The program sushw (pronounced, SU set header word) is an all purpose utility for setting the value of seismic trace headers. This program permits the user to set one or more trace header words. A common use of sushw is to just set a particular field to one value. For example, sometimes data don't have the ``dt'' field set. Let's say that the data are sampled at 2 ms, By using sukeyword, we see that dt is in microseconds
% sukeyword dt ...skipping unsigned short ns; /* number of samples in this trace */ unsigned short dt; /* sample interval; in micro-seconds */ ...This means that the following command sequence will set all dt values to 2000 microseconds
% sushw < data.su key=dt a=2000 > data.out.suA more tangible example can be seen by piping suplane data into sushw
% suplane | sushw key=dt a=2000 | sugethw key=dt | more 161 wenzel> suplane | sushw key=dt a=2000 | sugethw key=dt | more dt=2000 dt=2000 dt=2000 dt=2000 dt=2000 ...
From the selfdoc for sushw we see that the following optional parameters are defined
Optional parameters (): key=cdp,... header key word(s) to set a=0,... value(s) on first trace b=0,... increment(s) within group c=0,... group increment(s) d=0,... trace number shift(s) j=ULONG_MAX,ULONG_MAX,... number of elements in groupThese extra options permit more complicated operations to be performed. This is necessary, because often there is a relationship between header fields and the position of the trace within the dataset. The value of the header field is computed by the following formula
i = itr + d val(key) = a + b * (i % j) + c * (i / j) where itr is the trace number (first trace has itr=0, NOT 1)where percent % indicates the ``modulo'' function and / is division.
For example if we want to set the sx field of the first 5 traces to 6400, the second 5 traces to 6300, decrementing by -100 for each 5 trace groups
% sushw < data.su key=sx a=6400 c=-100 j=5 > data.new.suAgain, piping in suplane data into sushw
% suplane | sushw key=sx a=6400 c=-100 j=5 | sugethw key=sx | more sx=6400 sx=6400 sx=6400 sx=6400 sx=6400 sx=6300 sx=6300 sx=6300 sx=6300 sx=6300 sx=6200 sx=6200 ...
As another example, if we wanted set the ``offset'' fields of each group of 5 traces to 200,400,...,6400
% sushw < data.su key=offset a=200 b=200 j=5 > data.out.suAs before, piping suplane data into sushw yields the following
% suplane | sushw key=offset a=200 b=200 j=5 | sugethw key=offset | more offset=200 offset=400 offset=600 offset=800 offset=1000 offset=200 offset=400 offset=600 offset=800 offset=1000 offset=200 ...
We can perform all 3 operations with one call to sushw, via:
% sushw < data.su key=dt,sx,offset a=2000,6400,200 b=0,0,200 c=0,-100,0 j=0,5,5 > newdata.suOr with suplane data piped in
% suplane | sushw key=dt,sx,offset a=2000,6400,200 b=0,0,200 c=0,-100,0 j=0,5,5 | sugethw key=dt,sx,offset | more dt=2000 sx=6400 offset=200 dt=2000 sx=6400 offset=400 dt=2000 sx=6400 offset=600 dt=2000 sx=6400 offset=800 dt=2000 sx=6400 offset=1000 dt=2000 sx=6300 offset=200 dt=2000 sx=6300 offset=400 dt=2000 sx=6300 offset=600 dt=2000 sx=6300 offset=800 dt=2000 sx=6300 offset=1000 dt=2000 sx=6200 offset=200 dt=2000 sx=6200 offset=400 ...As you can see, it is natural to use pipes and redirects to control job flow, but this becomes ungainly on a single command line. Later in this document, we will see how to construct complicated processing sequences in the controlled environment of shell scripts.