Abstract:
Seismic borehole techniques offer the opportunity to characterize the nearsurface
aquifer properties sensitively. Knowledge of rock-physical relations at
the field scale is essential for interpreting geophysical measurements. However,
transferring the results of existing lab-scale studies directly to the field scale
remains challenging due to the use of different frequency ranges. To address
this issue, we developed an experimental monitoring setup for gas and heat
injection experiments to study rock-physics relations at the field scale. We successfully
studied the dependence of temperature and gas saturation on seismic
properties, respectively. The integration of geophysical measurements into a
hydrogeological problem allows us to demonstrate the applicability of theoretical
rock-physical concepts at the field scale, providing an essential link to the
discipline of hydrogeophysics. After a thorough preliminary survey, which revealed
a detailed picture of the subsurface conditions, we were able to define
suitable test site areas for our injection experiments. With controlled injections
of heat, CH4, and H2 at depth ranges between 7 - 18 m, we obtained controlled
changes in sediment parameters such as temperature and water saturation. We
monitored the temperature and saturation changes in a time-lapse experiment
for at least twelve months at observation depths between 8 - 18 m. In each case,
we analyzed P-wave velocity and amplitude change, including energy level and
quality factor. A subsequent comparison of our seismic data with in situ water
content measurements obtained from the gas injection experiments using
different computational approaches resulted in a fit. It confirmed the method
of seismic cross-hole measurement for gas leakage detection. With a comprehensive
monitoring layout we furthermore detected parameter changes inferred
from temperature variations in the subsurface and compared the results to in
situ temperature measurements. We demonstrate in our experiments that we
verify rock physics relationships at the field scale with our experimental setup
and quantify relative water content changes in the subsurface.
Hydrogeophysics