Introduction to Geophysics — DU B.Sc. (Hons.) Geology Previous Year Question Papers

(b) Solve the following numerical. (i) A seismic wave crosses from a rock with a velocity of 2 km/sec to another rock with 4 km/sec, such that it runs along (parallel to) the interface after refraction. At what , angle did it approach the interface? (ii) A seismic reflection survey is carried out over a 2500 m thick horizontal layer with ! a P-wave velocity of 2000 m/s. Calculate the travel time of a reflected wave at a detector placed 1000 m from a source. 1. Answer the following : (a) The strength of the total magnetic field at the pole is of the strength of the total magnetic field at the equator. (b) A surface on which the potential is constant is called (c) What is the name of the phenomenon in which Earth's magnetic field periodically reverses its direction? (d) What would be a VES curve over a three-layer i sequence comprising: moist soil (top), freshwater saturated coarse sand (middle), and Clay (bottom)? (e) The resistivity of the orebody is . than the resistivity of the host rock. (f) When the movement of the particles is parallel to the direction of wave propagation, the wave is called Age (Ma) Declination Inclination Age (Ma) Declination Inclination 7. (a) How does the anomaly arise in the resistivity survey? With illustrations, briefly describe the Wenner and Schlumberger configurations. (b) A Schlumberger array with a current electrode separation of 80 m and potential electrode separation of 10 m is placed over an inhomogeneous medium. If the measured potential difference is 40 mV and the computed apparent resistivity is 120 Sm. Calculate the magnitude of current passing through the subsurface. (Use t = 3.14) 8. (a) Write a short note on the seismic refraction survey. (b) Write short notes on: (i) Geographic poles (ii) Magnetic poles (iii) Geomagnetic poles 6. (a) Describe the role of paleomagnetic study in establishing the seafloor spreading theory. (b) A small area of a continent has rocks of a range of ages. Their palaeomagnetic directions were measured, giving the data below (dates are accurate to about t4 Ma, palacomagnetic directions to ‡2°). Plot the palaeolatitudes against time. What can you deduce about the movement of the continent? (g) P-waves always travel than shear waves in the same medium. (h) Which type of rocks are commonly used in paleomagnetic studies due to their ability to preserve remanence magnetism? (i) Sedimentary rocks (ii) Igneous rocks (iii) Metamorphic rocks (iv) Organic origin rocks (i) During the reflection survey when each reflection point on the reflector is sampled more than once, the method is called 2. Briefly explain the following : (i) Free-air correction (ii) Apparent polar wander path (iv) Crossover distance (or) 3. (a) Briefly describe the following : (i) True gravity and absolute gravity. (ii) Define gravity reduction. (iii) International Reference Ellipsoid