Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 125

depth intervals. In practice it is possible to partly separate,
say, shallow, intermediate and deep responses.

Downward continuation should be used with caution,
but we recommend upward continuation as the filter of
choice for most low-pass filtering operations. A high-pass
filter (detail enhancer) can be obtained indirectly by sub-
tracting the resultant upward-continued data from the
unfiltered data to remove the longer-wavelength compon-
ent emphasised in the continued data. Figures 3.25c and
3.27c were produced in this way. Note the

3.7.3.2 Continuation filters
Particularly useful filters applicable to potential eld data
are the upward and downward continuation filters. In
physical terms, the filter transforms the data to what it
would have been if the measurements had been made at a
different height above the source. Increasing the height
'

'
appearance resulting from suppression of longer-
wavelength variations, the removal of the regional gradient
that is obvious in Fig. 3.27a and the reduced interference
between adjacent anomalies, especially in the northern and
eastern parts of the Las Cruces data.

' sharper

the potential field upwards, i.e. upward con-
tinuation, and the reverse for downward continuation.
Upward continuation is equivalent to increasing the
survey height, downward continuation to decreasing it.
Continuation is possible because the frequency spectrum
(see Appendix 2 ) of potential fields varies in a predictable
manner with distance from the source. The data are usually
continued between planar horizontal surfaces, but it is
also possible to work with irregular surfaces; this is the
basis for the aeromagnetic drape corrections mentioned
in Section 2.4.1.1 .
The effects of changing source

continues '

3.7.4 Gradients/derivatives

Gradients or derivatives of magnetic and gravity fields are

more sensitive than the measured TMI and normal gravity
to changes in the physical properties of the subsurface, so
they are a detail-enhancement filter. Derivatives emphasise
shallow bodies in preference to the deeper-seated broader
features, which produce small changes (gradients) in the
fields. Magnetic and gravity derivatives are calculated from
the TMI and gravity data (see Gradients and curvature in
Section 2.7.4.4 ) , if they were not directly measured with a
gradiometer as described in Section 2.2.3 . The use of the
vertical gradient is especially common. It can be visualised
as the difference between the upward- and downward-
continued (see Section 3.7.3.2 ) responses at equivalent
locations, and normalised (divided) by the difference in
height between the two continuation responses.
Vertical and horizontal gradients are very sensitive to
the edges of bodies and are

detector separation are
shown in Fig. 2.4 . Increasing the separation causes a
decrease in amplitude and increase in wavelength of the
response. Upward continuation is a form of low-pass
filtering. The amplitude of the whole data spectrum is
attenuated with height, but the rate of attenuation is
wavelength-dependent. Shorter wavelengths (high fre-
quencies) associated with near-surface sources attenuate
more rapidly with height than the longer wavelengths
(lower frequencies), so the shallow-sourced responses, plus
any short-wavelength noise, are suppressed. Longer-
wavelength components dominate the filtered data so they
have a smoother appearance ( Fig. 3.25b and 3.27b ) . Con-
versely, downward continuation amplifies the spectrum
with decreasing height, with shorter wavelengths (high
frequencies) ampli

-

. Note how the
relevant responses in Figs. 3.25 and 3.26 are concentrated
at the edges of the prism. For vertical dipping contacts, and
with vertical magnetisation in the case of magnetic sources,
the derivative responses coincide with the contact. They
are displaced slightly from the body when the contact is
dipping or the source is narrow, although this displace-
ment is only normally signi cant for very detailed studies
(Grauch and Cordell, 1987 ) .
Figures 3.25d and 3.27d show the first vertical deriva-
tives of the gravity model and the gravity data, respectively,
with the equivalent responses for the magnetic data shown
in Figs. 3.26b and e , and Figs. 3.28c and d . As shown in
Figs. 3.25 and 3.26 , the relationship between the derivatives
and their respective sources is quite simple for gravity, but
can be more complex for magnetics. The form of the

' edge detectors '

ed more than longer wavelengths
(lower frequencies) so the near-surface response is
enhanced. In practice, data cannot be downward-
continued very far because the transform becomes
unstable. This is because the shorter wavelengths are inher-
ently low in amplitude, or at worst unavailable owing to
inadequate sampling (cf. Section 2.6.1 ) , in measurements
made at greater heights. Also, wavelengths with amplitudes
similar to the noise level cannot be accurately downward-
continued. Instability in the filter is seen as extreme
variations in amplitude over short distances. Downward
continuation is only effective when applied to very high
quality datasets.