Fraser Filter

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A Fraser Filter, named after Douglas Fraser, is typically used in geophysics when displaying VLF data. It is effectively the first derivative of the data.

If f(i) = f_i represents the collected data then average_{12}=\frac{f_1 + f_2}{2} is the average of two values. Consider this value to be plotted between point 1 and point 2 and do the same with points 3 and 4: average_{34}=\frac{f_3 + f_4}{2}

If \Delta x represents the space between each station along the line then \frac{average_{12}-average_{34}}{2 \Delta x}=\frac{(f_1 + f_2)-(f_3 + f_4)}{4 \Delta x} is the Fraser Filter of those four values.

Since 4 \Delta x is constant, it can be ignored and the Fraser Filter considered to be (f_1 + f_2)-(f_3 + f_4).


Telford, W.M.; L.P. Geldart; R.E. Sheriff. Applied Geophyisics (2nd ed.). pp. p480+.