PROCESS T2F ------- --- Parameters, alphabetically: addwb coords etime fftlen fno ftr lno lprint ltr stime window winlen Document Date: 1 October 2009 Updates: Oct. 2008 - Add coords AMPL PROCESS T2F converts traces from the time domain to the frequency domain. Each trace is windowed, padded with zeroes to a power of 2, transformed, and optionally converted to polar coordinates. Traces left in rect- angular coordinates are in complex form such that the imaginary part of each frequency immediately follows the real (real-imaginary pairs). Traces in polar form have the amplitude spectrum in the first half of the output trace while the wrapped phase spectrum is in the second half of the trace. Each output trace will contain a power of 2 + 2 number of samples and is the number of samples used in the FFT (fftlen+2). There are fftlen/2+1 frequencies in the output trace representing frequencies 0 (dc) to the Nyquist frequency (1./(2*Sample interval)). Any seismic process may be used on frequency data, but some post T2F processes may not make geophysical sense. Frequency domain plots may be made using polar coordinates and plotting the first half of the trace. Bandpass filter and deconvolution should be done in the time domain using processes filter and decon before the frequency domain processes. A good review of the frequency domain is "A Guided Tour of the Fast Fourier Transform" by G.D.Bergland in "IEEE Spectrum", July 1969. THE PARAMETER DICTIONARY --- --------- ---------- STIME - Start time, in seconds, of the data to transform. The use of stime causes the the deep water delay to be set to 0. and the first time sample on an inverse transform (PROCESS F2T) to be 0. Preset = the delay of the trace. e.g. stime 3.3 ETIME - End time, in seconds, of the data to transform. The data after etime will be padded with zeroes in order for the data length be a power of 2. Preset = the entire data trace. e.g. etime 4.0 ADDWB WINDOW - The type of window to apply before computing the fft. = hamm, Hamming = hann, Hanning = bart, Bartlett (triangular) = rect, rectangular (box car - no window) = blac, Blackman = ebla, exact Blackman = blha, Blackman-Harris Preset=hamm e.g. window rect WINLEN - The window length, in seconds. A window length of zero causes the entire time domain gate to be windowed. A non zero length indicates that winlen data will be modified at both ends of each data gate. Preset=0 e.g. winlen .2 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 use of rectangular coordinates is slightly faster than polar coordinates, and is understood by frequency domain processes such as f2t. = polr, polar coordinates. The first half of the output trace will be the full (-pi to +pi) amplitude spectrum and the second half of the trace will be the phase spectrum. = ampl, amplitude spectrum. Only the positive portion ( 0 to pi ) of the amplitude spectrum is output. Preset = rect e.g. coords polr FFTLEN - The number of points to use in the FFT. T2F will make this a power of 2 if it is not. Preset = the number of points in the trace. e.g. fftlen 1024 END - Terminates each parameter list. Written and copyrighted (c) by: Paul Henkart, Scripps Institution of Oceanography, January 1984 ALL RIGHTS RESERVED.