750 and 4650m east of SP.II would move westward and downward, and the cross at 4650m east of SP.II would move eastward and downward, i.e. the basement page 13 would bend downward more rapidly when approaching SP.III from the west.

A mean sedimentary velocity of 2600m.s was used for analysis of the basement arrivals on spreads 3-7 from SP.II but for the remaining two spreads (8 and 9) the three plane layers were used. The arrivals from shot points III and IV were analysed with 2600m.s⁻¹ sediment to the west and the plane layer case to the east of SP.III.

The basement velocity of 5655m.s⁻¹ was obtaining by averaging the slopes of the segments where a basement refraction was recorded from both directions on the same spread. A simplified interpretation by the second author, using plane layer interpretation theory, is given in Appendix IE.

Conclusions

This survey has defined the shape and depth of the basement-sediment interface for a line east from Butter Point. Nearshore the surface is gently domed, averaging 400m below sea level, but dip increases eastward reaching up to 65° at 6km from Butter Point. Beyond this distance the data is insufficient but indicates a depth to basement greater than 1.8km.

The present interpretation shows that SP.II (nearshore)is underlain by 300m of sediments (2600m.s⁻¹) overlying basement (5655m.s⁻¹). SP.IV 8km east is underlain by a thin (60m) layer of low velocity sediment (2210m.s⁻¹) another sedimentary layer of 250m (2730m.s⁻¹) and a further and thicker sedimentary layer (3547m.s⁻¹).