Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 14

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GUÐMUNDUR PÁLMASON
to use reversed profiles. Where a sufficient number of profiles is
available in a certain area, this is not serious as they give a consi-
stency check on each other. In several cases where an anomalous
structure was found on unreversed profiles, detailed studies were
made later using reversed profiles or a series of such profiles (e.g.
Snaefellsnes). This made possible a more accurate determination of
velocities and depths.
3. INSTRUMENTATION
As mentioned earlier, a set of long-distance refraction equipment
of the same kind as used in 1959 was purchased in 1961. It was made
by AB Elektrisk Malmletning (ABEM) in Stockholm, Sweden, and
has been described in detail by Báth (1960). It has twelve channels
and seismometers with a natural frequency of 6 c/sec. The record-
ing is on photographic paper. For a more detailed description of the
instrumentation the reader is referred to the paper by Báth (1960).
The shot instant was transmitted by radio as a break in an oscil-
lator signal of 700 c/sec frequency, which was recorded on one of the
channels. The oscillator was switched off by an auxiliary detonator
connected in parallel with the detonator in the dynamite charge.
4. TREATMENT OF SEISMOGRAM DATA
4.1. Travel times.
In crustal studies seismic refraction travel time data are often
interpreted in terms of plane, horizontal or dipping boundaries be-
tween layers of constant velocity. In particular this applies when
sparse data are available. When a larger amount of data exists and
more detail is required this is not a satisfactory method. Velocities
are then known with greater accuracy and this permits a more de-
tailed structural interpretation.
For most of the travel time data treated here, apparent velocities
have been computed by the method of least squares. This is useful
as a first step towards a more detailed structural interpretation. The
near-surface structure at the shot point is obtained satisfactorily in
this way. In the earlier work in northern and eastem Iceland (Pálma-
son, 1963) the interpretation was made only in terms of horizontal
boundaries down to layer 3. As more data were gathered, and in
particular after detailed studies of selected areas. it became clear