A mathematical basis for the application of power spectrum analysis to aeromagnetic map interpretation is developed. An ensemble of blocks of varying depth, width, thickness, and magnetization is considered as a statistical model. With the use of the fundamental postulate of statistical mechanics, a formula which can be used to analyze the power spectrum of an aeromagnetic map is developed. The influences of horizontal size, depth, thickness, and depth extent of the blocks on the shape of the power spectrum are assessed. Examples which include power spectra of maps from Canada and Central America demonstrate the application of the approach. In the cases studied a double ensemble of blocks appears to best explain the observed power spectrum characteristics.

Formulae of the energy spectrum and autocorrelation function have been derived for magnetic anomalies due to four classes of bodies which can be represented in turn by (a) poles, (b) line of poles, (c) dipoles and (d) line of dipoles. The effects of the depth and dimensions of the magnetized sources on the spectrum and the autocorrelation function have been studied. In the case of the dipole models, the orientation of the polarization vector casts a significant influence on the characteristics of the two functions. In all cases, however, the functions change in size and shape as the inclination and declination of the geomagnetic vector are varied.

Recent geological work within the Contract Area of P.T. Riotinto Bethlehem Indonesia in West Sumatra, including aerial photographic and aeromagnetic surveys by Lockwood Survey Corporation, have yielded much new data on the Sumatran Fault Zone and have revealed its intimate connection with volcanism. Three major fault zones each characterized by complex wrench, thrust and subordinate normal faulting can be recognized from aerial photographic interpretation. During the Quaternary significant di splacements along the Sumatran Fault Zone occurred and are still continuing, and in consequence morphological features and especially drainage patterns are in the closes t correspondence to the fracture systems. Strong E-W magnetic lineaments thought to represent major intermediate intrusives at depth are found associated with volcanic centres. Where these lineaments are cut by strands of the fault zones volcanoes are located. Igneous activity along original E-W a lignments and dextral tranSCUlTent faulting along NNW trending zones interact to produce a barrier range of volcanic rocks along the active fault zone with evenly spaced active volcanic centres at 75-100 km. distances along it. The total horizontal shift along the fault is approximately 130 km. since the inception of the present volcanic cycle. The fault system is thought to be a fundamental fracture whose full explanation is to be sought within the concept of a plate tectonic interpretation of the geology of Southeast Asia. 12 0 Volcano wilh hiltorlcal .rocord 01 activity. } From Mop by B. G. Esdl!r (111511) \'flth odditions from Von B.mm.Hn (1949) Alter. J. Kat~r. 1969. Figur •. I .

Aeromagnetic, ground magnetic, and gravity data, together with all available drillhole data and physical property measurements, were used to map the Precambrian geology of an area in Minnesota that is virtually devoid of outcrop. The work was done for purposes of land use planning and to encourage minerals exploration and mostly consisted of the analysis of profiles of aeromagnetic data to map magnetic/ lithologic contacts, to infer structure, and to determine thickness of overburden cover.Two greenstone belts were resolved. They comprise higher density rocks separated by nonmagnetic metasedimentary intervals. The belts are deformed into synclinal structures that, according to modeling, range from 1 km to as much as 5 km in depth. Lithologic predictions were confirmed in five out of six

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