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This document provides an overview of gravity and seismic geophysical exploration methods. It begins with introductions to gravity, its units of measurement, and factors that cause gravity variations. It then discusses gravity data acquisition, processing steps like tidal and elevation corrections to derive anomaly maps, and interpretation. For seismic exploration, it describes data acquisition using common midpoint gathers and factors like fold, followed by processing steps like normal moveout correction and stacking to improve signal-to-noise ratio and imaging resolutions. It concludes with discussions on filtering, migration, and how these improve subsurface representations.
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- 1. Surface and Subsurface Exploration using Geophysical Methods. Presented By, Ali Ahmad SP17-R15-002
- 3. INTRODUCTION WHAT IS GRAVITY? standard gravity g= 9.80665 m/s² 9.81 m/s² gravity unit, gu = micrometer per second, μm/s2 c.g.s., unit of gravity, milligal mgal = 10-3 gal= 10-3 cms-2 = 10 gu-3
- 4. Gravitational Acceleration The gravity method is based on two laws derived by Sir Isaac Newton, which he described in Philosophiæ Naturalis Principia Mathematica (July 1967): • Universal Law of Gravitation • Second Law of Motion Units standard gravity g = 9.80665 m/s² = 9.81 m/s² gravity unit, gu = micrometer per second, μm/s² c.g.s. unit of gravity, milligal =1 mgal = 10-3 gal= 10-3 cm m/s²
- 5. Gravity variations over Earth’s surface Variations are due to: (A) Variations in gravity with latitude 1. Shape of the Earth (gE < gP) 2. Variations in mass distribution (gE > gP) 3. Rotation of the Earth (gE < gP) (B) Topography (elevation) The Earth is not smooth (C) Heterogeneities within the Earth (local geology) 1.Local variations in mass distribution 2.Observable changes in surface g 3.Material property is density
- 6. Igneous and metamorphic rocks Granite ρ = 2.52 – 2.75 g/cm³ Basalt ρ = 2.70 – 3.20 g/cm³ Gneiss ρ = 2.61 – 2.99 g/cm³ Sedimentary rocks Water ρ = 1.00 – 1.05 g/cm³ Clay ρ = 1.63 – 2.60 g/cm³ Shale ρ = 2.06 – 2.66 g/cm³ Limestone ρ = 2.50 – 2.80 g/cm³ Dolomite ρ = 2.28 – 2.90 g/cm³ Sandstone Cretaceous ρ = 2.05 – 2.35 g/cm³ Triassic ρ = 2.25 – 2.30 g/cm³ Carboniferous ρ = 2.35 – 2.55/cm³
- 7. Gravity Exploration Methodology 1. Planning of gravity survey. 2. Data Acquisition a. Gravity measurement b. Elevation determination c. Establishment of base station d. Profile layout 3. Data Processing (Reduction) 4. Interpretation 5. Gravity modeling
- 8. Gravity data Processing (reduction) • What to do with the gravity data that you have collected? • Series of processing steps to remove unwanted signals from the data (i.e., gravity variations that are not caused by local density variations) 1.Temporal corrections a. Tidal effects b. Instrument drift 2.Spatial corrections a. Latitude b. Elevation
- 9. 1. Tidal effects – gravitational pull of sun and moon cause bulges in the Earth’s surface - ocean tides (water) and Earth tides (land) -effects are well-understood and can be accurately corrected 2. Instrument drift – due to temperature-induced changes in the spring and temporal changes in the elastic properties of the spring
- 10. Latitude correction • The theoretical value of gravity with latitude is given by the Geodetic Reference System for 1967 (GRS67) equation: g(θ) = 978031.846(1+ 0.0053024sin2 θ − 0.0000058sin2 2θ)
- 11. Elevation corrections • g depends on the distance from the centre of the Earth (r): Free air correction F.A.C = 0.3086 Δh Bouguer Correction B.C = 0.00004193 ρ Δ h Summary of Elevation Corrections Site elevation F.A.C B.C Below reference level Subtract Add Above reference level Add Subtract
- 12. Free Air Anomaly Determination • Free air anomaly is the difference between Observed gravity and theoretical gravity after applying necessary corrections on observed gravity. F.A.A = (gobs ±D.C ±F.A.C) – Th. Value of “g” Bouguer Anomaly Determination • Bouguer Anomaly is the difference between observed gravity and theoretical gravity after applying necessary corrections on observed gravity. B.A = (gobs ± DC ±F.A.C ± L.C ±T.C) – Th. Value of g
- 13. Interpretation 1. Bouguer anomaly map 2. Free Air Anomaly Map Gravity modeling
- 15. Seismic Exploration 1. Planning of Seismic survey. 2. Data Acquisition Seismic data are recorded in the field a. on magnetic tape. b. Cartridge c. Now a days on CD’s 3. Data Processing (Reduction) 4. Interpretation 5. Seismic modeling
- 16. Acquisition involves many different reciever configurations, including laying geophones or seismometers on the surface of the Earth or seafloor, towing hydrophones behind a marine seismic vessel, suspending hydrophones vertically in the sea or placing geophones in a wellbore (as in a vertical seismic profile) 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 as seismic data. Seismic acquisition
- 17. Acquiring data (CMP Gather)
- 18. Shotpoint # 1Hydrophone groups #1#2#3#4#5#6 Separation between midpoints is 1/2 separation between hydrophone groups Midpoints CMP Method
- 19. Shot point # 2Hydrophone groups #1#2#3#4#5#6 Midpoints CMP Method
- 20. Shot point # 3Hydrophone groups #1#2#3#4#5#6 Midpoints CMP Method
- 21. Shot point # 4Hydrophone groups #1#2#3#4#5#6 Midpoints CMP Method
- 22. Shot point # 5Hydrophone groups #1#2#3#4#5#6 Midpoints CMP Method
- 23. Shot point # 6Hydrophone groups #1#2#3#4#5#6 Midpoints CMP Method
- 24. Shot point # 7Hydrophone groups #1#2#3#4#5#6 Midpoints CMP Method
- 25. Shot point # 8Hydrophone groups #1#2#3#4#5#6 Midpoints CMP Method
- 26. Shot point # 8Hydrophone groups #1#2#3#4#5#6 Midpoints CMP Method
- 27. Hydrophone groups #1#2#3#4#5#6 Midpoints Shot point # 1 CMP Method
- 28. Hydrophone groups #1#2#3#4#5#6 Midpoints Shot point # 1 Shot point # 2 Shot point # 1 Shot point # 2 CMP Method
- 29. Hydrophone groups #1#2#3#4#5#6 Midpoints Shot point # 1 Shot point # 2 Shot point # 1 Shot point # 2 Shot point # 3 Shot point # 3 CMP Method
- 30. Hydrophone groups #1#2#3#4#5#6 Midpoints Shot point # 1 Shot point # 2 Shot point # 1 Shot point # 2 Shot point # 3 Shot point # 3 Shot point # 4 Shot point # 4 CMP Method
- 31. Hydrophone groups Midpoints Shot points # 1-8 1 2 3 45 6 7 8 138 CMP Method
- 32. Midpoints 1 2 3 45 6 7 8 13 Fold or Multiplicity is the number of times that the same midpoint is sampled by different shots and different receivers. or It is number of reflections from one common depth. Fold Common Midpoint Method (CMP Method)
- 33. Midpoints 1 2 3 45 6 7 8 138 Maximum Fold is achieved after the 6th shot Fold Common Midpoint Method (CMP Method)
- 34. Fold The number of recorded channels The number of station intervals between geophones groups and source points The nominal fold and stacking density of the resulting CDP section The nominal fold of a given fold geometry can be derived from the equation; F=1/2 C . (dx)/(dg) . (dx)/(ds) Where F = Fold C = Number of recording channels dx = Spacing between Stations dg =Spacing between geophone groups ds = Spacing between source points
- 35. Seismic data processing Processing seismic data consists of applying a sequence of computer programs, each designed to achieve one step along the path from field tape to record section. Ten to twenty programs are usually used in a processing sequence, wisely selected from a library of several two hundred programs. In each Processing Software library there may be several programs designed to produce the same effect by different approaches. For example one program may operate in time domain while another works in frequency domain, yet they may yield similar results.
- 36. Objectives To improve the signal to noise ratio Isolation of the wanted signals Reflections isolated from multiples and surface waves To obtain a higher resolution by adapting the waveform of the signals To obtain a realistic image by geometrical correction To obtain information about the subsurface A near to Geological Section
- 37. Geometric Corrections A seismic trace on a field monitor shows reflected energy bursts from subsurface rock layer interfaces. We will measure the travel times from source down to reflector and back to geophone and use them together with average velocity to compute depths to the various reflectors. However before we use these reflected energy bursts and their travel times, we must apply several corrections to compensate for geometric effects. These corrections include 1. Static corrections 2. Dynamic corrections
- 38. Static Correction Often called statics, a bulk shift of a seismic trace in time during seismic processing. Removing near surface effects requires two corrections; 1.A weathering correction 2.An elevation correction A common static correction is the weathering correction, which compensates for a layer of low seismic velocity material near the surface of the Earth. Other corrections compensate for differences in topography and differences in the elevations of sources and receivers (Datum Correction or elevation correction).
- 39. Weathering Correction A weathering correction replaces the actual travel time through the weather layer by a computed travel time. A method of compensating for delays in seismic reflection or refraction times induced by low-velocity layers such as the weathered layer near the Earth's surface. The weather layer does vary in thickness If the thickness of the weathered layer is not known, then it can be determined by one or more uphole surveys.
- 41. • By definition, the weathering for the emerging wave path in fig. WC = - (dw)/(Vw) + (dw)/(Vc) Or WC = - dw (VC- Vw)/VwVc Since Vc>Vw the weathering correction is always negative.
- 42. Vc is known as the correction velocity. Vc may be • Known from previous experience. • Measured by uphole surveys. • Determined from first arrival refracted along the base of the weathered layer. The normal routine is to compute a weathering corrections at every shot hole and to estimate corrections for other stations by interpolation.
- 43. Elevation Correction (Datum Correction)
- 44. ECs = Ed – Es / Vc ECr = Ed - Er / Vc
- 46. Dynamic Correction One of the steps of processing the data is to rearrange the traces to make CDP gathers (Fig). The traces from different record which correspond to same depth point location are collected together into a single record. The traces are normally arranged with in this gather record in order of increasing offset distance. Then the reflected signals from a single horizontal interface align along a hyperbola as shown in figure.
- 48. Normal Move Out (NMO) The term normal move out or NMO means the variation in reflection arrival time with offset distance from source to receiver. Before stacking, the traces must be shifted to its original place by NMO. A reflection typically arrives first at the receiver nearest the source. The offset between the source and other receivers induces a delay in the arrival time of a reflection from a horizontal surface at depth. A plot of arrival times versus offset has a hyperbolic shape. Move out correction is time correction applied to each offset.
- 49. a) Gather Data b) Gather Data after NMO correction
- 50. The common reflecting point on a reflector, or the halfway point when a wave travels from a source to a reflector to a receiver, is shared by numerous locations. Move out corrections and stacking, or summing of traces, result in redundancy of the data that improves the signal-to-noise ratio. Stacking
- 51. After Gather After NMO After Stacking
- 52. Filtering of Seismic Data Seismic data, in general, • Contain noise signals along with seismic reflection signals. These noise signals interfere • With the interpretation of the seismic signals • Degrade the quality of the subsurface images • That can be obtained by further processing. It is, therefore, • Very desirable to suppress the noise • That is present in the recorded data • Before processing it for imaging.
- 53. Types of Filters 10 HZ 125 HZ 50/60 HZ 50/60 HZ
- 54. Migration Seismic migration is the process by which seismic events are geometrically re-located in either space or time to the location the event occurred in the subsurface rather than the location that it was recorded at the surface. Thereby creating a more accurate image of the subsurface. This process is necessary to overcome the limitations of geophysical methods imposed by areas of complex geology. Such as: fault, salt bodies, folding, etc.
- 55. Time Section Before Migration
- 56. Time Section After Migration