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
=
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
=
f
3
+
f
4
2
If
Δ
x
represents the space between each station along the line then
a
v
e
r
a
g
e
12
−
a
v
e
r
a
g
e
34
2
Δ
x
=
(
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
)
.
