Jökull - 01.12.1970, Page 41

LEGEND:
NKP______ LINE OF GRÁVIMETRIC SURVEY, JUNE 1967
REEERENCE POINT IN CALCULATIONS.
Fig. 1. Location of gravity survey lines on Tungnárjökull in 1967. Results of J. P. Martin’s
seismic survey in 1955.
Mynd 1. Lega þyngdarmcelilína á Tungnárjökli vorið 1967. Niðurstöður jarðsveiflumælinga J.P.
Martins frá vorinu 1955.
h2 the elevation of the underlying bedrock and
Q2 the estimated density of the bedrock. We
assurne that the interface between ice and bed-
rock is an infinite horizontal plane. Call an
anomaly calculated in this manner a tico-layer
Bouguer anomaly.
In contrast call it a one-layer Bouguer ano-
maly when we choose to ignore the existence
of the ice-layer and use (1) as it stands taking
q as the estimated density of the bedrock,
i. e. 02.
Variations in the two-layer Bouguer anomaly
will supposedly only be due to:
(I) Irregularities in the density of the under-
lying bedrock or mantle.
Variations in the one-layer anomaly will ad-
ditionally be due to:
(II) Variations in ice-thickness.
The difference between the two-layer and
the one-layer Bouguer anomaly is
0.04191-hi (02-01) (3)
The basic assumption behind the idea of
using gravity results to supplement seismic ones
is now this:
(I) is much smoother than (II) and can be
estimated from fewer points of observation.
Gravity stations near to points where we
know the ice-thickness from seismic survey are
used to obtain an estimate of the two-layer
Bouguer anomaly over the whole area of survey.
At all intermediate gravity stations we are able
to calculate the one-layer Bouguer anomaly and
subsequently the ice-thickness from (3).
JÖKULL 20. ÁR 39