University
of King Abdulaziz
Faculty of Earth Sciences
Department of Petroleum Geology and
Sedimentology
(444) (Seismic Stratigraphy)
Answer the following questions:
1.
Define the Seismic
Stratigraphy (5
marks)
Modal Answer
Seismic stratigraphy is that seismic reflectors
follow time surfaces rather than facies impedance boundaries. This technique is
based on contrasts in acoustic impedance between different materials;
reflections of sound or shock waves occur at transitions between different
types of sediment or rock.
2.
Why Seismic Stratigraphy? (5 marks)
Model Answer
1. Imaging
has improved to the point that the seismic line resemble geological cross
sections, with all the potential for interpretation.
2. Seismic
data allows characterization and quantification of lithology: Instead of grain
size and composition use amplitude, frequency, continuity, and velocity. These
features can be qualitatively described, and quantified easily when the data is
in digital format.
3. Layer
geometry: Seismic data show gross geometry (within the limits of resolution
over large areas.
4. 1-3
above comprise predictive tools that can allow you to interpret lithology,
depositional environment, sea level changes, and even age.
5. Industry
has been strongly support of Seismic Stratigraphy because of its predictive
nature
3. How to generate a seismic profile? (5 marks)
Modal
Answer
1.
Explosives or a
"vibraseis" truck generate shock waves at the surface. At sea a
compressed air gun is used.
2.
Shock waves travel through the
interior strata. Waves that encounter a boundary between different materials
are partially reflected because of differences in sonic velocity and density
(sonic velocity X density = acoustic impedance).
3.
Microphones called geophones (hydrophones
at sea) record reflected sound waves.
4.
Computer processing determines the
two-way travel time of each reflector surface.
5.
A vertical profile of reflectors
is drawn as a sinusoidal trace. Reflection peaks are filled in to make them
visible. Put together into successive columns, the vertical profiles create a
cross section with visible reflection planes.
6.
With current digital signal
processing technology, it is possible to image the subsurface in some detail
down to a depth of 5 km.
4. Write short note on similarities and differences Seismic
profiles related to geologic cross sections (5 marks)
Model Answer
Seismic
profiles seem to be synonymous with geologic cross sections. The two are
certainly related; one can identify layers, unconformities, faults, folds, and
other geologic features on a seismic profile. There are some important
differences when interpreting seismic profiles:
1.
Scale - a typical seismic wave
has a frequency of 100 Hz, which translates to a wavelength of about 15 m,
which is the lower limit to resolution of layers in a seismic profile.
Geologists typically focus on beds that are an order of magnitude or more
thinner than this. The units defined by reflectors are not individual beds, but
packages of strata.
2.
Different beds or packages of
beds will not show up on a seismic profile if there is insufficient contrast in
acoustic impedance. For example, sandstones and conglomerates would not be
resolved.
3.
The lithology of layers resolved
in a seismic profile can only be broadly guessed at, unless drill cores are
available from the subsurface that can be correlated with the seismic section.
4.
Depth on a seismic profile is
given as two-way travel time, rather than as thickness.
5.
Discuss briefly the Seismographs (5 marks)
Model answer
1.
Seismographs are the principal tool of scientists who study earthquakes.
2. Seismograph is a simple pendulum.
When the ground shakes, the base and frame of the instrument move with it, but
inertia keeps the pendulum bob in place. It will then appear to move, relative
to the shaking ground. As it moves it records the pendulum displacements as
they change with time, tracing out a record called a seismogram.
3. One seismograph station, having three
different pendulums sensitive to the north-south, east-west, and vertical
motions of the ground, will record seismograms that allow scientists to
estimate the distance, direction, Richter Magnitude and type of faulting of the
earthquake.
6. What are Wave Travel Times? (5 marks)
Model answer
Travel time is a relative
time, it is the number of minutes, seconds, etc. that the wave took to complete
its journey. The arrival time is the time when we record the arrival of a wave.
It is an absolute time, usually referenced to Universal Coordinated Time.
7. Discuss briefly the Seismic Wave
Speed in the earth's crust.
(5 marks)
Modal Answer
1. Seismic waves travel fast,
on the order of kilometers per second (km/s).
2. The precise speed that a
seismic wave travels depends on several factors; most important is the
composition of the rock.
3. Mostly the speed depends on
the rock type because it allows us to use observations recorded on seismograms
to infer the composition or range of compositions of the planet.
4. Temperature tends to lower
the speed of seismic waves and pressure tends to increase the speed.
5. Pressure increases with
depth in Earth because the weight of the rocks above gets larger with
increasing depth. The effect of pressure is the larger and in regions of
uniform composition, the velocity generally increases with depth, despite the
fact that the increase of temperature with depth works to lower the wave
velocity.
8. Write short note on Compressional or P-Wave and
S-Waves. (5 marks)
Model answer
1.
P -Waves
P-waves are the first waves to
arrive on a complete record of ground shaking because they travel the fastest.
They typically travel at speeds between ~1 and ~14 km/sec. The slower values
correspond to a P-wave traveling in water, the higher number represents the
P-wave speed near the base of Earth's mantle.
The velocity of a wave depends
on the elastic properties and density of a material. If we let represent the
bulk modulus of a material, the shear-modulus, and the density, then the
P-wave velocity (, is defined by:
2.
S-Waves
Secondary , or S waves, travel
slower than P waves and are also called "shear" waves because they
don't change the volume of the material through which they propagate, they
shear it. S-waves are transverse waves because they vibrate the ground in the
direction "transverse", or perpendicular, to the direction that the
wave is traveling.
The S-wave speed ( depends
on the shear modulus and the density:
S-waves are slower than
P-waves, the S-waves move quickly. Typical S-wave propagation speeds are on the
order of 1 to 8 km/sec. The lower value corresponds to the wave speed in loose,
unconsolidated sediment, the higher value is near the base of Earth's mantle.
9.
Discuss Refraction of seismic waves. (5 marks)
Model
answer
As a wave travels through
Earth, the path it takes depends on the velocity. Snell's law, which is the
mathematical expression that allows us to determine the path a wave takes as it
is transmitted from one rock layer into another. The change in direction
depends on the ratio of the wave velocities of the two different rocks.
Refraction has an important
affect on waves that travel through Earth. In general, the seismic velocity in
Earth increases with depth and refraction of waves causes the path followed by
body waves to curve upward.
10.
Write short note on Reflection of seismic waves. (5 marks)
Model
Answer
In seismology, reflections are
used to prospect for petroleum and investigate Earth's internal structure. A
seismic reflection occurs when a wave impinges on a change in rock type. Part
of the energy carried by the incident wave is transmitted through the material
and part is reflected back into the medium that contained the incident wave.
The amplitude of the
reflection depends strongly on the angle that the incidence wave makes with the
boundary and the contrast in material properties across the boundary.
11.
What are the Ways to Tie Well-Log and Seismic Data? (5 marks)
Modal Answer
There are three ways to tie well-log and seismic data:
1. Stacking Velocities derived from seismic data.
Stacking velocities are essential in frontier plays
where other data do not exist.
2.
Velocity
Surveys and Vertical Seismic Profiles (VSP) where it gives the best velocity
control. It uses a surface source and geophones downhole.
3. Synthetic Seismograms derived from
well data.
12. What is the Synthetic
Seismogram? (5 marks)
Model
answer
For a prospect with some 2-D or 3-D seismic data, the target level
on the seismic data must be identified. With a “bright spot” play, a guess may
be made by observation. If there are no wells, it, of course, is a guess. A lot
of dry holes result from guessing wrong – even on 3-D seismic data. What is
needed is a way to tie depth-based log data from key wells into time-based
seismic data. In other words, a time-depth chart, or a velocity function
(because depth = velocity x time), is required. This process is used to
generating synthetics and calibrating them to real seismic data.
13.
Why We Need Color for Seismic Display? (5 marks)
Modal
Answer
Variable-intensity color is needed
rather than variable-area wiggle for four reasons:
1. Balanced appearance of positive and
negative amplitudes.
2. No overlap -- and therefore, no
clipping of higher amplitudes.
3. No mislocation of higher amplitudes.
4. Better visual dynamic range.
14. Write short note on
Vertical Resolution of seismic data.
(5 marks)
Modal answer
1. Seismic
resolution is the ability to distinguish separate features; the minimum
distance between 2 features so that the two can be defined separately rather
than as one.
2. For
thicknesses larger than 1/4 wavelength we can use the wave shape to judge the
bed thickness.
e.g. Velocity = frequency x
wavelength
3. Vertical
resolution decreases with the distance traveled (hence depth) by the ray
because attenuation robs the signal of the higher frequency components more
readily.
15. Discuss
briefly the Reflector relationships. (5 marks)
Modal Answer
Onlap - the
successive deposition of stratal packages toward the shoreline, often
progressively covering an erosional surface. Onlap occurs during transgression
as depositional environments backstep shoreward.
Downlap - the
successive depositon of stratal packages over underlying strata toward the
basin center. This is generally a progradational pattern, occurring during
relative sea level fall as sediment packages build farther out into the basin.
Toplap - the pattern
made by the deposition of a horizontal strong reflector above a succession of
downlapped or inclined packages of strata.
Offlap - a pattern of
stratal packages and their reflectors the both prograde and aggrade, building
upward and outward into the basin.
16.
Discuss the main Seismic facies types. (5 marks)
Model Answer
Continuous
reflectors - suggest sedimentary strata deposited in a
relatively stable environment that change periodically through time. Example:
continental shelf.
Discontinuous
reflectors - suggest sedimentary strata deposited in
regionally heterogeneous environments. Terrestrial and shallow water carbonate
depositional environments tend to produce discontinuous reflectors.
Chaotic
reflectors - suggest crystalline rock such as evaporites,
igneous, or metamorphic bedrock.
17.
Complete the following sentences: (10 marks)
1. The Tools of
Subsurface Analysis are Satellite images, Gravity & magnetics, Well logs,
Cores and Seismic data.
2. Well logs reflects Great vertical resolution,
Delimit bounding surfaces and Establish lithology of sediments penetrated.
3. Seismic data reflects Great lateral continuity
and resolution and define gross sediment geometry
4. Facies analysis of subsurface data depends on
Well logs and Seismic data
5. To delimit stratigraphic surfaces & identify
sediments penetrated in wells, the most important well logs are Resistivity
Logs, Spontaneous Potential (SP) Logs, Gamma Ray Logs, Neutron Logs, Density
Logs and Sonic (acoustic) Logs.
6. The Resistivity Logs measures resistance of flow
of electric current and are functions of porosity & pore fluid in rock and
frequently used to identify lithology.
7. Spontaneous Potential (SP) Logs, measures
electrical current in well and are result of salinity differences between
formation water and the borehole mud and used to separates bed boundaries of
permeable sands & impermeable shales.
8. In a seismic profile, the patterns of the
reflectors include Continuous reflectors, Discontinuous reflectors and Chaotic
reflectors.
9. The main patterns of reflector relationships are
onlap, offlap, downlap, erosional truncation, toplap, and concordance.
10. Interpreting seismic profiles include Continuous
reflectors, Clinoforms, Erosional truncation and unconformities
11. Ways to tie Well-Log and Seismic
Data
are Stacking Velocities derived from seismic data,
Velocity Surveys and Vertical Seismic Profiles (VSP)
and Synthetic Seismograms derived from well data
12. The basic four type's different
seismic waves are Compressional or P (for primary), Transverse or S (for
secondary), Love waves and Rayleigh waves.
13. Gamma ray logs record radioactivity of a
formation where shale have high gamma radioactive response and Gamma ray logs
infer grain size (and so subsequently inferred depositional energy) and are
most commonly used logs for sequence stratigraphic analysis
14. Neutron Logs use quantity of hydrogen present
and measure porosity of formation and interpret lithology when used with
Density Log.
15. Density Logs measure formation’s bulk density
and used as a porosity measure and differentiates lithologies with Neutron Log
and used with Sonic Logs to generate synthetic seismic traces to match to
seismic lines.
16. Sonic (Acoustic) Logs, are measure of speed of
sound in formation, and are tied to porosity and lithology, and used with Density
Logs to generate Synthetic Seismic traces to match to Seismic lines.
17. Seismic stratigraphic interpretation used to
Define geometries of genetic reflection packages that envelope seismic
sequences and systems tracts, and also used to Identify bounding
discontinuities on basis of reflection termination patterns and continuity.
18. Terminations below discontinuity, or upper
sequence boundary are Toplap termination, Truncation of sediment surface and
channel bottom.
19. Terminations Above a discontinuity defining
lower sequence boundary are Onlap over surface and Downlap surface.
18.
Interpret and
write a short report on the following exercises:
(10 marks)
A.
Exercise 1 (5 marks)
B.
Exercise 2 (5 marks)
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