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Victoria University Antarctic Research Expedition Science and Logistics Reports 1980-81: VUWAE 25

APPENDIX I - SEISMIC REFRACTION SURVEY

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page 47

APPENDIX I - SEISMIC REFRACTION SURVEY

A. Details of Shots Fired.

* Distance to nearest geophone.

* indicates that no value was recorded.

Line 1
Shot Date Time Shot Depth Charge Shot instant Warning Fuse Burning Time Offset*
1 26/11 pm 5m 1.1kg AN60 16 sec. * 949m
2 26/11 2000 5m 1.1kg AN60 16 sec. * 949m
3 26/11 2120 3m 1.1kg AN60 9 sec. * 1898m
4 26/11 2130 5m 1.1kg AN60 5 sec. * 1898m
5 27/11 1230 5m 1.1kg AN60 * * *
6 27/11 1242 5m 1.1kg AN60 5 sec. 29 sec. 1898m
7 27/11 1349 5m 1.1kg AN60 6 sec. 32 sec. 994m
8 27/11 1624 5m 1.1kg AN60 12 sec. 31 sec. 2847m
9 27/11 1830 5m 1.1kg AN60 6 sec. 30 sec. 2847m
Line 2
1 28/11 1821 5m 1.1kg AM60 6 sec. 29 sec. 334m
2 28/11 1840 5m 1.1kg AN60 6 sec. 32 sec. 334m
3 28/11 2056 2m 1.1kg AN60 6 sec. * 3700m
4 28/11 2215 2m 1.1kg AN60 5 sec. 33 sec. 3700m
5 29/11 1901 5m 3.2kg AN95 5 sec. 32 sec. 8000m
6 29/11 2124 5m 3.2kg AN95 6 sec. * 6700m
7 29/11 2225 5m 1.1kg AN60 6 sec. 31 sec. 2700m
8 30/11 0045 5m 0.8kg AN95 7 sec. 33 sec. 2700m
9 30/11 1531 5m 1.1kg AN60 + 1.6kg AN95 5 sec. 32 sec. 5700m
10 30/11 1650 5m 1.6kg AN95 * 31 sec. 5700m
11 30/11 2249 5m 1.1kg AN60 7 sec. 36 sec. 1700m
12 30/11 2350 5m 1.1kg AN60 7 sec. 36 sec. 2300m
13 1/12 1310 5m 1.1kg AN60 6 sec. 32 sec. 670m
14 1/12 1430 10m 1.6kg AN95 8 sec. 35 sec. 4700m
15 1/12 1613 5m 1.1kg AN60 No warning. 33 sec. 3300m
16 1/12 1633 5m 1.1kg AN60 4 sec. 36 sec. 3300m
17 1/12 1940 5m 1.1kg AN60 * * *
18 1/12 2042 5m 1.1kg AN60 3 sec. 36 sec. 4300m
19 1/12 2240 5m 1.1kg AN60 5 sec. 32 sec. 4700m
20 2/12 1528 5m 1.1kg AN60 7 sec. 31 sec. 2700m
21 1/12 1630 5m 1.1kg AN60 No warning. 31 sec. 1000m
22 2/12 1642 5m 1.1kg AN60 6 sec. 30 sec. 1000m
23 2/12 1834 10m 1.6kg AN95 5 sec. 36 sec. 5300m
24 2/12 2107 10m 1.6kg AN95 6 sec. 32 sec. 6300m
25 2/12 2159 5m 1.1kg AN60 7 sec. 31 sec. 2000mpage 48
26 2/12 2316 5m 1.1kg AN60 7 sec. 30 sec. 1700m
27 3/12 0026 5m 1.1kg AN60 6 sec. 31 sec. 700m
28 3/12 0116 5m 1.1kg AN60 6 sec. 35 sec. 3000m
29 3/12 0310 10m 1.6kg AN95 7 sec. 33 sec. 7300m

B. Linear Regression Lines for the Refraction Arrivals at each Spread.

Line 1
Spread Offset Shot Point Apparent Velocity Intercept Comments
1 2832m II 5597 0.276
7820 0.484
2 1879m II 5065 0.242
953m I 6398 0.233
3 1912m I 4893 0.170
920m II 5759 0.295 Channels 1, 2, 12 only weak
3246 0.157
4904 0.258
4 2832m I 3839 0.018
Line 2
Spread Shot Point Angle Offset Speed Intercept Comments
1 IV 0 7888m 4985 0.604
III 0 3966m 4741 0.286
2 II 0 322m 3800 0.243
III 0 3633m 4658 0.275
3 II 0 1352m 3915 0.191
6631 0.355 Geophones 1-4 and 10-12 only
III 0 2617m 5071 0.363
IV 0 6531m 5616 0.776 page 49
4 II 0 2345m 4858 0.249
III 4° 1632m 6509 0.467
IV 0 5545m 5904 2.799 Probably not refraction arrival
8721 1.208
5 II 0 3324m 4030 0.110
III 0 641m 2322 0.221
IV 0 4563m 2987 1.111 Bubble Pulse
9806 1.282
6 II 0 4287m 3398 -0.074
IV 0 3608m 5377 0.850 Very weak
3000 0.373
22400 1.556
7 II 3.6° 5293m 3146 -0.205
III 17.8° 1041m 2692 0.223
IV 7.3° 2636m 2200 0.217
2893 0.308
3881 0.453
8 II 3° 6269m 4619 0.384
III 9.5° 1995m 3253 0.532
3930 0.404
IV 8.5° 1671m 3482 0.380
2281 0.220
9 II assume = 0 7221m 3844 0.089
III assume = 0 2954m 3389 0.512
3368 0.633
2641 0.063 Very Week
IV assume = 0 659m 2356 0.237
page 50
C. Time/Distance graph for Line 1.

C. Time/Distance graph for Line 1.

D. Time/Distance graph for Line 2.

D. Time/Distance graph for Line 2.

page 51
E. Plane Layer Interpretation

E. Plane Layer Interpretation

page 52

F. Shot Instant Detection

In the survey we used plain No. 6 blasting caps and safety fuse. Because of the uncertainty in burning time of the safety fuse, (over a 24cm length of fuse the burning time ranged from 29 seconds to 36 seconds) a more accurate method of predicting the explosion time was needed so the recorder could be started in time without wasting too much paper.

To give a 6 second warning we taped an ordinary silicon diode to the safety fuse, 8cm from the detonator - as the fuse burned past this diode the temperature was raised, changing the forward voltage drop over the diode. This change in voltage triggered an oscillator tone which was transmitted to the recorder.

When the detonating cord fired, a piece of wire taped to the cord was broken, terminating the tone. This termination was recorded and gave us our shot instant on the records.

The operation was satisfactory, although there were problems with feedback when using the radio on high power but this could be easily remedied with better screening. Also if the wire broken by the explosion was engulfed in sea water thrown up by the explosion, the tone returned, sometimes in a few tens of milliseconds. Uncertainties in shot instant times are probably less than 0 milliseconds.

G. Ray Tracing Analysis

The analysis used here is suitable for this situation where the refracting interface being mapped is sharply curved, so ray paths don't travel along the interface and leave at the critical angle, but instead cut through the bulk of the material.

You need to know the structure above the interface and the depth of the interface either under the spread or the shot point, as well as the velocity below the interface. In the present case the interface was the basement surface, and was fairly flat near shot point II (SP II) and its depth ZA was obtained from the time intercept as SP II.

METHOD

Using the plane-layer structure already determined for the overlying sediments, and the apparent velocity of the spread, the ray path can be calculated back to the point B on the bottom plane refractor, and the co-ordinates of B, the time to get from B to G (TBG) and the angle θn-1 at which the ray arrived worked out.

Also for the point A co-ordinates and time TSA can be calculated from the known structure at A, assuming critical refraction. For the case where q is large compared to the depth the point A is fairly constant for all spreads since the angle of the ray path along q changes very fast with a change in the incoming ray angle near the critical angle.

page 53
Diagram illustrating the Time Term Analysis Method.

Diagram illustrating the Time Term Analysis Method.

Having calculated the co-ords (X, Z, T) of points A, B, and the angle θn-1 you can set up simultaneous equations and solve a quadratic for Y and hence the position of the basement.

(5) is a quadratic in Y and is readily solved from which the sensible value of Y is taken and the co-ordinates of the basement refraction point R can be worked out.

page 54

This method was used for arrivals on spreads 3-9 from shot point II and on spreads 1-4 from shot point III to obtain the structure between shot points II and III. This structure was used, together with arrivals on spreads 1, 3, 4 and 4 from shot point IV, to locate the basement refraction point on the descending ray from shot point IV, assuming that the ray had the same angle in the water as the reverse rays from shot point II to spread 9, which was only 660m from shot point IV. The ray path from the shot point was traced down to the bottom plane layer refractor and the equation (5) solved.

The solutions gave the group of X's marked down at approximately 1500m depth on the profile diagram. Using the mean position of these points as the true position of the basement refraction point on the descending ray path from SP IV and using the single 2600 sedimentary layer in order to be consistent with the 3398 and 3146 arrivals, the equations were solved for the 22400 arrival and this gave the X marked at 1026m depth.

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