[GRASSLIST:3520] Re: Geophysics modules for GRASS

[Craig Funk]

Upward and downward continuation are essentially low and high pass 
filters.

Upward continuation will suppress signals from short wavelength sources 
at shallow depth,
and reveal the signals from long wavelength sources (regional or deep 
sources).

Downward continuation is used to enhance the response of the source at 
depth h by effectively
bringing the surface of measurement closer to the sources. This process 
magnifies the short
wavelengths relative to the long wavelengths thus brings the shallow 
sources more in focus. You could also think of this as a spatial 
de-convolution.

One has to be very careful with downward continuation with respect to 
the sampling rate of the survey and the sources of anomalies. For 
example you should not downward continue a surface below the source of 
an anomaly. There is no physical basis for this in the equations. Also 
if you have some near surface anomalies that where not adequately 
sampled, these signals will appear as lower frequency signals in the 
data (aliased) and these signals will tend to blow up when you downward 
continue. So you really have to "know your data" when you use these 
techniques.

Craig

On 27-May-04, at 12:09 PM, Benjamin Ducke wrote:

>
> What is upward and downward correction?
>
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