FAQ Answer

I downloaded data in SAC format. Is the unit of dependent variable in the data mm/sec, or nm/sec, or mm/sec/sec, etc.?

The units of displacement vary, depending upon the instrument response values and sensor type. These units are listed in the SEED volumes that you receive from us, and are available from the output of the program "rdseed" using the -R option. This option writes files that we call instrument response files, and look like this listing of the file RESP.PS.BAG..BHE, for example. If you look at this file, you will see listed something like this:

#         =======================================
#   +   +--------------------------------------------+    +
#   +   |   Response (Poles & Zeros),   BAG ch BHE   |    +
#   +   +--------------------------------------------+    +
B053F03     Transfer function type:    A [Laplace Transform (Rad/sec)]
B053F04     Stage sequence number:     1
B053F05     Response in units lookup:  M/S - Velocity in Meters Per Second
B053F06     Response out units lookup: V - Volts
B053F07     A0 normalization factor:   3948.58
B053F08     Normalization frequency:   0.02
B053F09     Number of zeroes:          2
B053F14     Number of poles:           4

where you will see that for this channel, its stated as velocity, in meters/second. Others may be acceleration or displacement.