Jökull - 01.12.1963, Síða 46

gives values of the parameters in the straight
line equation
.+
v ± s„
where t = travel time
t0 = zero distance intercept time
v = velocity
x = shot-detector distance
_ . / (2 A2) • (2 xa)
fo V (n — 2) (n • 2 x2 — (2 x)2)
:he standard deviation of t0
- V2 \/ (2A2)- n-
v _ V (n - 2) (n •
(n — 2) (n • 2 x:
the stanclard deviation of v
(2x)2)
A = deviation of measured travel time
from computed least squares line
n = number of observations used in com-
puting the least squares line.
For each profile a travel time graph was pre-
pared. Computation of structure was made, us-
ing a model with horizontal layers, according
to the formula
hi
‘o.i + l ' vi
i.i+l
i,i + l
k, i +1
where 1+ = thickness of ith layer
t0 j = zero distance intercept time for ith
line on travel time graph.
Vj = velocity of ith layer
cos aik = cosine of angle defined by
DISCUSSION AND INTERPRETATION
OF RESULTS
Grouping of layers.
The layers found in the present investigation
can be grouped according to seismic velocities
into four groups, as has been done in Table 1.
The mean velocities and their standard devi-
ations, assuming the layers in each group to be
the same, are given in Table 2. The number of
values used in computing each mean value are
given in parenthesis.
TABLE 2.
Layer P velocity km/sec
0 2.82 ± 0.07 (17)
1 4.16 ± 0.04 (29)
2 5.06 ± 0.04 (33)
3 6.32 ± 0.04 (30)
S velocity km/sec
2.32 ± 0.03 (18)
2.74 ± 0.03 (23)
3.51 ± 0.02 (19)
Transverse waves from layers 1 to 3 were
usually observed together with longitudinal
waves. In the Neovolcanic zone the transverse
waves were less clear, possibly due to noise from
the inhomogenous surface layer. In all cases,
where both P and S waves were observed, Pois-
son’s ratio was computed and is given in Table
1. The mean value within each group and its
standard deviation, assuming the layers to be
everywhere the same, is given in Table 3. The
number of values in each group is given in
parenthesis.
TABLE 3.
Layer Poisson’s ratio
1 0.276 ± 0.006 (18)
2 0.281 ± 0.003 (23)
3 0.272 ± 0.004 (18)
The differences in values found for the three
layers can hardly be considered significant.
For a surface layer the first term only in the
formula for h( applies.
Only P waves were used in computing the
structure, except in one case, mentioned later.
Table 1 gives a summary of profile locations,
layer velocities and thicknesses, and Poisson’s
ratio as computed from P and S velocities. The
travel time graphs with computed structures for
all profiles are given in Appendix.
Geograpical dislribution.
Layer 0 is generally found at the surface in
the Neovolcanic zone and its thickness is com-
monly a few hundred meters. The surface rocks
of this zone are of Quaternary age and this layer
is therefore identified as Quaternary volcanic
rocks. The velocity values found for this layer
show greater diversity than is found for the lower
layers. Postglacial basalt lavas may have a P
42
JOKULL 1963