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Introduction to Seismic Method


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Introduction to Seismic Method utilized during Survey

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Introduction to Seismic Method

  1. 1. Introduction to Seismic Method:
  2. 2. • Measurement of seismic-wave travel time is one of the most common geophysical method. • Seismic exploration is divided into refraction and reflection surveys, depending on whether the predominant portion of the seismic waves’ travel is horizontal or vertical. • Refraction seismic surveys are used in engineering geophysics and petroleum exploration. • Seismic reflection surveys, on the other hand, detect boundaries between different kinds of rocks; this detection assists in the mapping of geological structures. • Seismic energy is detected on land by using devices called geophones, which react to on-site ground motions. SEISMIC METHOD
  4. 4. OBJECTIVES REGIONAL EXPLORATORY INVESTIGATIONS • Delineation of structural and stratigraphic anomalies FIELD DEVELOPMENT • To mark suitable locations for field development and for reservoir studies PROSPECT DELINEATION • To mark the depth of prospective horizon and trap closures
  5. 5. PREPARATION AVAILABLE BUDGET & TIME AVAILABLE DATA Geological Data Geophysical Data Other Information
  6. 6. PLANNING & DESIGNING 1. ACQUISITION PARAMETERS 2. BID INVITATION a. Minimum Offset b. Maximum Offset c. Source Interval e. No. of Channels d. Receiver Interval g. Source (Dynamite or Vibroseis)
  8. 8. INSTRUMENT FOR SEISMIC SURVEYING 1. Geophone. 2. Seismic Cable. 3. Seismograph. 4. Energy Source. 5. Seismic Crew.
  9. 9. Weight drop Dynamite SOURCE TYPE Vibroseis Hammer
  11. 11. •The blast of explosives produces a very strong energy, •Placing of the explosives to the bottom of a hole from 1 to 35 meters,)and the energy is better distributed. •In order to make the holes, a hammer perforator, an auger or drilling rig is used. •In the sand, a simple metallic tube is very efficient. •In some desert regions, where the drilling is impossible, explosives can be suspended and exploded in the air about 1.5 m from the ground •This method, very noisy & dangerous
  12. 12. VIBROSEIS
  13. 13. FIELD PROCEDURE • Planting geophone along a straight line and detonating. • Generating seismic waves by detonating explosives on one end of a line. • Recording seismograph. • Picking 1st arrivals. • plotting of data to determine V1V2h1
  14. 14. Trimble Man Pack - 4700 GPS • GPS used by the Survey Crew for locating the Source & Receiver Points of the Seismic lines, carried on their shoulders External Batteries and Receiver • Its is connected with two external batteries and a receiver which receives the signal from the satellite.
  15. 15. Shot Points Receiver Points • After locating the points, the crew mark the places of source and receiver points finally marking a seismic line Source Points with red paint Receiver Points with white paint
  16. 16. Drilling Jackhammer • Drilling Crew uses Jackhammers for shallow holes Man Portable Drilling Rig • For deep holes, Man Portable Drilling Rigs are used
  17. 17. Dynamite Stick for Deep Shots • After drilling shot holes dynamite is loaded in them, using dynamite sticks Detonator • Detonators are placed in the dynamite stick, used to initiate the blasting of dynamite
  18. 18. • Lowering of Charge in a Drilled Hole with detonator in them • Hole is loaded and filled with Sand • Loading is done and place is marked
  19. 19. Geophone Stings Takeout Point of a Takeout Cable • This crew first of all layout the Geophones on the seismic lines as spread. • These geophones are connected to each other by the Takeout Cable
  20. 20. CABLE & GEOPHONE Link cableGeophone string
  22. 22. CABLE LAYOUT
  26. 26. Station Unit Box • Takeout cable is then connected to Station Unit Box Crossing Station Unit Box • All the Station Unit Box is further connected with one Crossing Station Unit Box (of an array) • Then yellow cable called Jumper Cable is finally connected to Recording Truck through which all the information goes into it
  27. 27. RECORDER
  29. 29. Color Line Monitor Recorder Tape Driver • Monitor on which the Observer control all the activities on a seismic line • The recorder which records all the data coming through cables • The tape driver records all the data on the magnetic tapes or cartridges
  30. 30. Camera Seismic Source Synchronizer • The thermal plotter used to make a hard copy of the recorded data • Device which synchronizes the blaster. Before shooting, recorder presses ARM button to charge the detonators, at the same time the blaster operator presses the ARM button in the blaster • Then recorder presses GO button to blast the dynamite
  31. 31. Portable recording system SEISOMOGRAPH
  32. 32. Blaster • Bag carried by the shooting crew is called The Blaster, it works in coordination with the Seismic Source Synchronizer • “Blast view” on seismic line
  33. 33. A recorded result of shot point after the blast SEISOMOGRAM
  34. 34. A recorded result of shot point after the blast SEISOMOGRAM
  35. 35. • After all the shots are recorded every thing is wounded • The data is send to the Operator Company • The place is restored by the Green Team and inspected by the Client Company • Then Operator Company sends data to DGPC
  36. 36. SPREAD GEOMETRY The most common recording type consists to send seismic waves from one shot point and to record reflected waves with many (48, 96 or more) geophones regularly disposed on the ground along a seismic cable. The CDP is the reflection point in depth, then CMP is the midpoint on the ground between shot and a geophone. With a switch (roll-along switch), the observer can "displaces" the recording geophones along the line with a constant number of actives geophones (24, 48 or more). The aim of this process is to add the travels having a reflective common point, in order to improve the signal / noise ratio. With 48 active geophones, there are 24 travels adding on a common reflector (Fold 24).
  37. 37. Basic Spreads
  38. 38. The shot or the vibration point can be situated: • At the extremity of the active geophones (End shot) • At the center of the geophones. (Split spread shot) • At a certain distance (offset) of first geophone The last geometry allows to have a long distance of offset, The second is more adapted if the reflectors are tilted.
  39. 39. NOISE Noise is the information on the seismic record which one does not wish to use. TYPES OF NOISE: 1. Random Noise or Incoherent Noise 2. Coherent Noise
  40. 40. RANDOM NOISE OR IN COHERENT NOISE It is the seismic energy that does not align up from trace to trace or record to record on seismic record It displays no systematic pattern. This noise is uncorrectable. We can overcome random noises by recording more than one traces from the same location. Sources of Random Noise: i. Wind Noise ii. Water Flow Noise iii. Small Movement With in the Earth iv. Local Noise v. Bad Geophone Noise
  41. 41. COHERENT NOISE It is seismic energy which aligns from trace to trace or record to record on seismic record. • Often, it is very similar to the signal and usually more difficult to overcome than the incoherent/random noise. • By examining the patterns of coherent noise, we can devise field procedures to reduce it. Source size and source depth , Electric Filtering, Receiver Arrays, Source Arrays, Deconvolution and Editing and Muting. Sources Of Coherent Noise: i. Multiple Reflections ii. Direct Arrivals and Refracted Waves iii. Diffraction Events iv. Ground Roll
  42. 42. • Head waves are always the first events seen on a record. • They are linear and often very visible and strong. • Sometimes they can disappear with large offset. • Many geophysicists try to remove head wave using •surgical mute, •FK filtering •adapted NMO stretch mute value, • but in some case, uppermost reflection and refraction can be very close and difficult to distinguish.
  43. 43. Refracted Waves In Seismic reflection survey the refracted events comes as first arrival.
  44. 44. Ground Roll Ground rolls are Rayleigh waves which appear as second arrival on seismic section.
  45. 45. • Surface waves (Rayleigh, Love) primarily Rayleigh are – low velocity, – low frequency signal, – With relatively higher amplitude seen below shots. They override the useful reflections. • They can be reduced using – frequency filtering – FK filtering.
  46. 46. Diffracted Events At faults and some unconformities diffracted waves are generated on seismic section.
  47. 47. • The seismic reflection takes an interest essentially to the P compression waves, in the setting of seismic studies for the geo-technical; • the recording of the S waves can to prove out to be useful for the shearing modules calculation. • In knowing the velocity of the P waves and S, it is possible of calculating the Poisson’s ratio:
  48. 48. Refraction Methods Advantages Disadvantages Refraction observations generally employ fewer source and receiver locations and are thus relatively cheap to acquire. Little processing is done on refraction observations with the exception of filtering to help in the process of picking the arrival times of the initial ground motion. Because such a small portion of the recorded ground motion is used, developing models and interpretations is no more difficult than our previous efforts with other geophysical surveys. Refraction seismic observations require relatively large source-receiver offsets (distances between the source and where the ground motion is recorded, the receiver). Refraction seismic only works if the speed at which motions propagate through the Earth increases with depth. Refraction seismic observations are generally interpreted in terms of layers. These layers can have dip and topography. Refraction seismic observations only use the arrival time of the initial ground motion at different distances from the source (i.e., offsets). A model for the subsurface is constructed by attempting to reproduce the observed arrival times.
  49. 49. Refraction Survey Advantages – Amplitude of refractor are large. – Best for exploration of shallow structures. – Easy to process and interpret. Disadvantages – Exploration for deeper structures is difficult because of large XCrit distance, – Blind to low velocity layers.
  50. 50. Reflection Methods Advantages Disadvantages Reflection seismic observations are collected at small source-receiver offsets. Reflection seismic methods can work no matter how the speed at which motions propagate through the Earth varies with depth. Reflection seismic observations can be more readily interpreted in terms of complex geology. Reflection seismic observations use the entire reflected wave field (i.e., the time-history of ground motion at different distances between the source and the receiver). The subsurface is directly imaged from the acquired observations Because many source and receiver locations must be used to produce meaningful images of the Earth's subsurface, reflection seismic observations can be expensive to acquire. Reflection seismic processing can be very computer intensive, requiring sophisticated computer hardware and a relatively high- level of expertise. Thus, the processing of reflection seismic observations is relatively expensive. Because of the overwhelming amount of data collected, the possible complications imposed by the propagation of ground motion through a complex earth, and the complications imposed by some of the necessary simplifications required by the data processing schemes, interpretations of the reflection seismic observations require more sophistication and knowledge of the process.
  51. 51. Seismic Reflection Survey Advantages – Geophone spread needed is relatively shorter. – Not blind to low velocity layers. Since acoustic impedance exist for every layer. – Convenient for deeper exploration. Disadvantages – Difficulty in recognizing reflected waves. – Small amplitude. – Obscured by overlapping refracted and surface waves.