Universidade Fernando Pessoa
Porto, Portugal
Seismic-Sequential Stratigraphy
In addition to rays that return to the surface after reflection at single interface, known as primary reflection, there are many paths in a layered subsurface by which rays may return to the surface after reflection at more than one interface (Plate 68). Such rays are called: multiple reflections or simply multiples. These events must be eliminated from the recorded section in order to allow a better understanding of the geology of the area.
Plate 68- The primary reflection from the marker M (the first rebound to the left) may be followed by a second arrival. This would be generated by the downward bounce of the residual energy on the upper interface and by its reflection on M.
Multiples are the principal set of spurious events interfering with seismic data (Plate 69):
- Their presence on a recorded section is due to the fact that an acoustic impedance contrast between two adjacent layers, reflects down-going waves up as well as down.
Plate 69 - This old seismic line comes from offshore Labrador (an Atlantic-type divergent margin), which, locally, overlies late Cretaceous rift-type basins. Admittedly, this line is affected by numerous multiples. Most of them have higher amplitude than their corresponding primaries. Three successive rebounds within the water layer, if not more, are visible and can be readily accounted for. Several other multiples however appear to be generated by the strong dipping horizon at 1 second on the left of the section. Probably, they are secondary rebounds of this primary event: (i) within the water layer, (ii) downward on the water bottom or (iii) a combination of both.
A certain amount of seismic energy, therefore, is not transmitted from one layer to the next through the stratigraphic series along a simple two-way path. It remains trapped within a given formation producing an arrival on the recorded section at each rebound:
(i) Surface multiples (generated by the downward rebound at the air-ground interface) are the most common.
- They may be readily distinguished from primaries.
- They appear on the seismic section at double the time of primary reflections.
(ii) Generally, a multiple has weaker amplitude than its primary as most of the reflected energy is in fact transmitted through. However, this is not always the case. If the acoustic impedance contrasts generating multiples are higher than the contrasts generating primary reflectors, for a given time, the spurious events will be stronger on the seismic section.
(iii) Other situations may also occur, such as resonance within a layer or fortuitous summing of a multiple with later primary arrivals within the same frequency range.
The travel path of internal multiples, generated anywhere within the stratigraphic sequence by the trapping of energy within a layer or a combination of layers, is much more difficult to reconstruct.
In water layer reverberations, as sketched in Plate 70, the rays from a marine source are repeatedly reflected at the seabed and sea surface (ringing in water).
Plate 70- Reverberation or ringing in water is illustrated on this sketch. Reverberation is also known as singing, that is to say, seismic resonance produced by short-path multiples in a water layer. Indeed, as said previously, repeated travel paths within the water layer are summed to the travel time to the indicated deeper horizon.
This type of multiple (also visible in Plate 69), is possibly the most annoying but, fortunately, it is the easiest to eliminate. The two interfaces, at either side of the water level, are excellent reflectors.
- The air-water acoustic impedance contrast is as effective as the water-sediment contrast.
- Hence, bound by two strong reflecting interfaces, the water layer is an excellent multiple-generating medium. This is particularly true, on hard water-bottom areas where sediments are compacted and well consolidated.
In the example shown below (Plate 71), repeated travel paths, within the water layer, are summed to the travel time to the indicated deeper horizon.
Fig. 71- This marine section from offshore Labrador shows several multiples induced by the sea level. A strong reflection, culminating at approximately 2 seconds, is visible in the middle of the section. Because of the high acoustic impedance contrast at the water-bottom, 1st and 2nd order multiples are produced within the water layer (ringing / reverberations). The time difference between the primary and its first multiple is, every where, equal to the time through the water layer.
Each rebound generates successive multiples of this horizon. The marine section illustrated on Plate 71, from offshore Labrador, shows this kind of multiple. In this particular example:
- The multiple reflections have amplitudes comparable with the primary reflections.
- The strong amplitude is due to the high reflection coefficient of the interfaces.
The sketch illustrated on Plate 72 attempts to schematize the most probable paths, between shot point and receiver that an acoustic wave traveling through the sedimentary intervals may chose. Direct primary arrivals and different sets of multiples are indicated.
Plate 72 - On this sketch, in which the shots are underground, below the base of weathered layer, from left to right, we can recognize: (i) a reflection, (ii) a reflection refraction, (iii) a ghost, (iv) a short path multiple, (v) a long path multiple, (vi) a peg leg, (vii) an up hole time, (viii) a direct arrival, (ix) a shallow refraction, (x) an internal long path multiple, (xi) a diffraction and (xii) reflections on a fault plane.
The more frequent multiples, i.e. reflection events having suffered more than one reflection, as illustrated on Plate 72, are:
- Diffraction
A radial scattering of incident seismic energy.
- Reflected refraction
A refraction along a bedding plane or unconformity (reflectors) with appropriate velocity contrast that is reflected back from steep surfaces. Reflected refractions contain information only about the refractor.
- Ghost
A reflection event arriving a short time after the primary from the surface or the base of weathered layer (buried explosion on land).
- Short path multiple
A multiple-reflection involving only a short additional path length arriving very soon after the primary event.
- Long path multiple
It is a multiple-reflection involving additional path length sufficiently long that it is a distinct and separate event in the seismic record.
- “Peg leg”
A multiple-reflection induced by a ringing between two reflectors.
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