weqfl

NAME

SYNOPSIS

DESCRIPTION

weqfl computes and applies a continuously time varying filter to remove (or model) the effects of anelastic attenuation and dispersion (Q). The algorithm is based on phase-shift migration theory where the kernel of the phase term is modified to take into account dispersion. Essentially the filter explicitly extrapolats the wavefield at the surface to every depth sample, hence the name of the program weqfl. A Wiener approach was implemented for the inverse filtering to prevent amplification of higher frequency noise. With appropriate parameterization, you can apply both amplitude and phase Q compensation without introducing high frequency noise. Weqfl has the option to apply phase-only Q compensation as well.

weqfl assumes a seismic trace of the form x(t) = r(t)*w(t)*q(t), where r(t) is the earth response or reflection coefficients, w(t) is the time invariant wavelet, and q(t) is a time varying function describing the absorption-dispersion effects.

weqfl accepts input functions in card file format (1,...,9QTIM cards described below), in seismic trace format (i.e. analogous to a velocity tape), or constant Q as input on the command line. For the prestack case each Q function will apply to an input gather. For the case of input seismic format Q functions the Q traces must be in the same order on disk (or in a pipe) as the input seismic gathers (stacked or unstacked) and if you run short of Q-traces before the end of the input data the last Q trace will be used for the remainder of the run. Also for q traces if each Q trace is shorter than the input seismic traces then the last Q sample will be used to pad out the Q trace.

If the Q-trace input format is chosen the program checks to see if the input Q traces are sampled the same as the input data. If the Q-traces have a coarser sample interval they are interpolated on the fly to the same sample interval as the input data. This allows estimating Q (using program qest) at a coarser sample interval, say 8ms instead of 4ms, speeding up the estimation step. It is not critical to have the Q estimated at every input sample.

In the case of post stack data you may flag the program to assume each input data trace will be processed using each successive Q trace. In the case of prestack data each input record will be processed by each input Q trace.

Due to the realities of this type of numerical filtering it is next to impossible to strictly control the output energy of the filtered traces. This can result in variations in the overall energy of the output traces resulting in a striped appearance. The fix is to look at the energy of the input trace and try to force the output trace to have the same. The default is to use the ratios of the input and output envelopes as a gain compensation function.

In addition one should look at the output amplitude spectra (ampspec .. -db | usp xgraph) for a sample record. If the spectra after weqfl are too skewed to the high frequencies then consider running a true amplitude spectral shaping filter, e.g. program dafd -R -fl2 -fh100 -sf2 -ep-.5 ..., or an exponential version like dafd ... -E -fl5 -fh65 -fm10 -exp-.5 ... , or program rhof. All of these will serve to push down the highs to varying degrees.

The rho filter with a negative exponent will skew the spectra back to be more balanced with more equal contributions from both highs and lows. This rho filter process is entirely AVO friendly since it does exactly the same thing to every trace.

qest may be run on the seismic data to estimate the Q (you will have to editt off the first trace of each record to get the Q-traces.

weqfl gets its parameters from command line arguments and its Q-function from card images in a flat file. The cmommand line arguments specify the input, output, the modes of operation, the start and end processing limits, and verbose printout, if desired.  

Command line arguments

-N ntap

Enter the input data set name or file immediately after typing -N unless the input is from a pipe in which case the -N entry must be omitted. This input file should include the complete path name if the file resides in a different directory. Example -N/b/vsp/dummy tells the program to look for file 'dummy' in directory '/b/vsp'.

-O otap

Enter the output data set name or file immediately after typing -O. This output file is not required when piping the output to another process. The output data set also requires the full path name (see above).

-q q0

For non-seismic Q function input: enter the optional constant Q-value (Q-function inout file above is ignored). This is useful in building constant Q panels. Default = 0.0 (i.e. look for Q-function file).

-qt qtap

Enter the input data seismic format Q data set name or file immediately after typing -qt. The Q traces must be in the same order on disk (or in a pipe) as the input seismic gathers (stacked or unstacked) and if you run short of Q-traces before the end of the input data the last Q trace will be used for the remainder of the run. Also for q traces if each Q trace is shorter than the input seismic traces then the last Q sample will be used to pad out the Q trace. It is recommended that the Q traces be smoothed (e.g. using vsm) before input to weqfl.

-D cardin

For non-seismic Q function input: enter the name of the flat file containing the 1QTIM - 9QTIM card images of the Q functions (in the manner of TDFN cards for velocity functions). These have the format:
           nQTIM (cc 1-5)
           TIME (ms) (cc 6-10, 16-20, ..., 77-70
           Q (cc 11-15, 21-25, ..., 71-75), sequential rec # (cc 76-80)

 

Each function consists of a series of 1QTIM, 2QTIM, ..., 9QTIM cards with the last (or first if there is only 1) being a 9QTIM. Up to 64 T-Q pairs can be used for each function. The last 5 columns is the sequential record number for this function. Up to 70 functions may be specified along a line with linear interpolation of the functions between record control points (the same function will be applied to each record).

-null qnull

For mute zones where input Q values is zero replace zero Qs with this value. Default = 500.

-qmin qmin

Minimum effective Q values. Default = 20.0.

-qmax qmax

Maximum effective Q values. Default = 500.0.

-dq dq

Fractional Q increment. Default = 0.05.

-f0 f0

Dorminant frequency. Default = 50.0.

-prew sigma

Prewhiterning level for Wiener filter. Default = 0.01.

-md mode

Enter the mode. Mode = 0: do inverse Q filtering; mode = 1: do forward Q modeling. Default = 0

-phz phase only

Enter the command line argument '-phz' to turn on the phase only Q filter mode. The default is amplitude and phase Q filtering.

-ab absol

Enter the type of Q function: 0 = interpolate in time between time-Q control points; 1 = do a running average over time. Default = 0.