ENVIRONMENTAL ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 245

The measured time change in height difference in the period t 1 to t 2 is: dh t 2 A-B

dh t 1 A-B . In this value the effect of a uniform additional height change or a uniform
height change in time (subsidence rate) is not distinguishable. The time change in
height is in practice obtained from collected heights h t 1 and h t 2 as they have been
elaborated with respect to the unique stable reference benchmark. Hence, time
changes in the reference benchmark will affect the absolute height differences,
unnoticed. In some campaigns different reference benchmarks are used, which
gives space to additional errors if these reference benchmarks are subjected to
relative changes. Four types of errors can be distinguished
- measurement noise; no correlation/trend in space and time;
- benchmark motion; no correlation in space, a noticeable trend in time;
- model error; a noticeable correlation/trend in space and time;
- reference benchmark motion.
Usually the first epoch is used as a zero state. The model error is determined in
time and place using an integral error analysis and since it contains consistent
information it should be added to the signal (Fig 14.12b).
As an example, the development of a local subsidence bowl in five epochs is
considered. Benchmarks are selected above an expected subsidence bowl (Fig
14.13a). The relative benchmark heights per epoch as a function of the distance to
the bowl centre. Clearly in benchmark B something went wrong. Fitting congruent
bowls shapes in time provides a sound way to extract the most probable subsidence
(Fig 14.13b).

r r

r B r B

B B

benchmark B benchmark B

dh dh

(a) selected benchmarks (b) Fitting a standard congruent bowl shape
Figure 14.13

Modern methods for geodetic surveys involve GPS and InSAR (Hanssen).
Gravity force measurements allow to determine height changes in absolute sense.
GPS uses specific reflecting benchmarks. InSAR is based on radar-interfero-
metrics. Both use satellites. For InSAR reflection and scatter are crucial factors,
and not all surfaces are suitable (agricultural fields). Nowadays, in accuracy and
instant availability GPS and InSAR can compete with spirit levelling, the
traditional method.

application 14.1
In an area with uniform groundwater flow the groundwater appears to be
contaminated by a leak in a gas tank of a petrol station. By installing a well
downstream all the contaminants are caught, because, as calculation show, all