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I.
Answer the following questions: (12 marks)
1. Why Seismic Stratigraphy? (6 marks)
2. How to generate a seismic profile? (6 marks)
Modal answer
1.Why
Seismic Stratigraphy:
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.
2.How to
generate a seismic profile?
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 relector 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.
II. Write short note on
each of the following: (12 marks)
1.
Seismic profiles related to
geologic cross sections. (4
marks)
2. Waves Travel Times (4 marks)
3. Ways to Tie Well-Log and Seismic Data (4 marks)
Modal
answer:
1.
Seismic profiles related to
geologic cross sections.
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 that must be kept in
mind 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. Travel time is partly a function of thickness, but it
is also a function of acoustic impedance, therefore seismic profiles distort
true thickness. Also, angles of layering and of faults shown in the profile are
distorted. If the acoustic characteristics of the different layers in a profile
can be determined from drill hole data, then travel time can be converted to an
estimated depth in meters.
2. Waves Travel Times
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 (a 24-hour time system used in many sciences).
3.
Ways to Tie Well-Log and Seismic Data
There are
three ways to tie well-log and seismic data:
1.
Stacking Velocities derived from seismic data. It provides the poorest time-depth control. There are several reasons for
this, such as the processors’ need to avoid multiples and the limited offsets
of real 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. The
checkshot uses “first breaks” (first reception of energy downhole after the
shot), while the VSP analyzes the full sonic waveform over more closely-spaced
geophone positions.
3. Synthetic Seismograms (Synthetic Seismic
trace) derived from well data.
III. Answer the following: (12 marks)
1.
. What is the Synthetic Seismogram? (6 marks)
2. Why We Need Color for Seismic Display? (6 marks)
Modal
answer:
1.
Synthetic Seismogram (Synthetic
seismic trace):
1.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.
2.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.
3.The most common problem--the synthetic does not tie--may be
solved through use of other data and preparation of a suite of synthetics from
a range of parameters. Reasonable compromises may be made for inadequate log
data.
4.The process of generating synthetics and calibrating them
to real seismic data is as much an art as a science.
5. A synthetic
seismogram is created to simulate seismic data acquisition in the computer. The
unknown physical properties of the earth beneath a seismic survey are known
properties at a wellbore – P-wave acoustic velocity and bulk density. In
acquiring seismic data, at the simplest, a seismic compressional wave (P-wave)
is generated with a surface source; the wave travels at the acoustic velocity
of the rock, which varies with lithology; the wave bounces off surfaces across
which the impedance – the product of velocity and density – varies.
6. The strength of the reflection is measured with a
reflection coefficient, which is the difference in impedance over the sum of
the impedances. The wave then returns to the surface, where geophones detect
the P-waves returning vertically. The time from generation of energy to its
recovery at a geophone is the travel time; it depends on the velocities of the
units traversed. The amplitude of the recovered energy is governed by the
contrasts in velocity and density across the interfaces.
7. All the various geophone groups are recorded; data are
processed, and an output section is generated. The interpreter then tries to
identify target reflectors in time, analyze the seismic response to geology and
fluids, convert time to depth, and drill.
8. In other words, a source wave is sent through a velocity
field and a series of reflectors which yield seismic data, or:
(Source Wave)*(Velocity
Field)**(Reflection Series) = Data
9. Geology and hydrocarbons control both velocities and
reflections. The objective is to resolve the reflection series using the data,
knowing the source wave and estimating the velocity field:
(Reflection Series)=Data*((Source Wave)*(Velocity Field)**) -1
10. This is an “Inverse Problem.” There are direct and useful
ways to do this (called “seismic inversion”), if the phase of the data is
known, and the very low-frequency components of velocity and density that are
not captured in seismic data can be added back.
11. One of the simplest ways to work the inverse problem is to
take sonic velocity and density data from wells, run the seismic experiment
with the sonic-derived velocity field and the sonic- and density-derived
reflection series, assume a source wave similar to the seismic data, and
compare the result to the data.
12. The well data can be
varied to match what might exist away from the wellbore--but within the seismic
survey. This can be done before the survey is acquired to answer a question
such as:
2.
Why We Need Color
for Seismic Display?
Variable-intensity color is needed rather than
variable-area wiggle for four reasons:
·
Balanced
appearance of positive and negative amplitudes.
·
No
overlap -- and therefore, no clipping of higher amplitudes.
·
No
mislocation of higher amplitudes.
·
Better
visual dynamic range.
IV.
Answer the
following questions: (12 marks)
1. Write short note on Vertical
Resolution of seismic data. (6 marks)
2. Discuss
briefly the Reflector relationships. (6 marks)
Modal answer
1.
Vertical Resolution of
seismic data
One of the most common
questions made by people to me about reflection seismology is on the level of
detail that I can see. 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. For thicknesses larger than 1/4
wavelength we can use the wave shape to judge the bed thickness.
e.g. Velocity = frequency x
wavelength
There is a practical
limitation in generating high frequencies that can penetrate large
depths. The earth acts as a natural filter removing the higher frequencies
more readily than the lower frequencies. Vertical resolution decreases with the
distance traveled (hence depth) by the ray because attenuation robs the signal
of the higher frequency components more readily.
2.
Reflector relationships
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.
V. Discuss briefly each of the
following: (12 marks)
1. Structural
features in seismic cross section (6
marks)
2. Seismic facies
(6 marks)
Modal answer
1. Structural features in
seismic cross sectios
Folds and faults can be recognized
on seismic profiles. Fault surfaces do not show up as distinct reflectors. Even
if there is a distinct difference in acoustic impedance across the fault, the
generally high angle of many faults (>45°) results in a weak reflection
signal to the surface. Generally, faults are recognized as disruptions running
through a vertical sequence of horizontal reflectors.
2.
Seismic facies
Some general information as to the nature of
the rock in a seismic profile can be gleaned from the patterns of the
reflectors.
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.
VI.
Complete the following sentences: (14 marls)
1. The Tools of Subsurface Analysis are
…………………., ……………… & ……………, ……………………, ………………… and ………………….. data.
2. Well logs
reflects …………………, Delimit ………………….. and Establish ……………… of sediments
penetrated.
3. Seismic data
reflects ………………………. and ………………. and define …………………. 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 …………………. Logs, ……………………..Logs, ……………………………. Logs, ……………………. Logs,
…….…………….. Logs and ………………………………..…………. Logs.
6.
The Resistivity Logs
measures resistance of ……………………………………. and are functions of ……………… &
……………………… in rock and frequently used to identify …………………………………….. .
7.
Spontaneous Potential (SP)
Logs, measures …………………………… in well and are result of ………………………………. between
……………………… and the ……………………………… and used to separates bed boundaries of
permeable …………………. & impermeable ……………………...
8. Gamma ray logs record ………………………… of a
formation where shale have ………………. gamma radioactive response and Gamma ray
logs infer …………………… and are most commonly used logs for ……………………………………….
analysis
9.
Neutron Logs use quantity
of …………………………. present and measure …………………. of formation and interpret
…………………….. when used with Density Log
10. Density Logs measure …………………………………………….. and used as a
……………….. measure and differentiates lithologies with ……………………. and used with
Sonic Logs to generate ………………………………… traces to match to ………………………….. lines
11. Sonic (Acoustic) Logs, are measure of ………………………… in
formation, and are tied to ………………….. and ……………….., and used with ………………… Logs
to generate ………………………………… traces to match to Seismic lines
12. Seismic stratigraphic interpretation used to ………………………….. of
genetic reflection packages that envelope seismic sequences and ………………………, and
also used to Identify ………………………………… on basis of reflection termination patterns
and continuity.
13. Terminations below discontinuity, or upper sequence boundary are
……………………….., …………………………………….. and ………………………………….
14. Terminations Above a discontinuity defining lower sequence
boundary are …………………………………… and ………………………………………
Modal Answer
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.
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
9.
Neutron Logs use quantity
of hydrogen present and measure porosity of formation and interpret lithology
when used with Density Log.
10. 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.
11. 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.
12. 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.
13. Terminations below discontinuity, or upper sequence boundary are
Toplap termination, Truncation of sediment surface and channel bottom.
14. Terminations Above a discontinuity defining lower sequence
boundary are Onlap over surface and Downlap surface
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