Interpreting the results

view_cohe

Be sure to check the coherency before interpreting the results!

We recommend studying the coherence plots carefully before moving on the stage 0. These plots allow the end-user to estimate how robust the final solution will be and at which frequencies it will be most reliable. The more coherent the signals, the better/ more meaningful the results.

Ground noise must be “of sufficiently high amplitude to guarantee that one is recording above the” instrument noise “across the entire frequency band of interest”. - Pavlis and Vernon, 1994

“Coherence estimates are essential to appraise the validity of results.” - Pavlis and Vernon, 1994

“…when signals are highly incoherent, estimates of amplitude will tend to be underestimated, and phase estimates will become poorly determined under these conditions. ” - Pavlis and Vernon, 1994

stage 4

A transfer function can be approximated by many different combinations of poles and zeros. As the solution is not unique, respGen does not and cannot exactly reproduce the instrument manufacturers published poles and zeros.

A Note on the high frequency corner

An instrument response file is composed of two basic elements: 1) the response of the seismometer or accelerometer which controls the behavior of the lower frequency corner; and 2) the response of the digitizer which controls the behavior of the high frequency corner. The high frequency corner is generally very steep. It is controlled by the decimation routines commonly employed by digitizer manufacturers to downsample data (e.g., FIR, Butterworth, …). respGen sometimes cannot reproduce these high frequency corners because it is difficult to reproduce with a reasonable number of poles and zeros. Fortunately, in most cases the behavior of the digital antialias filter is known (or can be easily approximated) and the FIR filter coefficients can be manually appended to the resulting RESP file.