The signal amplitude and coherencies are used in evaluating data quality, and identifying the source(s) of noise in the data. For identifying the source(s) of noise in the data it is most useful to have these
parameters in the direction of the acquisition sensors, and for evaluating data quality it is most useful to have these parameters in the direction of the apparent resistivities.
The orientation of these parameters can be specified using the ROT option. This option may be ROT=NONE, meaning that the parameters
correspond to the measurement directions, ROT=RHOROT, meaning that the parameters correspond to
the rotation angles given in the >RHOROT block, or RHO=NORTH, meaning that the parameters have
been rotated to true north. The default is ROT=NONE.
The signal amplitude or power spectral density for any measurement can be delivered using a SIGAMP
data block. The signal level may be useful in diagnosing poor data. The default for SIGAMP data is
amplitude spectra. However, the option AMPPWR=PWR can be used to provide auto-power spectra
instead. The units are the same as those for >SPECTRA data blocks, except, of
course, for the appropriate amplitude/power conversion. The EUNITS or HUNITS option (as appropriate)
may be used to specify the units if other than the default.
The signal amplitude (or power spectral density) can either be calculated from the total field (including the
noise amplitude) or estimated using a remote field to eliminate the noise amplitude (Gambel, 1978). The
default for the signal amplitude is the total field; however, the option TYPE=SIGNAL or TYPE=NOISE can
be used to deliver the estimated signal or estimated noise.
Chave, A.D. and Jones, A.G. eds., 2012. The magnetotelluric method: Theory and practice. Cambridge University Press.
Wight, D.E., 1987. Society of Exploration Geophysicists MT/EMAP Data Interchange Standard.