Advantages and Disadvantages between these two Land seismic sources

1. LAND SEISMIC SOURCES
VIBRATORS VS. EXPLOSIVES
AUTHOR:
ENG. ALEJANDRO LEVY
UNIVERSIDAD PRIVADA DOMINGO SAVIO
TARIJA – BOLIVIA
2015

2. INTRODUCTION
Eng. Alex Levy
Experience:
Seismic Exploration Land/Marine
HSE Manager
Instrument Eng. Supervisor

3. ROCK TYPES
• Rocks can be classified into three main types, depending on the chemistry
of their formation
a. Igneous Rocks:
These rocks were formed by the cooling and
subsequent solidification of a molten mass of rock material, know as
magma.
b. Metamorphic Rocks:
Are those whose composition and texture has
been altered by heat and pressure deep within the Earth’s crust.
c. Sedimentary Rocks:
Sedimentary rocks are the weathered debris
derived by the slow processes of erosion of upland regions containing
other rock types.

4. SEDIMENTARY BASINS
• Sedimentary basins were formed over hundreds of millions of
year by the action of the deposition of eroded material and the
precipitation of chemicals and organic matter in the sea water.
• External geological forces then distort and modify the layered
strata.

5. • The following sequences of pictures show (exaggerated) the formation of
a typical basin.
• Sediment collects on the sea-bed, the weight causing subsidence.
• Different materials collected at the different times, so producing the
regular “layering” of strata in the basin.

6. • Volcanic action, or the movement of land masses, causes faults to
appear in the basin.

7. • These same forces cause rotation of the overall basin forming a
new mount range

8. • Erosion of the highlands, and additional subsidence forms yet another
area of low-lying land that is filled with water forming another ancient
sea.
• Additional sedimentation takes place, causing an “unconformity” in the
underlying strata.

9. • Finally, land mass movement causes folding and distortion of the
basin

10. OIL AND GAS FORMATION
• The temperature increases with depth within the Earth’s crust, so that
sediments, and the organic material they contain, heat up as they become
buried under younger sediments.
• As the heat and pressure increase, the natural fats and oils present in buried
algae, bacteria and other material link and form kerogen, an hydrocarbon that is
the precursor of petroleum
• As this source rock becomes hotter, chains of hydrogen and carbon atoms break
away and form heavy oil.
• At higher temperatures the chains become shorter and light oil or gas is formed.
• Gas may also be directly formed from the decomposition of kerogen produced
from the woody parts of plants
• This woody material also generates coals seams within the strata
• If the temperature and pressure gets too high, the kerogen becomes carbonized
and dose not produce hydrocarbons.

11. • The oil and gas produced by these processes may be in any
combination and are almost always mixed with water
• The minute particles of hydrocarbon are produced within the pores
of PERMEABLE ROCKS (i.e.: sandstone) and, being lighter than the
surroding material, move up through the rock until prevented from
doing so by an IMPERMEABLE ROCK.
• Although the initial source rock may only contain minute amount of
hydrocarbon, as the particles of oil, gas and water move or
MIGRATE, through the pore space within younger permeable rocks,
they coalesce into large volumes

12. • By the time this movement is stopped by the presence of a cap of
impermeable rock (or when they reach the surface) the total
hydrocarbon volume may be large enough to be a produce an oil or
gas field that will be profitable to develop.
• The ultimate profitability of such a field depends, of course, on
external economic forces and world demand as much as on ease of
extraction
• As seismic exploration is concerned with the imaging of sub-surface
structures, it is those structures that may indicate a potential
hydrocarbon trap that are of most interest to the explorationist

13. OIL & GAS FORMATION AND TRAPS

14. SEISMIC HISTORY
• The first use of explosive to delineate structures under the
earth was in the 1920’s and 1930’s in the Southern U.S. and
South America
• Digital processing and tape recording made a great
improvement in the seismic techniques in the 50’s

15. SO, WHAT IS SEISMIC EXPLORATION?
• An energy source produces sound waves that are directed into the
ground.
• These waves pass through the earth and are reflected at every
boundary between rocks of different types.
• The response to this reflection sequence is received by instrument
(geophones) near the surface, and recorded (Recording Truck) for
computer processing

17. SEISMIC TRACE
• The data recorded from one “shot” (one detonation of an
explosive or implosive energy source) at one receive position is
referred to as a seismic trace

18. • This seismic Trace is recorded as a function of time (the time
since the shot was fired)
• As this time represents the time taken for the energy to travel
into the earth, reflect, and then return to the surface, and the
vertical scale is measured in milliseconds.
• During the processing sequence these traces are combined
together in various ways, and modified by some fairly complex
mathematical operation.

19. MORE ABOUT SEISMIC TRACE
• When we are showing longer traces, or collection of traces, we will
resort to more conventional displays.
1. Shows a “Wiggle-Trace” display of a whole trace
2. A “Variable-Area/Wiggle-Trace” display, used for seismic sections
as it enhance continuity

20. SEISMIC PROFILE
• The display of many traces side-by-side in their correct spatial
positions produces the final “seismic section” or “seismic profile”
• The seismic profile provides the geologist with a structural picture
of the subsurface

21. THE IDEAL SEISMIC SOURCE
• Changes in the speed (velocity) of sound and the density within
particular rocks causes reflection and refraction of the sound
wave produced by a seismic source.
• Specifically, variation of these parameters at an interface
between two different rock types causes a reflection of some of
the seismic energy back towards the surface.
• It is the record of these reflections against time that produce
our seismic section.

22. • A seismic reflector can only reflect back to the surface an image of
the energy pulse it receives.
• If we send a complex pulse into the ground, that pulse will be
superimposed on every reflector we record.

23. • For this reason we wish to make the actual seismic source as
close as possible to a single pulse of energy - a spike.
• A spike of energy sent into the earth produces a set of clear
reflections.
• A more complex energy pule produces confused reflections

24. • In practice and ideal spike is impossible to achieve.
• As spike implies that an infinitely wide range of frequencies need
to be present in the source, all released over an infinitesimally
small time range.
• The earliest seismic surveys used explosives as a seismic source
with, for offshore exploration, up to 50 pounds (23 kg) of dynamite
being exploded just below the surface of the water.
• This is a very effective source, still used for onshore surveys, but is
environmentally obviously no longer a desirable source for offshore
acquisition.

25. ONSHORE SEISMIC SOURCES
• There are enormous logistical problems associated with
Onshore Seismic Exploration. (i.e.: Lakes, cities, etc )
• The seismic "line" must first be accurately marked out by
surveyors.

27. • This may mean painting marks on roads through residential
areas for example or cutting through dense jungle to mark shot
and receiver positions.
• In either case modern GPS equipment has simplified the
positioning

28. • Oil & Gas deposits tend to be in some of the more inhospitable
regions of the Earth, so the actual terrain conditions may limit the
available shooting / recording positions as well as define the costs of
the acquisition.
• Innovative seismic techniques are energizing exploration and
development activities in onshore areas, many of which have proved
difficult to image in the past.
• New seismic sources, acquisition methods and processing
approaches help illuminate reservoir hidden beneath complex near-
surface layers

29. SOURCES OF SEISMIC ENERGY
• Recent advances in source technology are further improving
data quality by putting more seismic energy into the earth at a
wider range of frequencies
• The ideal source for seismic exploration is an impulsive source
that concentrates its energy at a point in space and release it
instantaneously.
• In practice, sources have finite spatial size and emit signals
over a finite period, producing broadened wavelets that add
complexity to processing

30. • Land seismic data acquisition relies primarily on two types of
seismic sources – Explosives and vibrator trucks.
• Each has advantages and disadvantages.
• Surveys may be acquired using one type or both, and the choice
depends on several factors, including geophysical objectives,
cost and environmental constraints.

31. DYNAMITE
• Since the beginning of seismic exploration, dynamite has been
the universally accepted source for generating seismic energy
because it produces great quantities of energy.
• It is not expensive and it is safe when handled correctly.
• It is usable for both land and marine work in most climates and
field conditions.

32. CHARACTERISTIC OF SEISMIC DYNAMITE
• Explosive developed for seismic work use nitroglycerin and/or
nitrocellulose as active ingredients.
• The substances in their pure state are extremely dangerous and
highly volatile.
• However, when these highly explosive substances are absorbed by a
pores material such as wood pulp, kieselguhr, powdered chalk, or
roasted flour they are quite safe to transport, to store and use.
• Currently the gelatin dynamites are the most widely used in seismic
work world wide.

33. • They are classified into three general types:
1. Straight gelatins in which the nitroglycerin-nitrocellulose
colloid is the explosive ingredient
2. Ammonia gelatins in which ammonium nitrate is substituted
for part of the colloid
3. Semi gelatins which contain 60% nitroglycerin

34. • Some of the most important characteristics that seismic dynamite must
possess are:
High explosive power
High detonation rate
Great water resistance
Effective detonation under great water pressure
High density
Freezing resistance
Safety in handling

35. THE EXPLOSION
• Explosives are a very inefficient way to transfer energy
• The transfer from chemical energy to wanted seismic energy is
probably not more than a few percent of the available energy
• In land seismic work, most of the explosive energy is lost through the
fracturing of the shot hole walls, the generations of steam and by
heating the surrounding sediments
• Also, if the hole has not been tamped off properly it will lose energy
by blowing mud, water and debris out of the hole

36. EXPLOSIVES
• 1 kg of seismic explosive releases about 5MJ of energy almost
instantaneously.
• Charges can vary from a few grams to several tens of kilograms,
depending on the depth of the target reflectors.

37. • Where better surface conditions exist, or access is difficult, a
portable form of drilling rig may be used.
• Water & mud pumps, compressed air, emulsion and foam have all
been used to improve the circulation of the drill bit in different
conditions. The types of drill used extends from hand-held augers
to large truck-mounted hammer drills.
• Production rates for "conventional" (dynamite) exploration depend
almost entirely on the rate at which holes can be drilled.

38. • EMULSION HIGH EXPLOSIVE

43. SEISMIC EXPLOSIVES
• WesternGeco and Dyno Nobel developed dBX purpose-built seismic
explosive, the first explosive specifically designed for seismic use.
• The formulation offers significant geophysical benefits over
conventional explosive, optimizing energy transfer to the earth and
delivering higher S/N and greater bandwidth than dynamite.
• A comparison test in Canada demonstrated the capability of the
dBX source to improve imaging of deep reflectors

45. VIBROSEIS
• Vibrators are a Surface source
• In a Vibroseis survey, specially designed vehicles lift their weight
onto a large plate, in contact with the ground, which is then
vibrated over a period of time (typically 8-20 seconds), with a
sweep of frequencies.

46. • Seismic vibrators are the predominant source used in land seismic
exploration today
• The performance of a seismic vibrator is dictated by its actuator,
which is composed of a driven and a driving structure.
• The main element of the driven structure is the baseplate which is
pressed to the ground by weight of the truck
• The main element of the driving structure is the heavy reaction mass.
A piston inside the reaction mass is mounted above the baseplate
with a hydraulic system to drive the mass up and down

47. • During operation the vehicle moves into position and lowers the baseplate to the
ground, where it applies a compression to the earth. By controlling hydraulic fluid
flow around the piston inside the mass, the vehicle operator can make the piston
and base plate assembly move up and down at specific frequencies, transmitting
energy through the baseplate and into the ground.
• The base plate is often coupled with a large fixed weight known as the hold-
down weight
• During those parts of the cycle in which the reaction mass is moving down and
the base plate is moving up, the hold-down weight applies a compressive force
to keep the base plate in contact with the ground
• Harmonic distortions, or resonances, both in the vibrator and at the
earth/baseplate interface, can have the effect of additional upward-directed force
and must be considered in the selection of the desired vibrator output.
• Increasing the hold-down weight on the vibrator adds stability to the system and
helps establish optimal operating conditions.
• For coupling (base plate/ground) stability the hold down weight limit should be
between the 70-85%

50. • La energía desarrollada en un barrido pueden o no ser suficiente
para satisfacer el requisito propuesto (profundidad del objetivo que
quiero estudiar), y por lo tanto deberán ser emitidos otros barridos
(actuando los vibros en “flota” o grupos) cuyas energías serán
sumadas en el Sismógrafo, al cual llegan las respuestas que son
captadas por el dispositivo de recepción (ristras de Geófonos)

51. • The energy developed in a sweep may or may not be sufficient to
meet the proposed requirement (target depth), and therefore must
be issued other sweeps (acting the vibrators in "fleet" or groups).
• The fleet’s energy will be added in the Seismograph.

52. VIBS OPERATION

53. VIBS OPERATION

54. VIBROSEIS SYSTEM DESCRIPTION
• The Sercel Vibroseis System is composed of :
• A sweep generator,
• A vibrator to emit the sweep into the earth,
• A correlator to compress the long sweep into a short reflection
pulse,
• The correlator consists of a correlation process stage (FTP
board in the Central Control Unit) that detects the reflected
sweeps.

56. HOW TO GENERATE A VIBROSEIS BAND-
LIMITED SIGNAL
• The signal that makes it possible to have a band-limited component
amplitude spectrum, through Fourier Transform, is represented in
time like that shown in Fig. 1 - c.
• Unfortunately, the shape of that signal is not suited for the Vibroseis
technique that requires a long, low-power rather than short, high-
power signal.
• To describe the signal used in the Vibroseis technique, we have to
change the short, high-power signal (c) into a long, low-power signal
while preserving the limited bandwidth of the component amplitude
spectrum.
• This signal is virtually a sine wave, called sweep in the Vibroseis
terminology

58. • To expand a short pulse of high peak amplitude into a long sweep
of low peak amplitude you need to apply some frequency-
dependent delays. The energy in both forms of the signal (i. e.
pulse or sweep form) is the same. That’s why Vibroseis is not a low
energy system but a low power system.

59. • Naturally, in real-world situations we have to deal with multiple
reflectors, hence multiple reflections. If the reflection time is
shorter than the duration of the sweep, this causes the signals
picked up by the geophones to overlap:

60. • Where
• Trace (a) shows the sweep reflected from the first reflector,
• Trace (b) shows the same from the second reflector,
• Trace (c) is the signal detected by the geophone, i. e. the sum of traces (a),
and (b).
• Trace (c) is passed through the correlator to generate trace (d).
• The correlator boosts the signal and leaves the noise unchanged

61. • It should also be noted that for a given sweep amplitude in
Vibroseis the way of increasing the energy in the sweep is to
increase its duration or/and to increase the number of vibrators.
The fact is, it is the long duration of the sweep that allows us to get
the necessary energy into the ground. So, the peak amplitude of the
correlator output improves with the duration of the sweep.

62. • The side lobes of the auto-correlation function of a sweep can
be reduced by tapering the start/ends of the sweep.
• It is important to consider that the ground can be
mathematically consider as a “Low Pass Filter” since attenuation
is greater at higher sweep frequencies.

63. • In the Figure, an 8 seconds sweep length is plotted (Sweep
Length: 8 sec) where the time Seismic Data Acquisition is "12-
second" (Record Length: 12 sec).
• The final registration Correlated possess the length of time
called "listening time" which is equivalent to the time required
for the last component of the original sweep to travel to the
deeper reflector horizon, chosen as a target, and return to the
surface (in dynamite, because the duration of the event is
infinitesimal, then "listening time" is the "total time of
acquisition)

65. THE VIBROSEIS CONCEPT - SIGNALS USED
IN VIBROSEIS OPERATIONS
• One of the most important characteristic of the Vibroseis
method is the limitation of the bandwidth of the source.
• By this way, the Vibroseis technique allows us to generate only
those frequencies we actually need whereas with an impulsive
source like dynamite, some of the frequencies generated by the
blast are ignored during the seismic acquisition.

66. • The Explosive source develop its power in a very short time
(theoretically “cero”)
• Vibrational Sources (vibrators) distribute their power for a
sustained period of time, usually several seconds

67. CONCLUTIONS
VS
.

69. EXPLOSIVES
ADVANTAGES
Dynamite is a high –power source
of short duration
As such, it creates a compact
wavelet with a wide bandwith
Other advantages over vibrator
trucks are its light weight, low cost,
lack of required maintenance and
capacity for deployment in rugged
terrain unreachable by vehicles
DISADVANTAGES
 The process of drilling shot holes, burying the
dynamite and cleaning up after the operation is
labor intensive, and with this option the survey
geometry cannot be changed without drilling
new shot holes.
 The input signal can be neither measured nor
reliably repeated.
 Explosive sources are subject to strict security
regulations and permission for use and
transportation may be difficult to obtain in
some places.
 The potential for causing damage prevents their
use in populated areas.

70. VIBRATORS
ADVANTAGES
The energy spectrum can be
controlled easily.
The force applied to the ground can
be monitors and adjusted in real
time.
Can be used in urban areas and can
be equipped with special tires or
track for deployment in
environmentally sensitive areas,
such as sand dunes or arctic
snowpack.
DISADVANTAGES
The restriction of access in difficult
terrains like swamps, mountains
and coastal areas.
Fleets of vibs are expensive and
their maintenance as well.
The input signal is not impulsive,
so additional processing is required
to extract interpretable data. A
recorded trace is correlated with a
reference trace to extract the
reflected signal

71. REFERENCES AND ACKNOWLEDGEMENTS
• References:
 Seismic Course – Robertson Research International Limited – U.S. 1998
Training Course – Sercel – France 2008
Land Seismic Techniques for High Quality Data – Schlumberger – Norwa
 Acknowledgements:
Glen-Allan Tite – Seismic Auditor
 Ian Smith – Seismic Supervisor