Publisher Summary This chapter demonstrates that the use of cross-well seismic tomography in an onshore aquifer. Results of seismic survey suggested that the injected CO2 may migrate preferentially along the formation direction. The geophysical loggings of induction and sonic showed the CO2 accession to the observation well CO2-2. The pore pressure buildup measured at the observation well CO2-4 was about 6 % caused by the CO2 injection. It is interesting to note that there was not any significant change in S-wave velocity at the observation well CO2-2. Such results may enable us to predict quantitatively the location and amount of CO2 in the cross-well image plane. A series of observations and measurements composed of cross-well seismic tomography, geophysical logging, pressure and temperature measurement, and seismicity monitoring have been conducted at the test site to understand the CO2 behavior in the underground aquifer.