The Katsuraoka mine is situated in the southwestern part of Oshima Peninsula, Hokkaido. Pyrometasomatic magnetite ore deposits occur in the Palaeozoic formation (Katsuraoka formation).
The Katsuraoka formation is classified into 'five members based chiefly upon the remarkable change of the rock facies and yielded fossils such as Fusulinella bocki (MÖLLER) and Chaetetes sp. From these fossils the age of the formation is determined as Middle Carboniferous. The Katsuraoka formation is cut by many faults of NNW or NNE trend which are arranged in a fan shape.
The Miocene strata, covering the Paleozoic formations unconformably, are classified into two formations, the Asshinai formation (green tuff and andesite) and the Nakasuda formation (mud tone).
The ore deposits were formed in the lower chert member, especially in the area where limestone comes near contact with igneous rocks. It is clear that the localization of ore deposits is controlled by the faults and shear zones and intrusive masses.

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Ore deposits of the Kurosawa mine comprise six groups of small ore deposits, half of which are composed mainly of gypsum, and the rests are of composite ore bodies of gypsum, iron sulphide ore, yellow ore and black ore. The mine has been exploited for more than 20 years for gypsum at one time and for sulphide ores at another, following the successive discoveries of each different, ore body. Recent monthly production of crude pyritic ores amounts up to 2, 500 tons.
The ore deposits occur in the white gray fine-grained tuff, the uppermost bed, of the Kurosawazawa formation, and are underlain in some cases by rhyolite (R2) lava beds. The ore deposits are overlain by black mudstone. The next upper formation is characterized by dacite and tuffaceous beds with graded bedding and by intercalated basaltic agglomerate, and is in turn overlain by the pyroclastic formation which was deposited by the eruption of rhyolite (R3).
In the Sanjin pyritic ore deposits occur the zone (1) of very fine-grained pyrite and clay, the zone (2) of fine-grained pyrite and less amount of clay, and the zone (3) of medium-grained pyrite in the ascending order. Graine size of pyrite increases and the amount of clay decreases gradually in the ascending order. The zone (2) contains many small nodular balls of massive pyritic ore, while the zone (1) contains less abundant but much larger boulders of pyritic ore.
All ore deposits are deformed and folded irregularly by later tectonic movements, but the successive stratification of gypsum zone, pyrite zone, yellow ore zone and black ore zone in ascending order can be generally recognized in each ore body, Above the ore are locally observed hematite-pyrite rich zones with or without a intervening mudstone or tuff bed. Disseminations and stringers of hematite-siderite-pyrite mineralization are also observed especially in shear zones.
The sericitization is characteristic in the sulphide ore zones, while the chloritization is remarkable in the gypsum zone. The montmorillonitization seems to be restricted to the pyroclastic beds of the upper horizons than the hanging wall black mudstone bed.

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Usually, the interpretation of electrical anomalies in the electrical prospecting consists in solving the inverse potential problem. It is very difficult to solve the general problem of any resistivity distribution. Thus certain simplifying assumptions are often made about the types of resistivity distributions which occur in practice. Within the limits imposed by these assumptions, the standard curves and master curves have been calculated by many geophysicists.
In this paper, the author intends to explain the interpretation method by the analog computer which has been studied by the author and his assistants since 1954.
Many standard curves are obtained and the effects of topography and near-surface conditions are interpreted and the direct interpretation method is explained by using some electrical field surveys.

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