Болсуновская Л.М., Абрамова Р.Н., Матвеенко И.А. (ред.) Petroleum engineering. Teachers book. Нефтегазовое дело. Книга для учителя
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UNIT 3
SEISMOLOGY AND SEISMIC SURVEY
Lead in. Listen to the song “Seismic waves” and fill in the gaps with the
words from the tape.
1. happening, 2.terror, 3. drama, 4. building, 5. rebuild, 6. focus,
7. faster, 8. Richter
T. elicits the ideas from the students as they suggest things that can serve as
the answers to the questions given below. This activity will prepare Ss to
work with the topic of the Unit 3.
Is there any difference between seismology and seismic survey?
They represent different activities but seismology is actually a predecessor of
seismic survey. Their fundamental principles are the same. Unlike
seismology in seismic survey energy sources are controlled and they can be
easily relocated. Besides, the distance or spacing between shot points and
receiving points are relatively small. For seismic wave generation explosives
or vibrating machines are used. The generated vibrations are then registered
by means of special arrays of receivers.
What is the main aim of seismic survey?
The main aim of seismic survey is to get information about rocks, especially
the mode of their occurrence by timing wave arrival time and amplitude
variations as well as by determining frequency and signal type.
What devices are used for registration of vibrations?
The generated vibrations are registered by means of special recording
systems which may include seismometer cable assembly, arrays of receivers
arranged in different ways, central recording units, or recording trucks.
What types of waves do you know?
There are two major types of seismic or shock waves. They are body and
surface waves. The body waves travel through the interior of the Earth and
they are divided into primary or P-waves and secondary or S- waves. The
surface waves travel along the surface and they demonstrate the greatest
destructive power during the earthquakes. There two types of such waves:
love-waves or L-waves and Rayleigh waves or R-waves. Seismic survey is
mostly concerned with measuring body waves.
What is a seismogram?
Seismogram is data recorded on paper or tape during seismic survey, which
consist of several traces, each of which shows the ground vibration variations
registered by one receiver array depending on time that was required for the
energy to return to the surface.
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Exercise 5. Form nouns from the following verbs.
Reflect–reflection/ reflector, form–form/ formation, interpret–
interpretation/ interpreter, explore–exploration/ explorer, determine–
determination, record–record/ recorder, process–process/ processing,
produce–producer/ production, require–requirement, characterize–
characteristics/ characterization, influence–influence, calculate–
calculation/ calculator, apply–application, describe–description.
Exercise 6. Match the words in the right column with their synonyms in the
left one.
1. F , 2. E, 3. I, 4. G, 5. C, 6. H, 7. D, 8. A, 9. B.
Exercise 7. Match the words in A with the words in B to form collocations.
1.D; 2. K; 3. B; 4. J; 5. G; 6. E; 7. I; 8. A; 9. C; 10. H; 11. F.
Exercise 8. Choose the most suitable word or word-combination.
1. travel time, receptor; 2. reflection profiles; 3. propagates, encounters; 4.
source array, acquisition project; 5. data sets; 6. impact 7. variables; 8.
exponentially; 9. medium; 10. transition zone.
Exercise 9. Compile sentences.
1. Geophysicists use seismic technology to interpret data to map
structural maps.
2. Hydrophones and geophones are used to record the reflected energy
waves.
3. Non-geophysical issues are terrain, explosive use and storage
environmental concerns.
4. Seismic survey may reduce the number of unsuccessful wells.
5. We need to know the average velocity for absolute depth
determination.
Exercise 10. Reconstruct the order of operations that are performed in
seismic survey.
4; 2; 5; 3; 7; 1; 6.
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Exercise 11. State if these sentences are true or false. Add more
information if they are true and correct them if they are false.
1. F – Not necessarily to find oil because it gives information only on
subsurface structure of rock formations and can detect structural traps
that could potentially contain hydrocarbons.
2. F – It has been improved exponentially with development of 3D
modeling systems, satellite systems and so on.
3. F – The general principle of reflection seismology consists in recording
reflected sound waves using geophones or hydrophones onto a tape
over a predetermined period of time.
4. T – The most difficult of which is survey in the transition zone as it has
some characteristics of both land and marine environment and it
requires complex methods.
5. T – Because different parameters require particular techniques or a
combination of several methods.
6. F – In the land environment the choice of source array is made
between dynamite shot holes or mechanical vibrators.
7. T – As well as amount of energy and power required, security and
environmental protection issues.
8. F – all these should be taken into account because operations with
explosives in surveys can threat a number of injuries and sometimes
cause air or water pollution and so on.
Exercise 12. Match the pictures with the extract for illustration of which
they can serve.
A 6 (shows the general principle); B 7(shows how reflection rays
propagate); C 2 (shows vibroseis truck); D 4 (shows air gun array); E 1
(shows structural trap); F 9 (shows impedance equation); G 5 (shows
seismic survey in a borehole); H 8 (shows how distance (depth) is related
to velocity or time); I 3 (shows marine survey)
Exercise 14. Describe the method of geophysical exploration shown in the
diagram.
Suggested answer
In seismic survey sound energy waves are sent into the ground by means
of dynamite or Vibroseis. When they encounter the boundaries, different
rock layers within the Earth’s crust reflect back the energy. Then
hydrophones and geophones record the reflected energy waves over a
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predetermined set of time. The reflected signals are output onto a storage
medium. After that the recorded data are processed with special software.
As a result of data analysis seismic profiles are produced. These profiles
or data sets are then interpreted for possible hydrocarbon reserves.
Exercise 16. Render the information from the text into English
Suggested answer
Refraction method
The main difference between reflection and refraction methods grounds in
the fact that in refraction seismology the distance between shot point and
receptors is greater than the depth of mapping limits, while it is smaller or
equal to the depth of reflection boundaries in reflection seismology. That
accounts for the fact that in refraction surveys wave propagation lines are
mainly vertical. The head waves or refracted waves encounter a layer
characterized by high velocity leaving it at a critical angle. Analysis of
refracted wave propagation line can only detect a layer in which the wave
velocity is substantially lower than in overlying beds. In this respect the
refraction method has more limited application than reflection method.
As refracted waves are usually registered on much greater distances than
reflected waves, much more powerful source arrays are required in this
method. The arrangement of receivers along observation line results in
weakening of head waves that are affected by displacement factor. As a
consequence the receivers are arranged radially or perpendicular to the
line that crosses the shot and receiving points.
Exercise 17. Compare and contrast the methods shown in the pictures.
Mention such aspects as characteristics of wave propagation, source arrays
and application:
Suggested answer
The first picture shows the reflection method while the second illustrates
the refraction method.
The reflection method differs from the refraction one in the way that the
distance between the shot point and receptors exceeds the depth value of
mapping limits. In the refraction method the distance is the same or
smaller than the depth of reflection boundary. As a result a wave
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propagation line or incidence is vertical in reflection seismology and
predominantly horizontal in refraction seismology.
Because refractions are registered on the large distance, the energy source
should be more powerful than in reflection method.
As head waves are greatly influenced by displacement factor, they
become weaker in refraction method, so the receivers are usually placed
radially or perpendicular to the observation line. In reflection method they
are grouped along observation line.
The refraction method has much more narrow application than the
reflection one because it can only help to detect layer which is
characterized by much lower wave velocity than overlying bed. The
reflection seismology can be applied for hydrocarbon and mineral
exploration. The refraction seismology is commonly used to determine the
thickness of unconsolidated materials overlying bedrock (overburden
thickness) and depth to the water table. This method is used for
characterizing the geological framework of ground-water contamination
studies and for assessing geologic hazards and archaeological studies.
Exercise 20. Expand the given sentences in any way you can. Give details,
describe the definite situation.
Suggested answers
1. The capabilities of reflection seismology increased as a result of a
number of recent innovations.
2. This highly effective device reduces uncertainties of calculations in
seismic data interpretations.
3. Gas chimneys of industrial enterprises plaque the surrounding area.
4. The hand becomes insensitive to temperature when it is frozen.
5. The terrible event broke the natural continuity of historical
development.
6. The full-vector wave-field imaging was performed on the basis of
shear and compressional wave recordings.
7. The ground penetrating radar captured wrong data due to its
unqualified application.
8. 3D seismic technology advanced efficiency of reservoir modeling.
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Exercise 22. Match the words in A with the words in B to make
collocations.
1. H, 2. D, 3. A, 4. F, 5. B, 6. I, 7. J, 8. E, 9. C, 10. G.
Exercise 23. Fill in the blanks with correct prepositions.
1. for, of; 2. for, up to; 3. across, of; 4. by, in; 5. among, in.
Exercise 24. Compile sentences.
1. Reflection seismology is at present the most accurate method.
2. 3D-seismic technology is applied to reduce uncertainties.
3. Full-vector wave-field imaging includes shear and compressional
waves.
4. Gas chimneys plague economically important areas.
5. Reflection seismology is extensively used in exploration for
hydrocarbons.
Exercise 25. Fill in the gaps with words which are made from the word
given at the end of each line.
1. advancement;
2. uncertainty;
3. acquisition;
4. insensitive;
5. executive;
6. capabilities;
7. compressional;
8. valuable;
9. continuous;
10. inaccuracy
Exercise 26. State whether the following sentences are true or false, correct
the false ones.
1. F – 3D-seismic technology
2. F – gas chimneys plaque them
3. F – shallow
4. F – the most accurate
5. T
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6. F – combining P- and S- waves
7. T
Exercise 27.
You will hear the report on seismic waves. Match the terms
with their descriptions. One of them is extra.
1. C, 2. F, 3. D, 4. E, 5. A, 6. B, G is extra (it is about surface waves)
Tapescript
Seismic Waves
Earthquakes generate two main types of seismic or shock waves: body
waves and surface waves.
Body waves travel through the interior of the Earth. The fastest of these
are primary or P-waves. These compressional waves move faster in dense
rock and slower in fluids thus their speed and direction change. Because
they are deflected by the Earth core, P-waves are not seen in the so-called
shadow zone.
The slowest body waves are secondary or S-waves. S-waves are elastic
shear waves that move sideways at right angles to the direction of travel.
Because S-waves travel only through solids, they do not penetrate the
Earth’s outer molten core. For this reason there is also an S-wave shadow
zone.
The slowest seismic surface waves do not penetrate the Earh interior but
follow the surface. One type of surface waves, the love-wave travels in a
circular motion and causes downage by displacing surface material or
residing. Love-waves can be very damaging. This crack pavement and this
twisted railway clearly demonstrate their destructive power. Earthquake
downage also results from rayleigh waves, surface waves which deform
material vertically.
Exercise 29. For the gaps 1–5, 6–10, choose one of the words on the right
(1–6) that best completes the gap in the text. You can use each word only
once. For every 5 gaps, there is one extra word.
1–3 (conductivity);
2–2 (decreases);
3–6 (strength);
4–4 (penetrate);
5–1 (limestone);
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6–3(high-frequency);
7–4(encounters);
8–5 (variations);
9–6 (reflection);
10–2 (impedances).
Exercise 34. Choose the necessary word from the given ones.
1. undertook;
2. submerged;
3. two-way travel time;
4. trough;
5. spaced;
6. convention;
7. deflection;
8. bursts.
Exercise 35. Give English equivalents of the words in the brackets.
1. conventions;
2. trough, peaks;
3. is intercepted;
4. sequence, vessel;
5. bursts, separation;
6. common depth point;
7. echo-sounding was undertaken;
8. plotted.
Exercise 36. Fill in the gaps using the terms from the text.
1. geological structures,
2. gravity and magnetic surveys,
3. echo-sounding,
4. bursts,
5. two-way time,
6. ray path,
7. wiggle traces,
8. magnitude of geophone movements,
9. right, black,
10. shades of gray
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Exercise 40. Read the text and state if the sentences are true or false.
1.T; 2. F; 3. T; 4. F; 5. T; 6.T; 7. F; 8. T; 9. F; 10. T.
Exercise 42. Match the terms with their definitions.
1. F, 2. I, 3. J, 4. C, 5. B, 6.E, 7. D, 8. G, 9. A, 10. H
Multiple
A repeated reflection within a rock formation
Ringing
Reflections between the water surface and the seabed
Diffractions
Hyperbolic reflections from the broken end of a
reflector which mimic arched formations
Random noise
Unwanted reflections from within rock layers,
horizontally propagated and refracted sound, bubble
pulsations from the airguns
Deconvolution
Processing technique which aims to counteract the
blurring of reflected sound by 'recompressing' the
sound to the clean 'spike' emitted from the source.
Muting
Technique that cuts out parts of traces embodying
major defects such as non-reflected signals
Fltering
Processing step which removes undesirable noise to
enhance the best reflections
Migration
Technique that corrects distortions caused by plotting
inclined reflectors
CDP gather
Wiggle traces from a single CDP collected into groups
and displayed side by side in sequence
Normal move
out (NMO)..
Correction which is needed to bring the pulses to a
horizontal alignment, as if they all came from vertically
below the sound source
Exercise 50. Replace the underlined words or word-combinations with
synonyms from the vocabulary.
1. slantwise;
2. deduced;
3. adjacent;
4. 'isopach' maps;
5. multiple;
6. drained;
7. enable;
8. revealed;
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9. interrelationship;
10. installed.
Exercise 51. Reconstruct the jumbled sentences.
1. Improved resolution and reduced acquisition time enabled shooting
seismic at different time intervals over the same area.
2. Modern streamers have multiple global positioning system sensors that
constantly record the position of the streamers.
3. It can be economic to permanently install an array of hydrophone
beneath the seafloor.
4. The paravane reduces the drag of the streamer array, which can be
sufficient to stop a powerful vessel.
5. These techniques allow to acquire a swath of seismic data in a short
time.
6. Increased computing capacity led to the first reliable seismic images.
7. Seismic sections are digitized and the digital files entered into a
gridding and contouring program.
8. The dynamic changes with time will clarify how a field is behaving.
Exercise 52. Match the parts of sentences.
1. B, 2 C, 3. E, 4. A, 5 G, 6 D, 7 F, 8. I, 9. H.
Exercise 53. Reconstruct the order of sentences.
1. I, 2. F, 3. D, 4. J, 5. A, 6. B, 7. H, 8. E, 9. G
Seismic sections picked by hand are digitised and the digital files entered
into a gridding and contouring program.
Some rock layers produce wiggles with a distinctive character that can be
followed right across a section.
Other layers may be identified by comparison with synthetic
'seismograms'.
The interpreter studies the maps to identify areas that are shallower and
form an anticline or a shallow area surrounded by faults.