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Seismic pulses obtained from the analysis of direct waves

A comparison to simulated pulses obtained with modeled notionals

Petroleo Brasileiro S/A, Petrobras, Brazil

Corresponding author: adelson@ep.petrobras.com.br

The first 20% of the full text of this article appears below.

This paper discusses a deterministic signature-estimation method for marine seismic data. This method assumes that the direct wave is known and all notionals can be represented as a scaled version of an effective notional.

"Notional" refers to the individual source signatures in a source array (see Ziolkowski et al., 1982). This effective notional is defined so that a modeled direct wave fits the direct wave actually observed.

Signatures observed in reflections are then straightforwardly determined as well as the directivity properties of the acquisition apparatus. This paper focuses on the validity of the basic hypothesis: that the behavior of a set of air guns in a marine seismic source can be described as if all air guns emit the same effective notional (properly scaled).

	Motivation and description of the method

 
Statistical signature-estimation methods usually yield a poor representation of the signature's frequency spectrum and almost nothing about its phase. More precise estimation is needed for several modern processing and interpretation techniques.

However, the homogeneity of sea water is ideal for deterministic signature estimation because the theory of acoustic wave propagation states that in a 3-D homogeneous and nondispersive medium, the wavefront does not change from point to point. Hence, the source signature will be identical at every point (with amplitude proportional to geometrical spreading).

Actually, more than one source is used in an array of sources, and each has its own signature, the notional. Arrays make the observed signature dependent on the direction of observation. This directional dependence is usually referred to as the directivity of the source. Other factors contribute to the directivity. Proximity of the sea surface to the array means energy reflected back from the surface (ghosts) is not sufficiently delayed to be distinguished from energy that propagates directly. Thus, it is necessary to compute the source array as composed of the sources . . . [Full Text of this Article]