Fraser Filter

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 formula_1 represents the collected data then formula_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: formula_3
If formula_4 represents the space between each station along the line then
formula_5 is the Fraser Filter of those four values.
Since formula_6 is constant, it can be ignored and the Fraser Filter considered to be
formula_7.