Jökull - 01.12.1976, Page 27

Problems in the Interpretation
of Drill Hole Magnetic Data
LEÓ KRISTJÁNSSON,
SCIENCE INSTITUTE, UNIVERSITY OF ÍCELAND, REYKJAVÍK
ABSTRACT
Examples are given to illustrate liow the
interpretation of magnetic field. measurements
in deep drill holes in Iceland may be com-
plicated by lateral inhomogeneities. Suscepti-
bility logging of drill chips from these holes is
shown to be useful in sucli interpretation.
INTRODUCTION
Research towards subsurface stratigraphic
mapping in geothermal drill holes by magneto-
meters has been going on in Iceland for several
years, but not much data on the remanence of
tliese strata is available yet. Kristjánsson (1972,
1975) and others have presented theoretical and
experimental work relevant to this problem,
and the present paper reviews new measure-
ments on practical aspects of interpretation of
drill hole field data.
LATERAL VARIATIONS
IN MAGNETIZATION
Before interpreting magnetic anomalies, one
must always subtract an estimated regional field
(i. e. the field that would be measured if near-
by strata were removed) from the data. In drill
holes, this field will be particularly difficult
to estimate because of lateral variations in rock
magnetization, which may vary greatly with
depth and which cannot be obtained from
above-ground surveys. These variations are part-
ly original, and partly due to topographic ef-
fects, intrusives, alteration etc. Also, the drill
holes may not have the assumed cross-sections.
All these effects, and those discussed by the
author (1975) will cause a margin of uncer-
tainty in the remanence of strata, as deduced
Fig. la. City engineer’s map of the vicinity of
drill hole K1 (modified).
Mynd la: Kort borgarverkfrceðings af nágrenni
borholu K1 við Stálborg (breytt).
Fig. Ib. Total-field magnetic profiles on six
lines shown in Fig. la. Location of drill hole
indicated by -f.
Mynd lb. Segulmcelinganiðurstöður á sex mceli-
linum úr Mynd la.
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