Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 46

46
GUÐMUNDUR PÁLMASON
As one approaches the farm Saurbaer on the north shore of Hval-
fjördur the travel time of the P3-wave decreases. At a seismometer
station (No. 1253), which is about midway between the farms Saur-
baer and Brekka, the total delay time reaches its lowest value, 0.71
sec. The station delay time here is then 0.71-0.47 = 0.24 sec. This
corresponds to a depth to layer 3 of 1.2—1.9 km, depending on the
relative thicknesses of layers 1 and 2 under this station. Farther
along the profile to the east the delay time increases again to a
similar value as was found west of the anomaly.
This delay time anomaly is probably related to a positive gravity
anomaly which was found by Einarsson (1954) in the same area.
Both are probably caused by the same geological structure. A positive
magnetic anomaly, which probably has a genetic relationship to the
seismic and gravity anomalies, has also been found on aeromagnetic
profiles over Hvalfjördur (Sigurgeirsson, 1969).
The third profile (49a and b) in the Esja-Hvalfjördur area runs
from Hvalfjördur near Hvaleyri in a southeast direction towards lake
Thingvallavatn. It is reversed with shot points in Hvalfjördur and
Stíflisdalsvatn. It is located between the two above mentioned ano-
malies in the Esja-Hvalfjördur area, and should therefore show
whether they are connected or not.
The total delay time for the P3-wave is given in Fig. 12 for both
directions. For profile 49a (shot point in Hvalfjördur) the total delay
time is 0.65 to 0.73 sec. For profile 49b it is about 0.10 sec higher.
This is due to the elevation difference between the shot points, which
is about 180 meters. All travel times are reduced to the level of the
shot point of the line in question.
No major travel time anomalies are seen on this profile. The delay
time anomalies on profiles 47 and 50 do not therefore appear to be
connected.
The offshore profile Kjalames-Faxaflói (48) mns in a westward
direction from a station on land, which is very close to the shot point
of profile 47. There appear to be some irregularities in the structure
of layers 1 and 2 along the first 10-15 km of the profile, which may
be related to strong magnetic anomalies in that area. On the basis of
the travel times of profiles 47 and 48 the most likely station delay
time for the P3-wave at the end station of profile 48 on Kjalames is
about 0.30 sec. Beyond a distance of some 15 km the delay time in-
creases to 0.40-0.45 sec. The depth to layer 3 imder Kjalarnes is
1.8-2.2 km, depending on the relative thicknesses of layers 1 and 2.