Misan University – Engineering College
Petroleum Engineering Department
Subject: Geophysics Class: 3rd
Exam: Final 2019-2020, First Attempt Date: 12 /8/2020
Examiner: Dr. Haider H. Dahm Time: 3 hours
NOTE(S): *Answer All questions unless otherwise specified, Full Mark is (42)
* for MCQs, You MUST write the correct answer on your answer sheet. And you MUST
show your calculations if the answer involves numbers. Otherwise, will be given ZERO.
* The total pages for this questions sheet are 5 pages.
Section #1Conceptual, Physics, and Principals: MSQs: Answer ONLY 15 of the following (30
Marks):
1. Before stacking reflection seismic data, a normal move-out correction is applied. This correction is
applied to data that is sorted in
a. common shot gathers
b. common receiver gathers
c. common midpoint gathers
d. common offset gathers
2. A synthetic seismogram is generated by from an acoustic impedance log by
a. convolving it with an input pulse
b. determining the density of each layer
c. determining the seismic velocities of each layer
d. generating a reflection coefficient log and convolving it with an input pulse
3. A synthetic seismogram is important for interpreting reflection seismic data because it provides a
relationship between
a. travel times and depth to interfaces
b. density and seismic velocity
c. layer thicknesses and travel times
d. density and depth to interfaces
4. Within a given sedimentary layer, a change in the p-wave acoustic impedance could indicate a
change in
a. Porosity
b. Pore fluid content
c. Lithification
d. All of the above
5. In marine seismic exploration, a seismic source such as an air gun or water gun is used to
generate seismic energy. All seismic sources deployed in the water have what feature in common?
a. They generate P-waves only
b. They generate S-waves only
c. They generate both P and S waves
d. They generate both P and SV waves
6. Typically, seismic velocity increases with depth. The main reason for this is that, in general,
____________________ with depth
a. Density increases
b. Bulk and shear modulus increase
c. Bulk and shear modulus increase more rapidly than density
d. Density increases more rapidly than bulk and shear modulus
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7. For a reflection seismic survey, where the subsurface has an average P-wave velocity of 1000m/s
and we use a 10ms seismic wavelet, the best resolution we could obtain is?
a. 1 m
b. 2.5 m
c. 5m
d. 10 m
8. Seismic migration can be best described as
a. Converting the seismic section from time to depth
b. Adjusting the reflection time based on the hyperbolic travel time
c. Stacking the traces
d. Putting seismic reflectors in their correct location
9. In a seismic survey, an air wave can be best described as
a. A wave that travels through the air
b. A compression wave that travels through the air in the pore spaces in the rock.
c. A direct, compression wave that travels from the source to the geophones through the air.
d. All of the above
10. A seismic survey is conducted in a region with two layers. The top layer is shale, with density
2500 kg/m3, and seismic velocity of 2900m/s, and the bottom layer is sandstone, with a density of
2100 kg/m3, and a seismic velocity of 3000m/s. The reflection coefficient will be
a. Positive
b. Negative
c. Zero
d. Not enough information
11. A seismic survey is set-up with a source in a borehole, 100m below the surface, and an array of
geophones on the surface. There is an interface at 200m. The top layer has velocity 1000m/s and the
lower layer has velocity 2000m/s. Which of the following characteristics on a T-X plot will remain the
same if the source was now moved to the surface?
a. Slope of the refracted arrivals
b. Intercept times of the refracted arrivals
c. Arrival times of the direct ray
d. Arrival times of the reflections
12. When looking at first arrival times in a seismic refraction survey, what relationship between the
layer velocities allow us to determine the velocity of layer 3 and know that it is in fact the velocity for
layer 3?
a. V1 < V2 < V3
b. V2 < V1 < V3
c. as long as V2 > V3
d. V1>V2<V3
13. Which of the following statements about Snell’s law in the context of a seismic survey is
incorrect?
Here, θ1 is the incidence angle and θ2 is the transmission angle.
a. Snell’s law shows the angular relationship between the incident and transmitted waves at the interface
in terms of velocities
b. The angle of reflection can be different from the angle of incidence
c. When a seismic wave travels from a region of low velocity to high velocity, the wave is refracted away
from the normal vector of the interface
d. When θ2=90 degree, a head wave develops due to critical refraction
14. Which of these elastic moduli describes the ratio between volumetric stress and volumetric
strain?
a. Young’s Modulus
b. Shear Modulus
c. Bulk Modulus
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d. Poisson’s Ratio
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. 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
17. You carried out a single-shot seismic reflection 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 you 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
18. 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
19. “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
c. Collecting seismic measurement requires contact with the ground while collecting magnetic data can be
done at the surface or airborne.
d. All of the above
20. Acoustic impedance is used to describe the efficiency of seismic reflection and transmission. Its
value depends upon
a. critical angle and velocity
b. velocity and density
c. reflection and transmission coefficients
d. layer thickness and velocity
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Section #2, Applied Geophysics (Calculations) ANSWER TWO questions only (12
Marks):
Q2) Answer the Following: (6 Marks)
(a) The figure below shows a seismic bright spot due to a gas reservoir. Why does a
concentration of gas produce a strong seismic reflection ?
Use the data below to estimate the porosity of the gas reservoir
Reservoir (gas saturated sandstone) v = 2200 m/s
Sandstone with no gas v = 4200 m/s
Gas v = 300 m/s
(b) Explain what is meant by common mid-point profiling.
(c) Sketch the distribution of shot points and geophones for a 6-fold CMP gather.
Q3) The figure below shows a map of the Bouguer gravity anomaly measured over a
landfill. (6 Marks)
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(a) What does the negative Bouguer anomaly tell us about the density of the landfill?
(b) The density contrast between the landfill and the bedrock is 200 kg m -3. Estimate the
thickness of the landfill at the deepest point.
(c) Explain two assumptions that you have made in your analysis. How will these
assumptions influence your answer to (b).
Q4) Provide a full seismic interpretation to the following seismic section. Note that you
may use the basic knowledge of the DHI, Petroleum System Elements, vertical section,
horizontal slices, .. etc. to help you build your accurate petroleum seismic interpretation.
(6 Marks).
COVID-19 is NOT a JOKE !!
STAY HOME, BE SAFE !!
Best Of Luck ..
HOD Examiner
Dr. Hanoon H. Mashkor Dr. Haider. H. Dahm
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