Victoria University Antarctic Research Expedition Science and Logistics Reports 1980-81: VUWAE 25
APPENDIX I - SEISMIC REFRACTION SURVEY
APPENDIX I - SEISMIC REFRACTION SURVEY
A. Details of Shots Fired.
|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|
|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|
|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|
|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.
|Spread||Offset||Shot Point||Apparent Velocity||Intercept||Comments|
|920m||II||5759||0.295||Channels 1, 2, 12 only weak|
|6631||0.355||Geophones 1-4 and 10-12 only|
|IV||0||5545m||5904||2.799||Probably not refraction arrival|
|9||II||assume = 0||7221m||3844||0.089|
|III||assume = 0||2954m||3389||0.512|
|IV||assume = 0||659m||2356||0.237|
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.
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
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.