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Fig. 5.1 Artificial example to illustrate concept of combining two binary patterns related to
occurrence of mineral deposits; ( a ) outcrop pattern of rock type, lineaments, and mineral deposits;
( b ) rock type and deposits dilatated to unit cells; ( c ) lineaments dilatated to corridors;
( d ) superposition of three patterns (Source: Agterberg et al. 1990 , Fig. 1)
maps by incorporating new data originally had been used by Dowds ( 1969 )who
updated probabilities of oil well occurrence maps by including later discoveries. The
method of weights of evidence modeling was invented by Good ( 1950 ). In its original
application to mineral resource potential mapping (Agterberg 1989a ), extensive use
was made of medical applications (Spiegelhalter and Knill-Jones 1984 ).
5.1.1 Basic Concepts and Artificial Example
Figure 5.1 illustrates the concept of combining two binary patterns for which it can
be assumed that they are related to occurrences of mineral deposits of a given type.
Figure 5.1a shows locations of six hypothetical deposits, the outcrop pattern of a
rock type ( B ) with which several of the deposits may be associated (Fig. 5.1b ), and
two lineaments that have been dilatated in Fig. 5.1c . Within the corridors around the
lineaments, the likelihood of locating deposits may be greater than elsewhere in the
study area. In Fig. 5.1b-d , the deposits are surrounded by a small unit area. This
permits estimation the unconditional “prior” probability P ( D ) that a unit area with
random location in the study area contains a deposit, as well as the conditional
“posterior” probabilities P ( D | B ), P ( D | C ) and P ( D | BC ) that unit areas located on the
rock type, within a corridor and both on the rock type and within a corridor contain
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