Measurement of Anelastic Strain Recovery
(ASR)
After the drill core is extracted from its host rock, strain recovery begins
immediately (expansion of the core). Two parts of strain recovery process can be
distinguished:
- instantaneous recovery of elastic strain, and
- time dependent recovery of anelastic strain
The time dependent Anelastic Strain Recovery (ASR) can be
measured up to 50 h after core extraction. The sample should be available for the analysis
as soon as possible. The measurements are carried out close to the drill site. Two samples
per core run are measured simultaneously. This leads to an increased compactness of strain
data and to a more reliable interpretation of in situ stress conditions.
The strain measurements in four radial directions lead to an
overestimation, which allows the calculation of four solutions of the strain ellipse and
the determination of four values of the maximum horizontal strain (E1), respectively. In
the axial direction there is also a transducer, which measures the axial strain recovery.
It can not be seen in the photography beside.
The orientation of maximum horizontal strain recovery (E1)
of the core coincides with the maximum horizontal stress (SH) in the ground. The diagram
beside gives the strain history of a core sample (depth: 3237 m) from a well in Germany
(Ev = vertical strain recovery, E1 = maximum horizontal strain recovery, E2 = minimum
horizontal strain recovery).
The process of anelastic strain recovery is connected
with the emission of acoustic signals, caused by the formation of microcracks during the
stress relief of the core and the expansion of it. The diagram on the right side shows
the cumulative number of acoustic signals (AE), which occurred during the anelastic strain
recovery process of the core shown above.
The ASR-measurements are carried out at constant conditions of
temperature and moisture within a thermostatic chamber. The deviation of the measuring
temperature is less than 0,1 degree Celsius.
