We derived an equation giving a relationship between the average depth of a stratum boundary and the power spectrum of theg gzz component of a gravity gradient tensor. It was found that the relationship was nonlinear to the wave number on a semi-logarithmic scale and that the average depth was estimated to be shallower than the actual one if a conventional linear approximation would be employed. We applied the equation obtained in this study to the gzz component of the gravity gradient tensor observed in the Kuju geothermal area of central Kyushu, Japan, and obtained results that are consistent with the average depth estimated by spectrum analysis of gravity anomaly. On the other hand, we found that the equation could not estimate the average depth of the stratum boundary at the lowest wave number range. In addition, it was shown that the wave number range giving the same average depth as that given by spectrum analysis of gravity anomaly shifts to the higher wave number range, and its range becomes broader.
The Taiwan Power Research Institute plans to perform a field experiment on sedimentary rocks at 2500 m depth at a CO2 geological storage site near a coastal area in Taichung-city. Application of electromagnetic methods is being planned to monitor changes in resistivity caused by injected CO2. It is necessary to measure the initial resistivity of the layers, before large-scale CO2 injection tests. For the CSMT (Controlled Source Magneto-Telluric) method, the exploration depth has been previously limited to less than 1000 m at most sites in Japan, because the distance between an electric current source and a survey site cannot be separated enough because of the influence of artificial noise. In this study, we conducted simulations using numerical models in order to evaluate the effectiveness to monitor the CO2 reservoir at a depth of 2500 m. As a result, the total amount of injected CO2 reached 100 Mton, apparent resistivity increased 5%. We conducted a field test at the site using the great depth CSMT method. We set the current source at a distance of 15 km away from the site and we measured apparent resistivity and phase data at the frequencies of 8192 - 0.015625 Hz using a new electromagnetic exploration instrument with high spectrum resolution controlled by GPS synchronization (Johmori et al., 2010). We could obtain good data by processing with a digital filter and average E/H vector for adjacent survey stations, regardless of the noises at the site. We could also measure the data at the low frequency of 0.0625 Hz by performing a near-field correction considering the signal source. Furthermore, the resistivity profiles obtained by the 1D and 2D inversion techniques matched well with the resistivity of boring core samples and electrical logging data from 2000 to 3000 m depth at the site. In conclusion, we confirmed the effectiveness of this technique.