A2.4VF2 Applied Environmental Geoscience Lecture 3

1. A2.4VF2 Applied Environmental GeoscienceLecture 3 SEISMIC REFLECTION METHODS

2. Contents • Introduction • Reflection surveys on land • Reflection surveys on water • Interpretation of seismic reflection results • The synthetic seismogram • Conclusions

3. INTRODUCTION

4. Reflection methods make use of the energy that is reflected from a subsurface layer. • The relative strength of this energy is determined by the reflection coefficient and thus by the impedance contrast. • There is no requirement for the lower layer to have a higher velocity than the upper layer (unlike refraction methods). Thus any sequence can be imaged.

7. Due to the geometry of the ray paths, a reflected signal will always arrive after a direct ray and possibly after the refracted ray. • Thus they cannot be detected using first arrival methods. The full waveform must be recorded and analysed. • Reflection and refraction surveys are not mutually exclusive. The same geophone records (provided they are a full time-record) can be used for either or both analyses.

9. REFLECTION SURVEYS ON LAND

10. Traditionally, reflection surveys have been used by the oil industry to investigate deep structures. They are capable of providing relatively detailed images from depths of 1000s metres. • Shallow reflection surveys have only recently been introduced, mainly in civil engineering. The results can be difficult to interpret due to weathering, noise and lateral variation. However, these problems are now being overcome.

11. Small explosive source for seismic reflection surveying

12. A reflection analysis is carried out on the full record from a geophone. • Secondary arrivals are identified, often marked in, and the records from adjacent geophones are aligned side-by-side. • The same record will also show the first arrival time and thus can be used in a refraction survey.

13. Geophone positions Time (mSecs)

14. This provides a visual image of the reflectors and their continuity can then be seen. • Superficially this appears to be a geological cross section. • There are, however, some cautions: • there may be a vertical exaggeration; • a strong reflector may not indicate an important geological change; • in general, structural interpretions are more reliable than lithological ones. • Ground truth is required, perhaps also a synthetic seismogram.

17. REFLECTION SURVEYS ON WATER

18. Reflection surveys are particularly useful on water. • This is because the source and hydrophone can be towed through the water, making continuous contact. • This removes the need to manually reposition the geophones between shots and is thus much faster. • The resulting record can be displayed as a continuous image on a chart or a display screen.

20. The chart record is continuous record of the arrivals at the hydrophones • Each pulse produces a series of arrivals and successive series are recorded along adjacent strips • This creates a continuous reflection profile or CRP • The CRP is often referred to as a seismic record or seismic profile.

22. It is important to remember that many of the features on a seismic record are influenced by the sound source. • Thus higher frequency sources produce more detail but less penetration, some sources ‘ring’ more than others. • Thus two records of the same feature can look rather different at first sight.

23. Sparker profile through lake sediments

24. Pinger profile through lake sediments

25. Seismic records appear to be geological cross sections. In fact, they have several characteristics that can be misleading: • the depth scale is exaggerated, leading to slopes appearing much steeper than in reality; • reverberations of the sound source can lead to reflections being repeated, thus leading to multiples; • individual objects give rise to so-called hyperbolic reflections that may appear to be structural surfaces; • not all reflectors need actually correspond to geological interfaces.

27. 1 km 20 m Base of continental slope west of Shetland. Vertical exaggeration is ~ 50x The actual slope angle is around 1º

28. Source ringing Multiple reflection Sparker cross section - sand wave, Pentland Firth

29. hyperbolic reflectors Seismic record from the outer continental shelf west of the Hebrides

30. CRP surveys can be used for a variety of purposes. • They are routinely used for engineering work and give detailed records down to 100m or so. • They are also used for offshore geological surveys, especially in shelf seas. In this case the depth may exceed 1000m but is seldom as deep as a land survey. • CRP surveys are difficult to operate in very shallow water due to the need to submerge the hydrophone below wave depth in order to reduce noise.

31. INTERPRETATION OF SEISMIC REFLECTION RESULTS

32. We can divide interpretation into two parts • The interpretation of structure using the geometry of the beds • The interpretation of lithology using seismic signatures and seismic attributes.

33. Structural interpretation is relatively straightforward and is largely visual. • the internal geometry of layered strata is revealed • sediment ‘packages’ can be identified • erosion surfaces can be identified • channelling can be identified • We must remember the various scale distortions that may exist in a seismic record.

34. Deep record from land survey TWT scale in seconds

35. Offshore sparker survey timescale lines 40ms apart.

36. It is possible to estimate the lithology (sediment type) from a seismic record, although this is less precise than determining the structure. • The key is the seismic signature of the material. This is the internal appearance of a bed, arising from the composite effect of numerous small reflectors within it.

37. A key issue concerns the sound source, since this influences the signature as well as does the sediment type. • The signatures obtained in marine surveys in particular are very sensitive to the sound source in use. • Thus, in a given material, a boomer may produce a different signature from a sparker. This is due to the differing frequency spectra and resolving power of the two sources. • This is less of a problem in terrestrial surveys since the higher frequencies (=details) are usually lost.

38. These signatures are both from identical lithologies False layering produced by non-lithological features

39. Signatures are broadly characteristic of the parent materials (with the above proviso). This leads to the idea of a seismic facies. • A seismic facies is a unit of sediment that has a consistent seismic appearance. It is often assumed that this implies a consistent lithology. • The full geophone record can be analysed statistically as a time series to obtain eg its frequency content, average amplitude, autocorrelation etc. • These are known as seismic attributes and can be characteristic of particular layers.

40. THE SYNTHETIC SEISMOGRAM

41. It is often instructive to compare the observed record with an artificial record obtained from the geological sequence found in a borehole through the profile. • Such an artificial record is termed a synthetic seismogram. This is produced from a mathematical model of the geological sequence and its interaction with a seismic wave. • This requires knowledge of the seismic velocity and density down the sedimentary sequence. This can be obtained from laboratory tests or from down-hole logs.

42. The model will incorporate the observed geological units, their thicknesses and their seismic impedances (=velocity x density). • From these a profile of reflection coefficients is calculated and from this is obtained the reflected signal that a given input would produce. • The mathematical procedure is termed convolution and is the converse of the deconvolution that is used to interpret the record.

44. CONCLUSIONS

45. Reflection surveys are potentially more powerful than refraction surveys in terms of providing a detailed picture. • Unlike refraction surveys, their interpretation is highly visiual and is often aided by attribute analysis. • They are routinely used offshore and for deeper land surveys. • Shallow land surveys are possible although the data requires considerable processing to remove artefacts and noise.

46. Summary • Introduction • Reflection surveys on land • Reflection surveys on water • Interpretation of seismic reflection results • The synthetic seismogram • Conclusions

47. THE END