PROCESS TX2FK Parameters, alphabetically: coords etime lprint nprestk nxpad stime twinlen weight xwinlen Document Date: 5 July 2007 version 2.1 a.j. harding Modifications: November 2006 - Change TWINLEN from time to number of samples. - Change TWINLEN preset to 25 samples. July 2007 - Only sort traces by range when nprestk > 2 - Abort if < 2 traces in tx. - Allow up to 32K samples per time trace. PROCESS TX2FK transforms data from the TX (time-space) domain into the F-K (frequency-wavenumber) domain by performing a 2-D FFT. The input is a set of normal (t-x) seismic traces and the output is a transformed set of F-K traces. The transformed output traces are in rectangular form unless polar form (amplitude and phase) is requested. The frequencies within each output trace are ordered from -Nyq to 0 to Nyq. Each output trace contains a power of 2 number of samples. The traces are ordered in wavenumber from 0 to K (Nyquist). Data in polar form are ordered with the modulus followed by the argument. The sample interval is in hundreds of microhertz in order to prevent truncation problems. e.g. an SEG-Y trace header interval value of 610 is really 610/10,000. or .0610. See doc/fk.forum for further discussion. The input traces must be sorted by increasing range (SEG-Y header word 10) and must be separated uniformly. DELTA X must be positive and must be constant. The only exception is when "super-gathers" are to be used (see parameter nprestk). Any SIOSEIS process may follow TX2FK, but care should be taken that it makes sense! The imaginary part of data in rectangular coordinates may be omitted from plot by decimating by a factor of 2. The steps used in the F-K transformation are: 1.) Each trace is windowed temporally in order to minimize edge effects along the time trace. 2.) The data are transposed so that all the data of constant times are adjacent. This sorts the data by x rather than t (or f). 3.) The 'range' traces are zero padded. Effectively adding dummy traces to the end of the dataset. The data must be padded to a power of 2, but additional padding may be desirable for migration etc. A user minimum number of dead traces is specified by nxpad. The data are also windowed in x if desired. 4.) The complex FFT is performed, transforming x to k, or from space to wavenumber. 5.) The data are transposed back to ordering by time. 6.) The forward FFT is performed, converting time to frequency. The maximum number of samples in time is 8192 real samples. 7.) The data are converted to polar coordinates if requested. Data in the fk domain may be processed by any other seismic process in SIOSEIS. The run time of TX2FK is governed by the number of traces, including pads, and the trace length, including pads. Both dimensions are a power of 2, so transforming 1500 points takes as long as 2000 points (2048 being the closest larger power of 2). The frequency-wavenumber domain is discussed in a paper "A Review of the Two-Dimensional Transform and Its Use in Seismic Processing" by D.W. March and A.D.Bailey in the "First Break", January 1983. Each parameter list must be terminated with the word END. The entire set of TX2FK parameters must be terminated by the word END. THE PARAMETER DICTIONARY --- --------- ---------- STIME - The start time of the data for the entire data set. Any trace that has an initial time (delay) greater than STIME will be zero padded so that the data starts at STIME . Any trace that has a delay less than stime will be shortened. Preset = the delay of the first trace. ETIME - The end time of the data for the entire data set. Data in excess of ETIME will be omitted from the transformation. Preset = the last time of the first trace. NXPAD - The number of dummy traces to insert at both ends of the seismic line. PROCESS FKMIGR needs dummy traces in order to prevent "wrap around". Preset = 10 TWINDOW - The type of window to apply before computing the temporal fft. = HAMM, Hamming = HANN, Hanning = GAUS, Gaussian = BART, Bartlett (triangular) = RECT, rectangular (box car - no window) = BLAC, Blackman = EBLA, exact Blackman = BLHA, Blackman-Harris Preset=HANN e.g. window rect TWINLEN - The window length, in samples. A window length of zero causes the entire time domain trace to be windowed. A non zero length indicates that winlen data will be modified at both ends of each trace. Preset = 25 e.g. winlen 50 XWINDOW - Same as twindow but it windows the data by range. Preset = RECT ( No windowing of ranges ) XWINLEN - The window length in number of traces. A window length of zero causes the entire set of ranges to be windowed. A non-zero length causes that number of traces to be tapered at the binning and end. Preset = 10 COORDS - The coordinates of the output trace. = RECT, Rectangular coordinates. The output trace will be complex. The trace values will consist of real and imaginary pairs. The frequency dimension runs from 0 to Nyquist (pi) then the negative frequencies back to 0, as output from the FFT. The number of samples in the output frequency trace is a power of two. The k dimension is also ordered as it comes directly out of the FFT, 0 to Nyquist followed by the most negative k. = POLAR, Polar coordinates. The first half of the output trace will be the amplitude spectrum and the second half of the trace will be the phase spectrum, each ordered 0 to Nyquist back to 0. The k dimension is reordered so that the first trace is the negative Nyquist, then increasing to 0, then increasing to the positive Nyquist. The total number of traces output is a power of two plus 1. = POLARU, "user friendly" polar coordinates. This produces output data as a series of wavenumber traces running from -Nyq -> +Nyq and only positive frequencies in polar form (magnitude followed by phase). This format is useful if it is desired to look at the FK data using for example a contouring program. The data are not used internally in this form but is converted on input/output from SIOSEIS. Preset = RECT e.g. coords POLAR OHDRPATH - If processes TX2FK and FK2TX are called in the same job then the original TX headers will be used as the trace headers of the the processed TX data. This will preserve all RP/Shot numbering as well as GMT information. However, if these two processes are done in separate jobs then the user may specify a permanent disk file to hold the original TX headers. This filename can then be given to FK2TX when the inverse transform is done. PATH1 & PATH2 - Before TX2FK transforms the input traces it accumulates them them in a scratch datafile. The size of this file is the number of input traces prior to padding in range * the sample length of each trace (etime - stime) prior to padding in time. The size of the data after transformation is equal to the number of traces * trace length after padding of each to the nearest power of 2. This is stored in a second scratch dataset after which the first scratch file is deleted. If there is a FK2TX in the procs list then it will use this second scratch file as its first scratch file. The default location of these files is implementation dependent and may be of insufficient size. The PATH parameters allow the user to specify the location of the two scratch files. PATH1 - The location of the first scratch file. PATH2 - The location of the 2nd scratch file. Since this is the same as the first scratch file used by FK2TX (if present) it should be given only here or in FK2TX. If contradictory definitions are given the first on processed will be used. NPRESTK - The number of rps to use in each fk transformation. Each rp is terminated with a -1 in SEG-Y trace header word 51. Processes SORT and GATHER set this "end-of-sort" flag. Shot data may set this flag with process INPUT or DISKIN parameter NTRGAT. The data are sorted by increasing range prior to transforming x to k. Process fk2tx will automatically "unsort" the data into the original order when nprestk is greather than 1. END - Terminates each parameter list. Copyright (C) the Regents of the University of California All Rights ReservedGo to the list of seismic processes. Go to SIOSEIS introduction.