Survey objectives

A geophysical survey should be undertaken with a clear objective in mind since this critically affects both the

geophysical method that should be used and also the type of survey, for example airborne or downhole. The two most common objectives of mineral geophysical surveys are to map the local geology and/or to measure responses originating from the mineralised environment. Mapping surveys provide essential geological context, and in areas of poor outcrop the data may comprise the only useful form of‘geological’ map available. Surveys designed to target the mineralised environment may be intended to detect or to define the geological/geophysical features of potential sig- nificance. Detection simply involves ascertaining whether ‘something’ is there or not, and obviously the ‘something’ has to produce a detectable geophysical response so that its presence can be detected. Surveys designed to characterise the source of a response are required when information about the nature of the source is required, usually to design a drilling programme to sample it.

Putting aside the inevitable influence of costs and budgets (see Section 1.2.3), the decision over whether to use geophysics in an exploration or mining programme, and if so which method to use, depends on the geophysical detectability of the features of interest. If significant phys- ical property contrasts do not occur in the survey area then the chances of a successful outcome are much reduced. Clearly, some understanding of the likely physical proper- ties of the geological environment being surveyed is para- mount. This might be based on petrophysical data from the survey area; more probably, especially in the early stages of exploration, it will be based on data from other areas or from published data compilations. Successful out- comes are more likely when there is a good understanding of petrophysics, one reason that we emphasise this subject throughout this text.

2.5.1

Geological mapping

Mapping the local geology seeks to identify geological settings conducive to the formation of orebodies. In poorly explored areas the primary intention may be simply to create a geological‘base map’, which will form a basis for assessing the area’s prospectivity. In better known areas particular deposit types may be sought and the features of primary interest will depend on the exploration model being applied. A common example is seeking to map major faults which may have acted as conduits for mineralising fluids. Alternatively, if the mineralisation being sought is strata-bound then mapping the prospective lithotypes or lithological contacts may be the aim of the survey.

We reiterate the important point made in Section 1.1

that the contrasts in physical properties that control geo- physical responses do not necessarily have a one to one correlation with contrasts in lithotype. This is because it is the rock-forming minerals that are the basis for assigning lithological names, and these in turn are controlled by rock chemistry. In contrast, the physical properties of relevance to geophysics are not entirely, or sometimes even slightly, controlled by the rock-forming minerals. For this reason, a ‘geological’ map created using geophysical measurements should be referred to as a pseudo-geological map. Compilation of the pseudo-geological map involves iden- tifying the near-surface and deeper responses and classify- ing the remaining responses according to their possible sources (seeSection 2.11). The integrated analysis of mul- tiple data types, i.e. magnetics, radiometrics, conductivity etc., helps to produce a more reliable and accurate model of the subsurface geology.

Surveys designed for geological mapping should provide a uniform coverage of geophysical data across the area of interest. With large areas to cover this will probably require an airborne survey and survey specifications typical of reconnaissance objectives (see Section 2.6.3). As explor- ation focuses on areas considered to be most prospective, more detailed surveys may be undertaken and ground surveys may be used. The most common types of geophys- ical surveys for geological mapping are airborne magnetic and radiometric surveys. Airborne gravity surveys are becoming more common as surveying technology impro- ves. These geophysical methods produce responses that distinguish a wide range of lithotypes and are favoured in most geological environments. The more complex, diffi- cult to interpret and expensive electrical and electromag- netic methods tend to be used less, although airborne electromagnetics is increasingly being used in a mapping role.

2.5.2

Anomaly detection

The exploration strategy can involve surveys intended to detect localised responses distinctly different from their surroundings, i.e. anomalies. This approach is sometimes referred to as ‘searching for bumps’, i.e. looking for localised anomalously ‘low’ or anomalously ‘high’ values in various presentations of the survey data. This is a simple and effective form of targeting and is a valid approach when exploration targets give rise to distinct anomalies that are easily distinguishable from

the responses of unwanted features, such as the sur- rounding rock formations and mineralisation of no eco- nomic significance. The strategy can be applied at regional scale to select areas for detailed work, and at prospect scale to detect a target anomaly. It is also applicable when a deposit is being mined, with adjacent orebodies being the target. Also, surveys designed to detect faults or dykes ahead of coal mining are basically aimed at identifying anomalous parts of the geological environment.

Anomalous responses associated with the mineralised environment may be caused by the mineralisation itself, although not necessarily the actual ore minerals. Deposits comprising massive or disseminated metal sulphides and oxides are commonly targeted in this manner. Also targeted are alteration zones caused by mineralising fluids, which have the advantage of usually being much larger in area than the target mineralisation, so the geo- physical response from the alteration covers a corres- pondingly larger area, which may help to facilitate its detection. Porphyry style copper deposits are an example of deposits with extensive, geophysically distinctive, alter- ation haloes. Another form of anomaly targeting seeks to locate specific lithotypes in which mineralisation occurs, e.g. potentially diamondiferous kimberlitic and lamproitic intrusions, or ultramafic intrusions that might contain platinum group elements, or palaeochannels hosting placer deposits.

A survey designed to detect the responses from the mineralised environment requires a survey strategy based on the probability of making a measurement in the right place, i.e. within the bounds of its geophysical response (seeSection 2.6.4). This ensures that the anom- alous responses are both recorded and recognised as significant (see Section 2.5.3). Needless to say, some knowledge of the physical properties of the targets is required to ensure that responses are anticipated in the chosen form of geophysical data. The depth and volume of the source, and the magnitude of the physical prop- erty contrast with its host, are also important since the amplitude of the anomaly depends on this (see Section 2.5). In this context an appreciation of noise levels is required, especially geological noise. For example it may be comparatively easy to identify the magnetic response of a kimberlite intruding a weakly magnetised sediment- ary sequence, but the response may be unrecognisable in, say, a terrain comprising a variably magnetised succes- sion of basalts.

2.5.3

Anomaly definition

Surveys designed to improve the definition of an anomal- ous response are aimed at obtaining more information about the source of the anomaly, and are often used for designing drilling programmes. The surveys are conducted at prospect scale and during exploration in the mine environment. It may be a detailed ground survey to follow up an anomaly detected by an airborne survey or possibly a wider-ranging lower resolution ground survey. Informa- tion about the source such as its extent, shape, dip and depth can be obtained, usually by modelling the anomaly

(see Section 2.11). Accurately characterising a response

requires careful consideration of the survey configuration and the distribution of the measurements within the area of interest (seeSection 2.6).