9
King Fahd University of Petroleum & Minerals
Earth Sciences Department
GEOP 315
Seismic Exploration I
Final Exam
Student Name: Student ID#:
Instructions:
· Close your mobile
· This is a closed book & notes exam.
· Answer all the questions.
· Use figures and equations whenever appropriate.
· Define all your symbols.
Question 1 [25 points]:
Define the following briefly [5 points each]:
(a) The reflection coefficient at normal incidence.
(b) Head wave.
(c) Coherent noise.
(d) Huygen’s principle.
(e) DMO.
(f) The requirements of a seismic source and how they are satisfied.
(g) The ghost multiple.
(h) Effect of geometrical spreading versus absorption on wave amplitude.
(i) The dilatation ().
Question 2 [15 points]:
Are the following statements true (T) or false (F) [3 points each]?
(a) sxz is a strain that is caused by a force parallel to the x-axis acting upon a surface perpendicular to the z-axis.
(b)
The P-wave velocity (a) in a fluid is given by: a = (l/r)1/2; where l and r are the Lame’s constant and density of the fluid.
(c) T0 is defined as the minimum time in the T-X curve for any layer.
(d) When fitting a hyperbola to the T-X curve of multiple layers, the fit is only good at Z/X<1, where Z is the depth and X is the offset.
(e) The most commonly used method for determining velocity in seismic exploration is the best-fit method.
(f) Calculate the reflection coefficient (R) for a P-wave propagating in water and incident the water-air interface. [5 points]
(g) The T-X curve of multiple layers is not a perfect hyperbola.
(h) The T-X curve of a single dipping layer is a hyperbola centered about the axis x = 0.
(i) The NMO correction increases with offset.
(j) The DMO is constant for the same layer.
(k) The velocity that results from fitting a straight line to the T2-X2 curve of multiple layers is called the average velocity.
Question 3 [15 points]:
Fill in the blanks with the correct answer (2.5 points each):
(a) Incoherent noise is ------in nature and cannot be followed across adjacent traces.
(b) The ideal geophone spacing to attenuate a horizontally traveling wave of wavelength l is ------.
(c) A spread in which the source is in the center of regularly spaced geophone groups is called a ------spread.
(d) The ------is a layer near the surface, which is composed of loosely consolidated sediments.
(e) The limiting horizontal resolution on unmigrated sections is ------.
(f) A thin bed is that whose thickness is ------.
(g) a ------is the first reflection of the source wavelet from an interface.
(h) Incoherent noise is ------in nature and cannot be followed across adjacent traces.
(i) The limiting horizontal resolution on unmigrated sections is ------.
(j) The ------source usually generates a linear upsweep signal.
(k) In marine surveys, the ------generates seismic waves every time it expand or collapses.
(l) The ------is composed of a piezoelectric material.
(m) The main use of ------is to attenuate horizontally traveling ground rolls.
(n) A spread in which the source is in the center of regularly spaced geophone groups is called a ------spread.
(o) The ------is a layer near the surface, which is composed of loosely consolidated sediments.
(p) In a 3-D survey, a ------is the length over which sources are recorded without a crossline rollover.
Question 4 [20 points]
Draw a figure that shows the typical geometry of an orthogonal 3-D land survey and indicate the following (2.5 each):
(a) Receiver lines.
(b) Source lines.
(c) Inline direction.
(d) Crossline direction.
(e) Box.
(f) Bin.
(g) Patch.
(h) Swath.
Question 3 [30 points]:
The attached figure consists of several events:
(a) Define event A and compute its associated velocity [7 points].
(b) Define event B and compute its associated velocity [7 points].
(c) Define event C and compute its associated velocity and thickness [8 points].
(d) Use the above information to compute the critical angle qc [3 points]?
Question 4 [15 points]
(i) What are five important factors that control the design of a 3-D survey? [7.5 points]
(j) What are five important parameters that need to be set in a 3-D survey? [7.5 points]
Question 5 [25 points]:
Given the attached velocity model, compute the following:
(a) The interval zero-offset traveltime Dti of each reflector [5 points].
(b) The zero-offset traveltime to each reflector ti [3 points].
(c) The RMS velocity VRMS to each reflector [5 points].
(d) Compute the depth to each reflector using the RMS velocities and zero-offset traveltimes [5 points].
(e) Compare the depths you computed in (e) with the real depths by computing the error in depth to each reflector [5 points].
(f) Is it justified, in this case, to use the RMS velocities and zero-offset traveltimes to compute the depths [2 points]?
Layer / Velocity (m/s) / Thickness (m) / Depth (m)1 / 2000 / 200 / 200
2 / 3500 / 300 / 500
3 / 5000 / 500 / 1000
Question 3 [20 points]
(q) Define the NMO. Derive the approximate NMO formula. State at what condition the approximate formula is valid. [10 points]
(r) Given the following equation of a T-X curve that belongs to a single, homogeneous, horizontal layer: T2(X) = 0.25 + 1.610-7 X2. Calculate the layers velocity and thickness. [5 points]
(s) Given Figure 1 representing the T-X curve of a single, homogeneous, dipping layer. Find the layer’s dip, velocity, and thickness. [10 points]
Question 6 [30 points]:
Using the stacking chart in Figure 2, answer the following questions:
(a) What is the total number of receivers that the operator is using? [2.5 points]
(b) What is the total number of shots that the operator is using? [2.5 points]
(c) What is the number of receivers that the operator is activating with each shot? [2.5 points]
(d) What is the type of spread being used? [2.5 points]
(e) What is the maximum number of CDP-fold that we can get with this geometry? [2.5 points]
(f) What is the CDP spacing? [2.5 points]
(g) In tables, sort the traces in:
(1) CDP domain, indicating fold for each CDP [8 points].
(2) Offset domain, indicating fold for each offset [7 points].
1 2 3 4 5 6 7 8 9 10
A O X X X X X
B O X X X X X
C O X X X X X
D O X X X X X
E O X X X X X
Spacing between stations = 40 m.
O stands for shot location.
X stands for receiver location.
Exam duration 50 minute
Good Luck
Ali Al-Ghamdi