Survey design

An important consideration in designing a geophysical survey is the optimum number and distribution of the measurements to be made. Too many measurements are a waste of time and money, too few and the survey’s objective may not be achieved. In deciding the spacing between measurements it is important to consider the wavelength of the expected responses and the footprint of the measurement, i.e. the size of the area influencing the measurement. The distance between individual measure- ments should be sufficiently close to satisfy the require- ments of the sampling theorem (see Section 2.6.1). It is important that the survey extends across a large enough area to define the longest wavelength response of interest

(seeAppendix 2), and that the survey exceeds the limits of

the area of interest in order to determine the regional response (see Section 2.9.2), so as to facilitate its removal from the survey data. Survey design should also account for the need to minimise all sources of noise (seeSection 2.4). An example of a data acquisition variable directly related to improving SNR is the number of repeat meas- urements (if any) to make at each location. These allow suppression of random noise (seeSection 2.7.4.1), but also increase the time (and cost) to acquire the data. Ultimately, the survey budget will dictate how many measurements may be taken and their accuracy, and geological and logis- tical factors will also constrain their distribution.

Note that it is common for several parameters to be measured in a single geophysical survey, e.g. airborne magnetics and radiometrics. Characteristics of all the parameters being measured need to be considered when designing the survey, but usually the parameter of principal interest will control the setting of survey parameters.

2.6.3.1 Modelling as an aid to survey design

Whether the aim of a survey is mapping, detection or characterisation, the geological characteristics of the target and its geophysical response, with respect to the response

of the surrounding geology, need to be considered during survey design. Geophysical modelling, whereby the geo- physical response of a numerical model of the subsurface is computed (see Section 2.11), provides information about the nature of the geophysical response of the expected subsurface geology useful for survey design.

For a specific subsurface feature, including topography, the survey system and configuration most capable of detecting the feature and yielding the required information about it can be determined from modelling. This is par- ticularly important for the electrical and electromagnetic methods where a great diversity of survey systems is avail- able and a great diversity of survey configurations is pos- sible. For a particular survey system and configuration, a suitable sampling interval can be determined; the effects of changes in target parameters on the response can be assessed and those having the greatest or the least influence identified. The amplitude of the response can be used to estimate the acceptable level of noise.

Modelling is an important aid in the design of cost- effective and efficient surveys, and its role should not be underestimated. However, it is only as good as the assump- tions that are necessarily made about the geological envir- onment to be surveyed, which in the early stages of exploration may be highly speculative.

2.6.3.2 Survey height

The height of the survey system above the ground is an important parameter in all types of airborne geophysical surveys. Survey aircraft are equipped with radio altimeters to record the aircraft’s ground clearance. This is the survey height or flight height above the ground, which usually varies according to the topography, so it is specified as the mean terrain clearance (MTC) of the survey. Lower survey height reduces the system footprint

(see Section 2.6.2), improves spatial resolution and

increases the strength of the signal (see Section 2.3). Operational constraints, however, dictate the minimum MTC; and lower survey height is not always an advantage, as close proximity to near-surface sources increases their short-wavelength responses, which may be a source of noise (see Section 2.4.1). A small station interval is required to sample these properly (see Section 2.6.1). Even if this is achieved it can be difficult to correlate the responses between the survey lines, making gridding dif- ficult (see Section 2.7.2) and possibly masking responses from the underlying geology.

2.6.3.3 Survey configurations

Data acquired along a single survey traverse or in a drill- hole form a one-dimensional (1D) (spatial) data series, because variations in the measured parameter are depicted in only one direction. A time series is also a 1D data series. A group of 1D spatial series or a random distribution of measurements across a surface can be combined to form a two-dimensional (2D) data series. In this case, the data are presented as a map in terms of two spatial coordinates (easting and northing, or latitude and longitude). Most above-ground surveys are 2D and provide a map showing the spatial variation of the parameter measured.

Some common 2D geophysical survey configurations are shown in Fig. 2.10. The measurement locations are

a) b) c) d) Straight traverses Irregular traverses Square grid Tie line Random Survey/flight line

Figure 2.10 Some common configurations for geophysical surveys.

The dots represent data points. (a), (b), Various traverse or line configurations; (c) grid network; and (d) configuration typical of ground-based regional surveys.

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, has a 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 orflight lines are usually flown in alternate directions or headings. Where the topography is severe, they areflown as groups of lines with the same heading, referred to as racetrack flying

(Fig. 2.11b). Racetrackflying 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 forfixed-wing aircraft and helicopters, unless flying so low that the

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 significantly 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 configuration 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 ‘home-in’ on features of interest, as exploration progresses, produces an evolving dataset with

a) b) Tie-line spacing (10 x survey-line spacing) Survey-line spacing

Figure 2.11 Typicalflight paths of airborne surveys. (a) Survey with

adjacent survey linesflown in alternate directions, and (b) survey

flown in racetrack formation. In both cases, perpendicular tie lines

areflown with a spacing of usually 10 times the survey line spacing.

a range of station and line intervals (Fig. 2.10d). Here a small area has been surveyed with greater detail, as would be required to characterise a target (see Section 2.6.1). Multiple surveys of differing specifications and extent are a common occurrence and can provide useful information for designing subsequent surveys.

An uneven spatial distribution of samples can lead to aliasing (seeSection 2.6.1).Figure 2.10shows several survey configurations with uneven sampling. In these cases the degree of aliasing will vary with location and, in line-based configurations, with survey line orientation. The smaller along-line sampling interval allows short-wavelength

a) e) f) b) Anomaly maximum > 8000 nT Anomaly maximum > 1800 nT Anomaly maximum > 8000 nT Anomaly maximum > 1800 nT c) g) h) d)

Anomaly maximum > 2900 nT Anomaly maximum > 8000 nT

0 1

Kilometre Survey lines

Anomaly maximum > 8000 nT Anomaly maximum > 8000 nT 4000

500 TMI (nT)

Figure 2.12 Aeromagnetic data from the Marmora Fe deposit. The gridded, imaged and contoured data were created from various subsets of the full dataset to

demonstrate the effects of survey line spacing and position on anomaly resolution. The contour interval is variable. (a)–(d) Line spacing is 1,600 m, with the lines progressively laterally shifted 400 m in each case. (e)–(h) Line spacing progressively decreased by 400 m, with the central line optimally located directly over the deposit: (e) 1,600 m, (f) 1,200 m, (g) 800 m, (h) 400 m. Note the spectacular improvement in every aspect of the anomaly, i.e. its amplitude, shape, gradients and trend direction, with optimally positioned lines and decreasing line spacing.

TMI– total magnetic intensity. Based on diagrams in

variations to be properly represented (at least up to the Nyquist frequency of the sampling interval), but the much greater distance between lines, i.e. greater across-line sam- pling interval, means similar variations in the across-line direction are likely to be under-sampled (aliased). This is often less of a problem that it might seem because orientat- ing the survey lines perpendicular to strike ensures there will be a greater degree of variation in the geophysical response along-line than in the strike-parallel across-line direction. Of course, for responses that are equidimen- sional, the sampling interval should ideally be the same in both directions in order to produce a properly sampled anomaly. In reality, it is the (wider) line spacing that deter- mines overall survey resolution.

For airborne surveys, the nature of the topography in relation to the survey parameters will determine the survey platform and the cost of the survey. Terrains of moderate relief can be surveyed with smallfixed-wing aircraft, whilst helicopters are appropriate for low-level surveying of mountainous areas (Mudge, 1996). Cost and logistical considerations of course will ultimately determine thefinal survey parameters; and to that extent, the line spacing should be increased first and, if necessary, followed by limited increase in survey height.

Unmanned airborne survey platforms are likely to become available for commercial use in the near future (McBarnet,2005). The advantages of using an unmanned aircraft include reduction in cost, the ability to fly lower and in poorer visibility than is possible with manned aircraft, and the possibility of surveying at night when environmental noise levels are lower.