Jökull - 01.01.2011, Blaðsíða 55

Shear wave velocity in surface sediments
1

Dij DL
!R3

!R2
!R1

Figure 3. Rayleigh waves of different wavelengths propagating through soil. – Útbreiðsla Rayleigh bylgna með
mismunandi bylgjulengd í lagskiptum jarðvegi.
Field measurements
The first step in SASW measurements is to line up
geophones in a straight line on the surface of the soil
to be explored. The sensors only pick up vertical mo-
tion and it is therefore important that they are placed
vertically in the ground. The distance between the
sensors depends on the intended depth of penetration.
In the measurements of Icelandic soil up to ten geo-
phones have been used. The most common setup used
is shown in Figure 4. The length of this profile is 20
m and it can be used to determine the shear wave pro-
file down to 20 to 25 m depth. The sensors are of the
type SM-6 from Sensor in Netherlands, with a natu-
ral frequency of 4.5 Hz and a critical damping ratio
of 0.7. In the first measurements a data acquisition
station of the type HP 3852A controlled by a PC and
software developed in the HP-VEE (Hewlett Packard
- Visual Engineering Environment) programming lan-
guage was used. This system was later replaced with
a modern data acquisition card connected directly to
a laptop and software written in the Labview program
(National Instruments, 2009).
When the geophones have been lined up and con-
nected a wave is generated with impact load at one
end of the geophone line and the wave propagation
then recorded. The impact can be created by a sledge,
by jumping or using a falling load. Each measure-
ment takes typically 1 second and the sampling rate
is 1 to 2 kHz. Figure 5 shows time histories from one
such recording. The shape of the signal changes along
the traveling path due to the dispersion effect. For a
given line-up several measurements are taken for dif-
ferent types of impact load and which are generated
at different distances from the first sensor to the point
where a load is applied. The process is repeated at the
other end, i.e. using sensor no. 10 as the first sensor.
The main reason for using different impact loads is to
excite waves with different frequency contents.
1

1 2 3 4 5 6 7 8 9 10
! ! 2! 2! 4! 2! 2! ! !
16!=20m
Figure 4. Lineup of ten geophones in a 20 m long SASW measurement profile, ∆=1,25 m. – Fyrirkomulag tíu
hraðanema í 20 m löngum SASW mæliferli, ∆=1,25 m.
JÖKULL No. 61, 2011 55