SEISMIC METHOD

New introduction to seismic method

1. Measurement of seismic-wave travel time is one of the most common geophysical method. The main objective of this method is to map the structures of subsurface formations in order to infer the existence of possible petroleum traps . In addition, the method can be used to identify lithology (rock type), fluid content ( oil, gas, or water), and fine structures (fractures). In this method, seismic energy is generated artificially at the near surface and the generated waves travel in the subsurface and get reflected and refracted off layer boundaries . 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 is widely used in petroleum exploration; e.g. to detect boundaries between different kinds of rocks, this detection assists in the mapping of geological structures. The reflected waves are recorded at the surface and the travel times and amplitudes analyzed to map the subsurface. Almost all of petroleum exploration is done using the seismic method . Seismic energy is detected on land by using devices called geophones, which react to on-site ground motions. Compared to other exploration methods, the seismic method gives, by far, the best subsurface structural and lithological image. SEISMIC METHOD

2. SEISMIC METHOD

3. SEISMIC SURVEY OBJECTIVES PREPARATION PLANNING & DESIGNING PARAMETER SELECTION DATA ACQUISITION

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. PREPARATION AVAILABLE BUDGET & TIME AVAILABLE DATA Geological Data Geophysical Data Other Information

6. GEOLOGICAL DATA Surface maps Cross sections Contour maps Isopachs Formation depths/thickness Lithologies Well history Wire Line Logs MAPS WELL DATA

7. GEOPHYSICAL DATA Time / Depth contour maps Isochore / Isopach maps Velocity maps MAPS SECTIONS Field monitors Migrated / Unmigrated sections REPORTS

8. Isochore Map A contour map that displays the variation in time between two seismic events or reflections. Isopach Map An   isopach map  illustrates thickness variations within a tabular unit, layer or stratum. Isopachs are contour lines of equal thickness over an area.

9. OTHER DATA Topographic Maps Arial Photographs Cultural Information

10. KEY INFORMATION Depth of Target Horizon Dip of the Target Horizon Arial Extension of Target Accessibility in the Area Noise Type in the Area Thickness of the Weathered Layer Structural Settings of the Area Terrain Climate

11. 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

12. PARAMETER SELECTION SPREAD TYPE RECEIVER LAYOUT SOURCE TYPE

13. SPREAD TYPE

14. 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.

15. SYMMETRIC SPLIT SPREAD Maximum Offset Minimum Offset Minimum Offset Maximum Offset Shot point CH 1 CH 61 CH 60 CH 120 For a symmetric split spread, equal number of channels are laid out on both sides of the shot Minimum Offset: The distance between shot and very first live channel on either side of spread . Maximum Offset: The distance between shot and last live channel on either side of spread

16. RECEIVER TYPE GEOPHONES HYDROPHONES

17. SEISMIC DETECTORS • Land detectors (Geophone ): It is a device used to detect the seismic vibrations. A geophone is a transducer which transform mechanical energy (seismic vibrations) into electrical energy. It consists of a moving coil and a stationary magnet. The movement of the coil due to vibration creates electromagnetic flux proportional to the magnitude of vibration. • Marine detectors (Hydrophone ): It is a device used to detect the pressure waves . A hydrophone is a transducer which convert a sound signal into an electrical signal, since sound is a pressure wave. Most hydrophones are based on a   piezoelectric transducer that generates electricity when subjected to a pressure change.

18. Hydrophone

19. RECEIVER LAYOUT Linear Array Weighted Array Parallel array G1 G2 G23 G24 G1 G12 G24 G13 G1 G10 G11 G12 G13 G14 G15 G24

20. Weight drop Dynamite SOURCE TYPE Vibroseis Hammer LAND SURVEY

21. MARINE SURVEY Air Gun

22. SEISMIC SOURCES • Land Sources : • 1 ‐Impulsive sources: which are divided to Explosive sources such as Dynamite (common in Petroleum exploration), and Non Explosive such as Weight drop & Hammers (common in shallow seismic investigation). • 2 ‐Non impulsive sources: the main common is Vibroseis which is a designed vehicle lift its weight on large plate in contact with ground surface in sweeps. • Up Sweep: Frequency begins low & increase with time. • Down Sweep: Frequency begins high & decrease with time . • Marine sources : • Air gun: the common in offshore survey (first produced in 1960). This gun releases highly compressed air into water. It uses a compressed air at 2000‐ 5000PSI to produce an explosive blast of air. Several air guns with different sizes are fired to enhance their initial pulses & reduce their bubble effects.

23. DYNAMITE Shot Hole Depth Charge Size Shot point Interval Number Of Holes Source Array Pattern SP Interval Hole Depth Charge Size PARAMETERS:

24. The blast of explosives produces a very strong energy, P lacing 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

25. VIBROSEIS

26. ENERGY SOURCE USED IN SEISMIC WORK 1- Dynamite: %60 of seismic work used dynamite . Advantage 1- It has sharp peak 2- Decrease surface noise 3- It has wide range of frequency Disadvantage 1- It require drill hole 2- It is dangerous 3- Stack of charge 2- Dropping weight: It is composed of a rectangular steel plate, 2 to 3 tons usually, and dropped 3 to 4 times at height of 3 m, used in desert area. Advantage 1- Not dangerous 2- Easley used , cheap 3- Not distort the surface Disadvantage 1- Record is weak 2- Low frequency 3- Not used in mountainous area

27. 3- Vibroseis : It is used to generate seismic wave in the form of a wave train of control energy. Always 4 vibrators are used Advantage 1- The frequency is controlled 2- The energy is controlled 3- Not distort the surface Disadvantage 1- Not suitable when soil is too thick 2- Expensive 3- Not used in mountainous area 4- Hammer: It is used to generate seismic wave of small energy, and it is used for shallow investigation. Advantage 1- Very cheap 2- easily used 3- Not distort the surface Disadvantage 1- Give small energy 2- used only for shallow 3- Low frequency

28. SOURCE POINT Due to different types of sources the source point is referred as: Shot Point (SP) for Dynamite Sources Vibroseis Point (VP) for Vibroseis Sources

29. It is the generation and recording of seismic data. Acquisition involves many different receiver configurations, including laying geophones on the surface of the Earth or seafloor, towing hydrophones behind a marine seismic vessel to record the seismic signal. A source, such as a vibrator unit , dynamite shot, or an air gun, generates acoustic or elastic vibrations that travel into the Earth, pass through strata with different seismic responses and filtering effects, and return to the surface to be recorded by seismograph as seismic data. SEISMIC DATA ACQUISITION

30. INSTRUMENT FOR SEISMIC SURVEYING Geophone/Hydrophone Energy Source Seismic Crew Seismic Cables Recorder/Seismograph

31. REFLECTION FROM A HORIZONTAL REFLECTOR Offset CMP Surface Reflector Source Detector

32. SEISMIC DATA ACQUISITION ON LAND Shot Hole Geophones Recording Truck Seismic Waves Reflector

33. SEISMIC DATA ACQUISITION IN MARINE Hydrophones Seismic ship Seismic Waves Shot Reflector Streamer Sea surface

34. Survey Crew Drilling Crew Loading Crew Layout Crew Recording Crew Shooting Crew LVL Crew Safety Crew (HSE) SEISMIC CREWS

35. 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 It is connected with two external batteries and a receiver which receives the signal from the satellite. SURVEY CREWC

36. Trimble Survey Controller is connected with the GPS, it has the designed coordinates of each source & receiver points, given by the Operating Company Trimble Survey Controller When it gets the coordinates from the satellite it compares the designed and actual coordinates and gives the original values

37. 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

38. Drilling Jackhammer Drilling Crew uses Jackhammers for shallow holes Man Portable Drilling Rig For deep holes, Portable Drilling Rigs are used DRILLING CREW

39. 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 LOADING CREW

40. 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

41. Other then dynamite, Vibroseis is also a source in Seismic Acquisition Surveys

42. 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 LAYOUT CREW

43. CABLE & GEOPHONE Link cable Geophone string

44. LAYOUT(FRONT CREW)

45. CABLE LAYOUT

46. GEOPHONE PLANTATION

47. ROAD CROSSING

48. LINE SOURCE RECEIVER

49. 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

50. RECORDER RECORDING

51. 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 RECORDING TRUCK

52. 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

53. 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 SHOOTING CREW

54. Portable recording system SEISOMOGRAPH

55. A recorded result of shot point after the blast SEISOMOGRAM

56. Camera For LVL Blaster used For LVL Camera used for Low Velocity Layer showing the status of spread, it has built in floppy drive, printer and hard disk Refraction blaster which is connected with the camera for blasting the dynamite (REFRACTION) LVL CREW

57. LVL Spread A view of a spread for the Refraction Survey Recorded result of an Low Velocity Layer shot

58. Safety Caps Foot Wears Coveralls Loose Clothing Water Bottles Fire Extinguishers, Fire Alarms, Smoke Alarms Sign Boards The job of this crew is to provide the following things to all the crews SAFETY CREW

59. 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 client END OF SURVEY

60. Seismograph: It is the instrument that measure motions of the ground, including those of seismic waves generated by earthquakes, nuclear explosions, and other seismic sources . Seismogram: It is a graph output by a seismograph. It is a record of the ground motion at a measuring station as a function of time.

61. Two‐dimensional survey (2‐D): Seismic data or a group of seismic lines acquired individually such that there typically are significant gaps (commonly 1 km or more) between adjacent lines. A 2D survey typically contains numerous lines acquired orthogonally to the strike of geological structures (such as faults and folds) with a minimum of lines acquired parallel to geological structures to allow line‐to‐line tying of the seismic data and interpretation and mapping of structures. The seismic data recorded by 2‐D survey is seismic line . Three‐dimensional survey (3‐D): The acquisition of seismic data as closely spaced receiver and shot lines such that there typically are no significant gaps in the subsurface coverage. The seismic data recorded by 3‐D survey is seismic cube. LAND SURVEY

62. The 2-D method, which can be used to map structures along widely spaced traverses. Geometry 2‐D SURVEY

63. Result

64. The 3-D method, which can be used to accurately map subsurface structures in a three-dimensional sense . The majority (> 80%) of seismic exploration is done using the 3-D method. Geometry 3‐D SURVEY

65. Result

66. Seismic cube

67. 2‐D and 3‐D survey in marine differ from land survey by : ‐The seismic source is Air gun & not dynamite or Vibroseis ‐The seismic detector is Hydrophone & not a geophone ‐The sources and detectors are always at depth below the sea level & the depth of the cable is controlled by Streamer‐Birds ‐The receivers are connected together by streamer. MARINE SURVEY

68. 2‐D marine

69. 3‐D marine

70. Tail buoy: A floating device used in marine seismic acquisition to identify the end of streamer. It allows the seismic acquisition crew to monitor location and direction of streamers . Streamer Bird : A device connected with the streamer to control the depth of streamer .

71. Streamer Bird

72. SIGNAL AND NOISE Any events of interest are called signals while rest are called noise. Signals and noise are relative terms as in certain set of analysis an event may be considered as signal, while in another analysis it is considered as noise. In seismic acquisition and processing our major emphasis is to enhance the Signals and suppress the noise. Thus increase the signal to noise ratio (S/N).

73. Noise is the undesired information contained on a seismic record which one does not wish to use and these are to be filtered. There are two types of seismic noises encountered in seismic survey are; (1) Coherent Noise. (2) Incoherent Noise or Random Noise. SEISMIC NOISE

74. It displays some regular patterns on seismogram. 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. (1 ) COHERENT NOISE It has various sources such as: Ground Rolls Direct arrivals Refracted events Multiples reflections Diffraction events

75. 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. Direct and Refracted waves

76. Refracted Waves In Seismic reflection survey the refracted events comes as first arrival.

77. Ground Roll Ground rolls are R ayleigh waves which appear as second arrival on seismic section.

78. 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.

79. Diffracted Events At faults and some unconformities diffracted waves are generated on seismic section.

80. 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: S Waves

81. 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. It has various sources such as given below. i . Wind Noise ii. Water Flow Noise iii. Small Movement With in the Earth iv. Local Noise v. Bad Geophone Noise ( 2 ) INCOHERENT NOISE OR RANDOM NOISE

82. FILTERING A process or algorithm using a set of limits used to eliminate unwanted portions of seismic data, commonly on the basis of frequency or amplitude, to enhance the signal-to-noise ratio of the data. The common use of digital filter in data processing is to filter out unwanted frequencies. Types of filters : Band-Pass filter : A filter used to pass a defined band of frequencies. Any high or low frequencies outside this range will be attenuated . Low-Cut filter (High pass ): A filter that pass only the high frequencies and eliminate the low frequencies. High-Cut filter (Low-Pass): A filter that pass only the low frequencies and eliminate the high frequencies. Notch filter: It is used to filter out narrow band of frequencies within frequency range of data. The most common use of this filter is to attenuate noises caused by power lines.