Signature

Seismic ``signature" is defined to be a convolutional filtering on impulse-source data. This convolution models the nonimpulsive nature of real sources. Imagine the oscillation of a marine airgun's bubble. On land, the earth's near surface can have a very slow velocity. There Snell's law will bend all rays to very near vertical incidence. Mathematically, such reverberations in such layers are indistinguishable from source signature. For example, in California the near-surface soils often have a velocity near the air velocity (340 m/s) that grades toward the water velocity (1500 m/s). A buried shot typically has a free-surface reflection ghost whose time delay is virtually independent of angle. Thus the ghost is more like signature than multiple.

Synthetic data in Figure 7 shows the result of convolution before and after NMO. An event labeled ``G" marks the tail-end of the source signature.

 

deep Figure 7 Example of convolution before and after NMO. The raw data shows a uniform primary-to-tail interval, while the NMO'ed data shows uniform multiple reverberation. The letters F, G, and V are adjustable parameters in the interactive program controlling water depth, signature tail, and velocity.

The main idea illustrated by the figure is that some events are equally spaced before NMO, while other events are equally spaced after NMO. We will see that proper deconvolution requires a delicious mixture of NMO and deconvolution principles.

Figure 7 happens to have a short time constant with the signature and a longer one with the reverberation. The time constants would be reversed in water shallow compared with the gun's quieting time. This is shown in Figure 8.

 

shallow Figure 8 Model in water shallow compared to gun quieting time.

This figure shows an interesting interference pattern that could also show up in amplitude versus offset studies.