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Finally, it may be observed that all quantitative interpretation methods give good, reliable results on well-defined, well separated anomalies, as may be shown by testing their performance on independently calculated theoretical profiles rather than field curves.

A practical limitation to accuracy is that the choice of model may be incorrect. Excellent fit of observed and calculated profiles is no guarantee that the model choice is correct;

other models may provide equally good or better fits. The best support for interpretation

Figure 10.2 3D Euler solutions from analysis of aeromagnetic data over SW Sri Lanka (Perera, 1997).

comes from independent dedicated interpreter struggles on to reach this limit. Though the process may be time-consuming and tedious, the full value of the data should be realised in the interpretation.

10.3 Qualitative interpretation.

What is called 'qualitative interpretation' of aeromagnetic surveys is usually undertaken as an aid to geological mapping, often using the property of magnetic surveys to reveal the magnetic signature of the crystalline bedrock, even where it is covered by superficial deposits and thus invisible to the conventional field or photogeologist.

The first stage of this process is simply to prepare the geophysical data and the best available geological data at the same scale and projection (one of them preferably on a transparent overlay or both of them in a GIS platform) so that they may be directly compared. Usually there are some obvious correlations between the two data sets in the outcrop areas that allow geological deductions to be made from the geophysical data in the areas lacking in outcrop. This leads naturally into the process known as 'zoning' of the aeromagnetic map that is an important part of the qualitative interpretation process.

Zoning involves the outlining of those areas of well-defined physical expression on the aeromagnetic map that appear to the interpreter as distinct geological units. Note that one zone typically may contain many hundred individual anomalies that often have a

Figure 10.3 The Naudy ‘automatic’ depth interpretation method employed on a single magnetic profile.

common pattern, such as working with air photographs or satellite images. Figure 10.6 shows an area of an aeromagnetic map displaying several different types of

magnetic signature

associated with different rock types and its interpretation. forested and deeply weathered terrain. The aeromagnetic map of the same area shows intense detail, the qualitative interpretation of which provides a great deal of extra information (Figure 10.6), such as the location and extent of greenstone belts which might, for example, be explored for massive sulphides using airborne EM.

To assist in the zoning process - and in the delineation of structural features such as faults - considerable use has been made of equivalent earth susceptibility maps (see Section 7.3). The magnetic basement is assumed to be made up of semi-infinite vertical prisms with square tops centred on the nodes of the gridded magnetic data. By an inversion process, a magnetic susceptibility is ascribed to each prism. Commonly, the depth to the top of all the prisms has been a constant value below the survey flying height, either the ground surface or some deeper level. More recently, methods have been devised which allow the prism tops to constitute an irregular surface, such as may be determined from magnetic depth estimates or drilling information in the study area.

This allows the susceptibility boundaries, as revealed by contours of the individual prism susceptibility values, to be brought more sharply into focus in the susceptibility map, and hence to facilitate the zoning process for a metamorphic basement below nonmagnetic cover of varying thickness (see, for example, Pilkington 1989).

The zoning process is, of course, subjective. To ensure rigour it is best thought of in two phases:

1) dividing a map into geophysically similar units based on the physical description of anomaly patterns;

Figure 10.4 The absolute value of the analytic signal is known as the energy envelope. (From Roest et al., 1992).

2) ascribing geological names to the geophysical units with due reference to the information on any available geological map.

In principle, the geophysical map could be divided into an infinite number of geophysical units, each closely defined in terms of anomaly amplitudes, shapes of body outlines, spacing of magnetic lineations, and so on. In practice, the number of pigeon-holes into which identifiable zones may be placed has to be kept to a manageably small number, perhaps 20-30. We are concerned with an essentially descriptive process and description also implies simplification; description of every single anomaly is eventually less helpful than a few well-considered generalisations about anomaly pattern types. By the same token, a geological map is a generalisation and a simplification of field observations, designed to convey a general impression of the geology, and the geological legend usually contains only a few tens of entries.

The generalised geophysical units (or zones) can be defined in type areas (e.g. where geological exposure - or the quality of the geological mapping - is good), but outside the type area, other zones can show (e.g.) a majority of the same physical features, but also fail to show all of them. Is this a new or a different geophysical zone, or can it be categorized along with an existing type? Answering this question requires the establishment of thresholds of similarity - subjectivity is inevitable, but attempting to reduce it requires perseverance.

Experience seems to indicate, however, that any purely geophysical classification of rock units leads to the situation where, upon comparison with the geological map, a geophysical entity contains several distinctly different rock types and, conversely, that a single geological rock unit can contain several geophysically distinct units.

Figure 10.5 The concept of the ‘magnetic basement’ and its flaws. Magnetic anomalies often arise from rock units reaching the interface between the igneous and metamorphic rocks and the overlying (non-magnetic) sediments (e.g. unit A). Other units used as depth indicators, such as B, may not reach this surface, while others still, such as C, may reach a higher level giving erroneous indications of the basement depth.