MULTIPLES AND YOUR SEISMIC DATA

by Khristina Woodring and Roland Kirschner

Department of Geology and Geophysics

Louisiana State University

A. What are Multiples?

Multiples are multiplicative events seen in seismic sections. These events have undergone more than one reflection. They are produced in the data gathering process when the signal doesn't take a direct path from the source to the geologic event and finally back to the receiver on the surface. This causes the signal to arrive back at the receiver at an erroneous time, which, in turn, causes false results and can result in data misinterpretation.

B. What are the Different Types of Multiples?

There are various kinds of multiples ranging from various l ong path multiplesto short path multiples . Long path multiples appear as separate events in the seismic record, while short path multiples add tail to the primary reflection. Examples of long path multiples include simple multiples and interformational multiples. Short path multiples may be ghosts, near surface multiples, and peg-leg multiples.

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C. How can Multiples be Useful?

Ghosts, which are short path multiples, are useful in the interpretation of seismic data. In marine seismic surveys for each reflector three individual reflections are recorded. The first wave to arrive at the receiver is the primary reflection. It is followed by two ghosts (2a & 2b, in Figures 1 & 2 ) that are recorded at the same time, as they both bounce off the sea surface once. As the sea surface is an almost perfect reflector, the amount of energy lost during the reflection of both ghosts is negligible. Therefore the amplitude of these two signals is almost twice as large, as the amplitude of the primary reflection. Hence, it is this arrival that is commonly used in the interpretation of seismic data, rather than the primary reflection. A 180 degree phase shift is observed each time a seismic wave is reflected by the sea surface. This explains the reversal in amplitude for 2a & 2b in Figure 2. Finally, ghost no. 3 is recorded, which has the same amplitude and is in phase with the primary reflection, since it bounced off the sea surface twice.

D. What Types of Problems can Multiples Cause?

Multiples can present many

pitfalls

for the interpreter and cause problems in seismic data interpretation. This is because the features which multiples create may not reflect the true approximation of the geology of a structure or an area. For instance, multiples can enhance geologic features such as small anticlines, so that they appear larger than what they truly are. This is attractive to the oil/gas finder, but can lead to false conclusions concerning the location and amount of pay.

E. How can Multiples be Attenuated?

One of the most common methods of attenuating multiples is by using a stacking method referred to as common-depth point stacking during data processing. Multiples spend most of their time in shallower sections, bouncing off the interface at the low velocity zone. This situation causes an energy decrease in multiples relative to primary reflections of the same travel time.

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Thus, they have smaller stacking velocities and don't align on a continuous velocity log. This makes them relatively easy to identify. Stacking of the data allows the effect of multiples to be masked, so that a more true geological picture is attained.

Another possibility for attenuating multiples is by the use of predictive deconvolution. Deconvolution is the method by which seismic data may be filtered according to the processor's preference. Knowledge of the arrival time of primary reflectors allows the arrival time of multiples from the same surface to be predicted. By using the arrival time, the deconvolution operators can selectively pick multiples out of the data. The shortcoming of this method is that it primarily works only with simple multiples, as those with more intricate paths have a much harder to predict travel time. ³

References

1. Anstey, N.A. 1982. Simple Seismics. Boston: International Human Resources Development Corporation.

2. Comas, M.C., Zahn, R., Klaus, A. et al., 1992. Proc. ODP, Init. Repts., 161: College Station, TX (OceanDrilling Program).

3. Mountain G.S., Lorenzo J.M., Fulthorpe C.S., 1994: Underway geopysics. In: Mountain G.S., Miller K.G., Blum P., et al.,: Proc. ODP, Init. Repts., 150: College Station, TX (Ocean Drilling Program)

4. Sheriff, Robert E. and Lloyd P. Geldart., 1995. Exploration Seismology. Cambridge: Cambridge University Press.

5. Telford, Geldart, Sheriff, and Keys., 1976. Applied Geophysics. Cambridge: Cambridge University Press.