Geophysical data acquisition, processing and interpretation - Geophysics for the Mineral Exploration Geoscientist - page 29

usually referred to as stations, or simply data points, and
the distance between them is the station spacing, station
interval or data interval. For airborne surveys and down-
hole logs, the data are acquired using a moving sensor.
Measurements are made at a constant time interval so the
data interval is equal to the time interval multiplied by the
speed of the aircraft/downhole-probe.
When the data are acquired along parallel survey lines
or traverses, as is most commonly the case, the distance
between the lines, the line spacing or line interval,hasa
major influence on the resolution of the resultant map of
the measured response. Line spacing is normally much
larger than the station spacing, but if they are equal then
a regular grid network of measurements is obtained;
compare Figs. 2.10a and c . Survey lines are oriented so
as to be perpendicular to strike. Note that increasing
the survey height increases the system footprint (see
Section 2.6.2 ) and also reduces the short-wavelength
component of the measured response. In principle, this
means that station/measurement spacing and line spacing
can be increased.
In airborne surveying the survey lines or flight lines are
usually flown in alternate directions or headings. Where
the topography is severe, they are flown as groups of lines
with the same heading, referred to as racetrack ying
( Fig. 2.11b ). Racetrack flying tends to reduce the problems
associated with differences in ground clearance (see
Section 2.4.1 ) , so instead of artefacts occurring between
each survey line they mainly occur between the line-groups
flown in opposite directions. Data are also acquired along a
series of tie lines oriented perpendicular to the overall
survey line orientation, and at the same survey height.
The line intersections represent repeat measurements that
are theoretically made at the same point in space, and are
used to monitor noise and correct errors in the survey data.
For the same reasons, ground surveys may include tie lines
and/or repeat readings at selected stations.
Ideally, survey lines should be straight and parallel, with
line spacing and station spacing kept constant for the
entire survey ( Figs. 2.10a and 2.12 ) , but in practice any
number of factors may prevent this ( Fig. 2.10b ) . There may
be gaps in the data caused by, for example, equipment
problems, bodies of water, open pits, buildings, severe
terrain, areas of denied access or areas where there are
high noise levels. Logistical constraints on station distribu-
tion are more severe for ground surveys than in airborne
operations. There are few access limitations for fixed-wing
aircraft and helicopters, unless

a)

Survey-line
spacing

Tie-line spacing
(10 x survey-line spacing)

b)

Figure 2.11 Typical flight paths of airborne surveys. (a) Survey with
adjacent survey lines flown in alternate directions, and (b) survey
flown in racetrack formation. In both cases, perpendicular tie lines
are flown with a spacing of usually 10 times the survey line spacing.

topography appears extreme or tall vegetation and man-
made structures present a problem.
For ground surveying, there may be a requirement to
clear access routes, which can add signi cantly to the cost
of data acquisition. In densely vegetated areas and in very
rugged terrains, it may actually be easier to take
advantage of lakes and rivers, although of course meas-
urements may then be limited to shorelines. Figure 2.10d
illustrates a typical con guration of survey traverses
where measurements are made at relatively small spacing
along roads (possibly widely spaced and of different and
variable orientations), but time and access considerations
dictate that there are fewer measurements between the
traverses.
The tendency to on features of interest, as

exploration progresses, produces an evolving dataset with

' home-in '

flying so low that the