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 $f (i) = f_{i}$ represents the collected data then $a v e r a g e_{12} = \frac{f_{1} + f_{2}}{2}$ 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: $a v e r a g e_{34} = \frac{f_{3} + f_{4}}{2}$

If $Δ x$ represents the space between each station along the line then $\frac{a v e r a g e_{12} - a v e r a g e_{34}}{2 Δ x} = \frac{(f_{1} + f_{2}) - (f_{3} + f_{4})}{4 Δ x}$ is the Fraser Filter of those four values.

Since $4 Δ x$ is constant, it can be ignored and the Fraser filter considered to be $(f_{1} + f_{2}) - (f_{3} + f_{4})$ .

References

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Fraser filter

References

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