EOS 350 MIDTERM OCT 4, 2013 STUDENT NAME: TEAM #: Some equations which may, or may not, be useful: Distance from sensor to a dipole z ~ x ½, Distance to line of dipoles z ~ 0.75x ½ B = μh, M = κh Seismic equations: Z = ρv, R = Z 2 Z 1 Z 2 +Z 1, T = 2Z 2 Z 2 +Z 1 sinθ 1 = sinθ 2, d = Vt V 1 V 2 T i = 2Zcosθ or T V i = 2Z V2 2 V2 1 1 x cross = 2z V 2 + V 1 V 2 V 1 V 1 V 2 Note: The exam consists of 23 multiple choice questions each worth 1.5 points and 5 short answer questions that are worth 31 points. MULTIPLE CHOISE (34.5 points) 1. What is the correct seven-step process? a. Setup, surveys, data, processing, properties, interpretation, synthesis b. Setup, properties, surveys, data, processing, interpretation, synthesis c. Properties, setup, surveys, data, processing, interpretation, synthesis d. Setup, properties, surveys, data, processing, synthesis, interpretation 2. Geophysical surveys can provide information about the distribution of a physical property. What is the principle difficulty encountered when trying to use this information to identify a rock type? a. There aren t any real difficulties b. Different rock types can have different values of a physical property c. Different rock types can have the same value of a physical property d. A single sample of rock has multiple values of a physical property
3. Which statement in the list below is TRUE a. A buried object is detectable so long as it has a non-zero value of a physical property b. A buried object is detectable so long as it has a significant physical property contrast compared to the surrounding rocks. c. A buried object is detectable if its dimensions are larger than its depth of burial. d. A buried object is detectable if it has two physical properties that are not the same. 4. Which region of the earth would have the smallest magnetic total field strength as a background? a. Canada b. Brazil c. Antarctica d. Norway 5. What is the dimensional unit for magnetic susceptibility? a. nt (nano Tesla) b. Gamma c. Amperes/meter d. Dimensionless 6. The "Total field magnetic anomaly" can be best described as: a. A map of the amplitude of Earth's magnetic field. b. A map of the distribution of magnetic material. c. A map of the vertical component of the measured magnetic field. d. A map of the horizontal component of the measured magnetic field. e. A map of the component of the measured magnetic field anomaly that is aligned along the direction of Earth's magnetic field. 7. Which of the following situations CAN be corrected using data gathered at a base station? a. (i) Magnetic storms and (ii) regional trends b. (i) Regional trends and (ii) geologic features larger than the survey area. c. (i) Diurnal variations and (ii) magnetic storms d. (i) Geologic features larger than the survey area and (ii) diurnal variations
8. When choosing the station spacing for a magnetic survey which of the following items is the most important a. The inclination and declination of the earth s magnetic field b. The expected size and depth of the target c. The type of magnetometer used (that is, a total field magnetometer or a magnetometer that measures the individual components) d. The distance between your survey area and the base station 9. A magnetic survey using a total field magnetometer is carried out over an object at the equator. The total field magnetic anomaly is compared to the anomaly obtained when using a magnetometer that measures all three components of the field. (Bx, By, Bz). (with x, y, z respectively denoting northing, easting and vertical). The total field anomaly is a. the same as the Bx component b. the same as the By component c. the same as the Bz component d. None of the above 10. Which of these elastic moduli describes the ratio between volumetric stress and volumetric strain? a. Young s Modulus b. Shear Modulus c. Bulk Modulus d. Poisson s Ratio 11. A compressional wave a. Is also called an S-wave. b. Can travel only through solids. c. Depends on density, shear modulus, and bulk modulus. d. All of the above 12. In which of the following geoscience problems is a seismic refraction survey most likely to provide valuable information? a. geological mapping of British Columbia b. oil and gas exploration in the Gulf of Mexico c. locate a near surface pipeline outside EOS East Building d. geotechnical study for the site of the new earth system science building of UBC
13. Which geological model would lead to a failure of refraction survey? a. more than two layers overlying the basement b. dipping or irregular interface c. seismic wave travels faster in the surface layer than in any other layer d. all of above 14. Reciprocal times are total travel times a. from the first geophone to the last geophone of a spread b. from the first geophone to the last geophone of a reverse spread c. from the shot of forward spread to the shot location of reverse spread d. from the shot to the last geophone of a spread 15. If the incident angle is 45 degrees and the seismic velocity in the first medium is a reasonable value for sediments, and the seismic velocity in the second medium is 4 times slower than in the first medium, then the refracted angle a. Is less than 45 degrees. b. Is more than 45 degrees. c. Is 45 degrees. d. Cannot be determined given this information. 16. Suppose a seismic refraction survey was conducted over an area with 3 horizontal layers (ie 2 interfaces). Which of the following scenarios would produce first arrivals at surface geophones from all three layers? (Top layer: seismic velocity v1, middle layer: seismic velocity v2, lower layer: seismic velocity v3) a. V1 = 400 m/s, v2 = 2000 m/s, v3 = 1500 m/s b. V1 = 1500 m/s, v2 = 2000 m/s, v3 = 400 m/s c. V1 = 2000 m/s, v2 = 1500 m/s, v3 = 400 m/s d. V1 = 400 m/s, v2 = 1500 m/s, v3 = 2000 m/s 17. Suppose the correct scenario was found for the above question, for a refracted ray path traveling along the second interface (between the middle and lower layer), what is its incident angle at the first interface (between the top and middle layer)? a. Sin -1 (v2/v3) b. Sin -1 (v1/v3) c. Sin -1 (v1/v2) d. Sin -1 (v2/v1)
18. Assume that the earth is represented by a horizontal layer overlying a half-space. A simplest refraction survey with minimum instrument requires a. one shot and one geophone b. one shot and two geophones c. one shot and three geophones d. two shots with two geophones 19. Dave carried out a single-shot seismic refraction survey to find the depth to the basement and the velocity structure. Exact knowledge about the material above the basement and the shape of the basement interface was not known. After carrying out the survey he picked out the first arrivals and plotted the T-X graph as below. What conclusion can Dave make? a. he can only see one refraction so there is only one layer overlaying the basement b. he can determine the velocity of the basement by measuring the slope of the dashed line c. he can estimate the critical angle using the intercept time of the refraction arrival d. none of the above 20. In a seismic reflection survey each trace in the processed section a. Is the signal that would be received if the source and receiver were coincident b. Is the measured signal from the geophone that is closest to the receiver c. Is the signal from the common offset array d. Is the signal from a common midpoint array
21. Which of the following is probably TRUE about the situation yielding the TX data in the figure? a. The refracting horizon is dipping. b. The first interface has a change in dip somewhere. c. The velocities of two refractors are different. d. There are no direct arrivals 22. The acoustic impedance of a medium a. is equal to the value of the seismic velocity b. depends upon the product of density and velocity c. depends upon the ratio of density and velocity d. depends upon velocity and bulk modulus 23. Seismic reflections can occur when only when there is a change in seismic velocity. (This is a Right Wrong question for a final mark. Correct gets one mark; wrong loses a mark) a. TRUE b. FALSE SHORT ANSWER (31 points): 1.) (4 pts) A sphere has a radius of 10 meters and a magnetic susceptibility K=0.1. The center of the sphere is 20 meters beneath the earth s surface. If the strength of the earth s field is B=60,000 nt, what is: a) (2 pts) The magnetization of the sphere
b) (1pt) The magnetic moment of the sphere c) (1 pt) How is the magnetic moment affected if the center of the sphere is moved to 40 meters below the surface? 2.) (3 pts) The first picture below was processed to produce the second. What is the name of the processing step applied, what does it do, and why is it beneficial? (i) (ii)
3.) (10 pts) Suppose a sphere is buried below the surface of the Earth in a location where the inclination is -45 degrees and the declination is 0 degrees. The instrument used is a total field magnetometer. a) (4 pts) Which of the following images shows the responses that would be measured? In arriving at your answer please provide a series of diagrams that demonstrate your thought process. i. ii. b. iii. iv. b.
b) (3 pts) Use your chosen magnetic field data and consider the data that would be collected along a line that goes through the maximum and minimum of the anomaly. In the space below provide a rough sketch of the data. Use this information to estimate the depth of burial of the object and also estimate its location in (Easting, Northing) coordinates. c) (3 pts) If you were designing a field survey to find this object what survey design would you choose? That is, station spacing, line spacing etc. Show how you arrived at your answer.
4.) (8 pts) Consider the two plots below (i) P-wave (ii) SH-wave a) (2 pts) What source is used for generating a P-wave refraction experiment? What source is used for generating an SH refraction? b) (2 pts) In plot (i) what is meant by the air wave? Why is it not seen in plot (ii)? c) (3 pts) What is the thickness of the top layer that can be obtained directly for the SH reflection seen in (ii). Show your work. d) (1 pt) How would you determine if the refracted P and S waves were, or were not, coming from the same interface?
5.) (6 pts) Consider the three layer earth described below. All layers have the same density. We re interested in the seismogram that would be obtained at zero offset. a) (2 pts.) Consider the reflection from the top of layer 2: What is the travel time for that reflection event and what is the value of the reflection coefficient? b) (2 pts) Consider the reflection from the top of layer 3. What is the travel time for that reflection event and what is the value of the reflection coefficient? c) (2 pts) The seismic wavelet is sketched below. Use that wavelet and the information you derived in (a) and (b) to plot the recorded seismogram.