Land Seismic Sources - Explosives Vs. Vibroseis

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  • 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
  • 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
  • 16. 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
  • 17. • 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.
  • 18. 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
  • 19. 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
  • 20. 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.
  • 21. • 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.
  • 22. • 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
  • 23. • 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.
  • 24. 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.
  • 25. • 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
  • 26. • 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
  • 27. 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
  • 28. • 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.
  • 29. 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.
  • 30. 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.
  • 31. • 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
  • 32. • 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
  • 33. 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
  • 34. 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.
  • 35. • 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.
  • 37. 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
  • 38. 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.
  • 39. • 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
  • 40. • 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%
  • 41. • 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)
  • 42. • 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.
  • 45. 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.
  • 46. 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
  • 47. • 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.
  • 48. • 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:
  • 49. • 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
  • 50. • 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.
  • 51. • 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.
  • 52. • 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)
  • 53. 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.
  • 54. • 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
  • 56. 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.
  • 57. VIBRATORS ADVANTAGES The energy spectrum can be controlled easily. The force applied to the ground can be monitors and adju
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