Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 170

isolating the target response, and highlights some of the
problems in actually achieving this. It is based on data
from the northern Sierra Nevada, California, USA,
described by Chapman et al.( 1980 ).
The aim of the gravity survey was to trace the positions
of two Tertiary drainage features, called the Port Wine and
Brown palaeochannels, between locations where they were
known from previous mining or drilling. The locations of
the drill sections and the gravity stations, along with grav-
ity data, are shown in Fig. 3.70 . Part of the Port Wine
palaeochannel was worked from the Iowa Mine. The only
other workings in the area are the Hardscrabble Pit and
Brown Shaft, which are within the Brown palaeochannel.
The palaeochannels incise basement comprising a
range of lithotypes, including amphibolites,

course of the Port Wine channel as a northeasterly
trending gravity ' low ' extending from the drillhole inter-

A 0 through the Iowa Mine to tra- sections on traverse A -

B 0 . Note that the map of gravity stations shows
that the northwestern bank of the channel is not de ned as
the survey did not extend far enough to the northwest. This
allows the gridding algorithm to extend the low channel-
related values to the northwest, and possibly erroneously.
A subsidiary northwest-trending channel appears to inter-
sect the main channel near the Iowa Mine. Continuation of
the Port Wine channel to the northeast, and its relation-
ship with the Brown channel, are not clearly defined owing
to the effects of the shear-zone anomaly. The Brown
palaeochannel appears to become wider to the south, with
the Hardscrabble Pit/Brown Shaft lying on its western
edge.
The shear-zone anomaly presents a signi

verse B -

' greenstones '

(presumably ma c igneous rocks), and slates and

granodiorite. Dips are steep, and strike is northwest - cant problem

for regional removal. Its wavelength is similar to the
response of the palaeochannels. Moreover, it is geologically
quite likely that a basement shear zone would be preferen-
tially eroded and, consequently, in uence the drainage
system; so they are likely to be spatially coincident. It is,
therefore, unlikely that the shear zone and palaeochannel
anomalies can be completely separated based on their
wavelengths. It is uncertain whether the shear-zone anom-
aly should be treated entirely as part of the regional field, or
whether all or part should be considered residual in nature.
Three methods of regional removal have been applied to
the BA data. In all cases the residual is obtained by sub-
tracting the regional field from the Bouguer anomaly data.
Figures 3.70c and d show the data after low-pass wave-
length filtering with the cut-off wavelength (500 m) chosen
to include as much of the shear-zone anomaly as possible
in the regional variation, so that it is attenuated in the
residual data. The residual data con

southeast. The channel ll is Tertiary gravel which is

overlain by

flow breccias and alluvium. Drilling along

A 0 showed the Port Wine channel fill to be up
to around 200 m thick, and the channel to be about 500 m
wide. Drilling along the southeastern end of traverse B

traverse A -

B 0 -

has apparently intersected the margin of the same

palaeochannel.
Gravity measurements were made along a series of tra-
verses oriented roughly perpendicular to the palaeodrai-
nage, but access restrictions produced an uneven station
distribution. The resultant station locations are shown in
Fig. 3.70 . As noted previously, for surveys with irregular
station distribution it is good practice to display the sta-
tions so that features can be assessed in terms of artefacts
produced by the uneven data sampling and poorly
constrained interpolation by the gridding algorithm. The
data were reduced to Bouguer anomaly using a density of
1.85 g/cm 3 , and terrain corrections were applied to com-
pensate for the steep terrain.
The Bouguer anomaly (BA) is dominated by a north-
easterly negative gradient ( Fig. 3.70a ), culminating in a
prominent gravity

rm the course of the

Port Wine channel suggested by the first vertical derivative

data, and the tributary near the Iowa Mine is also con-
firmed. In addition, evidence is revealed of two palaeo-
channels east of the area. The Hardscrabble Pit/Brown
Shaft is located in the middle of the western one of these,
which trends north

anomaly in the eastern part of
the survey area. Chapman et al.( 1980 ) suggest that this is
due to a shear zone within greenstones. This is important
to the interpretation of the data and will be referred to as
the

' low '

trends
parallel to the shear-zone anomaly and may be due to this
and/or palaeodrainage.
Figures 3.70e and f show the results for a regional eld
obtained by upward continuation to a height of 400 m so
as to remove as much of the shear-zone anomaly as pos-
sible (compare Figs. 3.70e and c ). In the residual response
the Brown palaeochannel appears to be a single drainage

- south. The other gravity ' low '

. There is evidence of a decrease
in gravity related to the Port Wine channel along traverse
B

' shear-zone anomaly '

B 0 , but elsewhere any response from the channel ll is
disguised by the regional gradient.
The first vertical derivative of the gravity emphasises
shallow features. It is reasonably effective in defining the

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