{"id":259,"date":"2011-07-03T09:30:42","date_gmt":"2011-07-03T09:30:42","guid":{"rendered":"http:\/\/ahay.org\/blog\/?p=259"},"modified":"2015-09-07T20:46:08","modified_gmt":"2015-09-07T20:46:08","slug":"program-of-the-month-sfnoise","status":"publish","type":"post","link":"https:\/\/ahay.org\/blog\/2011\/07\/03\/program-of-the-month-sfnoise\/","title":{"rendered":"Program of the month: sfnoise"},"content":{"rendered":"

Starting from this post, once a month we will blog about one of the popular Madagascar<\/strong> programs. The 30 highest ranking programs<\/a>, according to admin\/rank.py<\/a>, are<\/p>\n

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grey window math dd spike put graph add cat transp scale pad spray smooth stack noise grey3 real dip ricker1 bandpass dots wiggle mask fft1 segyread contour pick mutter fft3<\/p>\n<\/blockquote>\n

This month’s randomly selected feature is sfnoise<\/a>. <\/p>\n

\"\" <\/p>\n

As stated in the self-documentation, sfnoise<\/strong> is used for generating random noise and adding it to the data. This is useful for generating synthetic datasets or for multiple realizations in inversion. Computer-generated random numbers are often called pseudo-random, because algorithmically generated sequences of numbers are not truly random. This is actually a useful feature if you want to reproduce previous calculations. For reproducibility, it is imperative to use seed=<\/strong> parameter to set the initial “seed” for the random-number generator. If seed=<\/strong> is not used, it is set by the computer clock. Therefore, different runs of sfnoise<\/strong> will produce different sets of numbers. <\/p>\n

Try the following on the command line: <\/p>\n

<\/i>
<\/i><\/a><\/div><\/div>
bash$ sfspike n1=5 mag=0 | sfnoise | sfdisfil \n0: -2.455 1.197 -0.145 0.2394 0.7676 \nbash$ sfspike n1=5 mag=0 | sfnoise | sfdisfil \n0: 2.203 -0.1106 -0.07494 -0.4916 0.2163\n<\/code><\/pre><\/div>\n
Your sets of numbers will be (most likely) different from the ones above and between the two runs. However, if you run  <\/p>\n
<\/i> 
<\/i><\/a><\/div><\/div>
bash$ sfspike n1=5 mag=0 | sfnoise seed=2011 | sfdisfil \n0: 0.1917 0.3379 -0.9459 0.5841 -0.02078\n<\/code><\/pre><\/div>\n
you should see exactly the same set of number as above. You should also get the same numbers if using  <\/p>\n
<\/i> 
<\/i><\/a><\/div><\/div>
bash$ sfspike n1=5 | sfnoise seed=2011 rep=y | sfdisfil \n0: 0.1917 0.3379 -0.9459 0.5841 -0.02078\n<\/code><\/pre><\/div>\n
The rep=<\/strong> parameter controls if the noise is replacing the data or being added to the data. The default operation is addition.  <\/p>\n
<\/i> 
<\/i><\/a><\/div><\/div>
bash$ sfspike n1=5 | sfnoise seed=2011 | sfdisfil \n0: 1.192 1.338 0.05412 1.584 0.9792\n<\/code><\/pre><\/div>\n
The algorithm for pseudorandom number generation that sfnoise<\/strong> uses is known as Mersenne twister<\/a>. It is a powerful algorithm that generates a nearly uniformly distributed sequence that does not repeat for a very large period of $2^{19937}-1$. <\/p>\n
\n
Matsumoto, M.; Nishimura, T. (1998). “Mersenne twister: a 623-dimensionally equidistributed uniform pseudo-random number generator”. ACM Transactions on Modeling and Computer Simulation 8 (1): 3\u009630.<\/p>\n<\/blockquote>\n
Uniformly-distributed numbers can be used to generate random numbers with other distributions. By default, sfnoise<\/strong> is using normal distribution<\/a>. It can be changed to uniform distribution by setting type=n<\/strong>. <\/p>\n