WAVELET TUTORIAL GENERATION OF DESIGNATURE OPERATORS

GNS Science Introduction This presentation discuss a suitable workflow for deriving and applying a Minimum or Zero phase designature operator from a supplied farfield airgun signature. The farfield signature example used in this tutorial was supplied by CGGVeritas, Along with near trace data from the 05CM (Crown Minerals 2005 survey) supplied by the Ministry of Economic development NZ. The signature was supplied at 0.5ms sample rate as an ASCII number dump. The receiver ghost for the expected cable depth was modelled in WAVELET. A debubbling filter was derived and applied to the signature.

GNS Science How to read a signature into the WAVELET application. Wavelet can read either SegY date or formatted ASCII text files. The Wavelet application would generally require any ASCII text file to be reformatted to the required WAVELET format of columns consisting of time & amplitude pairs as below: etc A simpler method is to use the xascsegy utility to read the ASCII text file and output a standard SegY dataset.

GNS Science XASCSEGY utility XASCSEGY takes input ASCII text files in a simple single column format. This matches the output from GUNDALF, outputs from other packages may need to be reformatted for input to XASCSEGY simple. Sample Rate and Nsamps can be parameterised via the form or integrated into the header of the input file.

GNS Science Wavelet The generated SegY dataset (farfield.segy) is read into the WAVELET application as follows. Start WAVELET from the launcher. Define a WAVELET session file. WAVELET application opens, click on the NEW button. Parameter form opens, select input dataset (farfield.segy) and define start and end times as required as SegY format does not support negative times. See example:-

GNS Science NEW wavelet param form

GNS Science Raw Farfield Signature

GNS Science Receiver Ghost.

GNS Science Raw Signature convolved with Rec Ghost

GNS Science Debubbling Operator The debubbling operator has to be created outside of the WAVELET application. Create an XSJE job flow that reads in the farfield signature using DISCREAD module. Use the DECONW module to generate the debubble operator. In the example shown a 150ms operator with 20ms Gap was used. DECONW has the facility to output the deconvolution operators to a SEGY discfile. This output file can then be input to WAVELET and applied to the farfield signature.

GNS Science Debubble Operator

GNS Science Anti-Alias Filter

GNS Science Signature with Debubble operator applied

GNS Science Signature with Debubble & AA Filter applied

GNS Science Signature resampled to 2ms

GNS Science Minimum Phase equivalent of Signature

GNS Science Minimum Phase conversion filter

GNS Science Signature with Minimum phase conv filter applied

GNS Science Zero Phase equivalent of Signature

GNS Science Zero Phase conversion filter

GNS Science Signature with Zero phase conv filter applied

GNS Science Raw Near trace data

GNS Science Min phase filter applied

GNS Science Zero phase filter applied

GNS Science Comments WAVELET session files can be saved and returned to at a later date. Any filters or wavelets created at the point the session file is created will be regenerated when session file is reopened. With the exception of the debubbling operator all required wavelets can be generated in the WAVELET application. Care should be taken in assigning the T0 point of the Farfield signature. An error at this point is likely to result in a time shift being introduced when the dephasing filter is applied.