Fraser Filter

A Fraser Filter 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$(f\_1\; +\; f\_2)-(f\_3\; +\; f\_4)$.

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