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Source signature determination by inversion of ministreamer data

Department of Petroleum Technology and Applied Geophysics, NTNU, Norway

Corresponding author: M. Landrø, mlan@ipt.ntnu.no

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

In experimental physics disciplines (e.g., particle physics, optics), it is essential to have detailed knowledge of the signal used to investigate or probe a target. For the seismic experiment, exact determination of the input signal has not been addressed to the same extent. In my opinion there are two major reasons for this lack of interest:

1. The input signal is strongly altered with respect to amplitude and frequency on its way through the earth down to the target level. Thus, many doubt that knowledge of the input signal before it enters the sedimentary rocks of the subsurface is useful.
   
2. It has not been sufficiently demonstrated that it is necessary to determine a directional source signature on a shot-by-shot basis.
   

In a deterministic perspective, we would understand our seismic data in the following way: First determine the source signal before it enters the sedimentary subsurface, then add absorption and scattering effects due to changes in medium parameters as the wavefield propagates through the earth, and in this way obtain a best possible fit with the measured data. I think deterministic approaches like this will be used more frequently in the future because of some recent developments, including:

• Time-lapse projects which, to exclude source variations between various surveys, require exact knowledge of the signature on a shot-by-shot basis.
  
• Processing four-component seabed data might need a directional source signature to determine angle ranges for maximum conversion of P to S energy.
  
• The new generation of surface-related multiple attenuation schemes often requires that the wavelet in the water layer is known.
  

The following presents the background and main principles of the ministreamer inversion method which was designed for far-field source signature determination.

	From near-field to mid-field measurements

 
The use of near-field measurements to compute the signature of an air-gun array was demonstrated nearly 20 years ago by Ziolkowski et . . . [Full Text of this Article]