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answerGEOPHYSICS (GEOL 4433)
EXAM 1- FALL 1998
YOUR BIG SEISMIC REFRACTION ADVENTURE
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It is the year 2025 and you have finally completed your Master’s Degree in Geology (we knew you could do it! Congratulations!). President Chelsea Clinton has declared that it's about time we explore the crustal structure of the ocean basins. You receive a phone call from the President informing you that she wants you to be the Chief Scientist on a cruise to the middle of the Pacific Ocean to discover the nature of Earth's internal layers there.
Initially, you panic because you know nothing about the Pacific Ocean, except that there is a lot of water there. However, you quickly come to your senses and recognize what a unique opportunity is being offered to you. Furthermore, you have taken GEOL 4433 four times and feel that you are sufficiently well-versed in the Principles of Geophysics to do a good job, so you accept your President’s offer.
Your first task is to familiarize yourself with the Pacific Basin and review what is presently known regarding its geologic history. To do this, you visit a number of World-Wide Web locations containing information on the Oceans. Among the more useful sites, you find:
http://www.ngdc.noaa.gov/mgg/global/seltopo.html
http://www2.ocean.washington.edu/
After several years of training and a 1-month journey to the center of the Pacific Basin, you are finally on station and ready to begin your study. The first order of business is to design and execute a seismic refraction survey along a 100 km track in order to gain some knowledge of the seafloor and its layered structure. To do this, you place a receiver array aboard one of the ships in your expedition. This vessel remains on station at your starting point throughout the survey. The boat you are on will serve as the source for seismic impulses, and is ordered to steam due west from the starting point for 100 km. Every kilometer along this track, an explosive charge is detonated and the resulting p-waves recorded by the receiver ship. The table at the bottom of the page represents the results of your survey listing offset distance (in meters) and time of first-arriving p-waves (in seconds) for each offset position.
As a result of this survey, you should:
1) Provide a plot of arrival times vs. distance (source-receiver offset).
2) Determine the number of layers in the Pacific Basin subsurface.
3) Determine the velocity of p-waves traveling through the Pacific Basin subsurface layers.
4) Determine the thickness of the Pacific Basin subsurface layers.
5) Determine the critical angles of refraction for each refractor.
6) Provide a summary SCALE diagram of the Pacific Basin crustal structure indicating the number of layers revealed by the refraction survey, the thickness of each layer, the p-wave velocity for each layer, the critical angle of incidence for each refractor, and for deeper refractors, the angles of incidence and refraction in overlying layers which yield critical refraction on the deeper interface.
7) Finally, speculate on the nature and composition of the Pacific Ocean layers (i.e. what specific type of geologic material is it?). I suspect you will need to visit our library for some insight on this.
OFFSET TIME 0 0.000 1000 0.357 2000 0.714 3000 1.071 4000 1.384 5000 1.593 6000 1.801 7000 1.971 8000 2.116 9000 2.261 10000 2.406 11000 2.551 12000 2.696 13000 2.841 14000 2.986 15000 3.131 16000 3.276 17000 3.421 18000 3.566 19000 3.711 20000 3.855 21000 4.000 22000 4.145 23000 4.290 24000 4.435 25000 4.580 26000 4.725 27000 4.870 28000 5.015 29000 5.160 30000 5.305 31000 5.450 32000 5.595 33000 5.740 34000 5.884 35000 6.029 36000 6.174 37000 6.319 38000 6.464 39000 6.609 40000 6.754 41000 6.899 42000 7.044 43000 7.189 44000 7.334 45000 7.479 46000 7.624 47000 7.769 48000 7.913 49000 8.058 50000 8.203 51000 8.335 52000 8.468 53000 8.600 54000 8.733 55000 8.865 56000 8.998 57000 9.130 58000 9.262 59000 9.395 60000 9.527 61000 9.660 62000 9.792 63000 9.925 64000 10.057 65000 10.190 66000 10.322 67000 10.454 68000 10.587 69000 10.719 70000 10.852 71000 10.984 72000 11.117 73000 11.249 74000 11.382 75000 11.514 76000 11.647 77000 11.779 78000 11.911 79000 12.044 80000 12.176 81000 12.309 82000 12.441 83000 12.574 84000 12.706 85000 12.839 86000 12.971 87000 13.103 88000 13.236 89000 13.368 90000 13.501 91000 13.633 92000 13.766 93000 13.898 94000 14.031 95000 14.163 96000 14.296 97000 14.428 98000 14.560 99000 14.693 100000 14.825