This document summarizes crosshole shear wave testing. It discusses that crosshole testing involves lowering a source and receiver down separate boreholes to measure seismic wave velocities between holes. Common applications include bridge/foundation analysis, material testing, soil/rock mechanics, and earthquake/liquefaction analysis. Key considerations are distinguishing refracted from direct energy arrivals, difficulty measuring shear waves, and deviations from vertical/straight boreholes leading to errors. Benefits are direct velocity measurements with depth, while limitations include inability to detect lower velocity layers and need for source power matching desired depth.
2. Introduction
Crosshole (Crosswell) seismic measures the
velocity of seismic waves between the boreholes
.There are two types of Crosshole approaches .
In the conventional approach, which involves
lowering a 3 component borehole geophone
down one hole and lowering a source down to
another hole .The source should be fired at
prescribed depth level and the source and the
geophone are always at same elevation and the
energy from each shot is measured at single
depth in each receiver hole .The travel time are
then converted to velocities by dividing them into
distance between the holes .
3.
4. Common applications
1) Bridge and foundation analysis
2) Insitu material testing
3 ) Soil and rock mechanics
4) Earthquake engineering
5) Liquefaction analysis
5. Considerations
1) Most mistakes made by inexperienced practitioners
of conventional crosshole seismic is mistaking
refracted energy for direct energy . Depending on
layer thickness ,distance between the holes and the
velocity contrast the first arrival energy is quite often
refracted rather than direct .
2) While downhole speakers are available and
generate good P-wave energy ,shear wave velocity is
difficult to measure in crosshole seismic .
3) It is difficult to achieve “perfectly “ vertical and
straight boreholes .There is always some derivations
in both parameters .Since crosshole is most often
done in high velocity materials and closely spaced
holes assuming straight and vertical holes can lead to
signification errors.
6. Benefits /Limitations
1) Seismic refraction requires that velocities increase with depth . A lower
velocity layer beneath the higher velocity will not be detected by seismic
refraction And will lead to errors in depth calculations .
2 ) seismic source employed must match the desired depth of penetration.
For hammer
and plate work, the maximum depth you can expect to explore to is about
15-20m;
however, this can vary significantly depending on geology, surface
conditions, cultural noise,
and the person swinging the hammer.
3)a relatively broad-brush technique – it looks at gross velocity differences, and
you should not expect to be able to map more than 3-4 individual velocity
layers.
4)Cultural noise can be a problem – it is more difficult to conduct a
seismic survey in an urban
environment than in a rural one. Surveying along busy roadways
should be avoided when
possible. Shooting at night is sometimes necessary in order to
achieve acceptable signal-to-noise
ratio in busy areas.
