Time-lapse or 4-D Geophysics
Four-dimensional geophysics requires measuring one or more physical properties of a fixed volume of the earth over a period of time. The surveys are either continuous or repeated at intervals at the same locations. Taking the difference between the two surveys allows any changes that have occurred during the interval to be observed.
For the first time, MT has been used to map resistivity changes in a geothermal reservoir.
Now, 4-D or "time-lapse" MT is a reality. NEDO (New Energy Development Organization) of Japan funded the 4-D MT study as part of a project to develop a practical technology for geothermal reservoir monitoring and characterization. NEDO chose MT to map subsurface resistivity changes in and around the 1 km-deep steam-producing zones at the Ogiri Geothermal Field in West Japan. NMC (Nittetsu Mining Consultants) of Tokyo, Japan was the prime contractor for the project; Phoenix worked as a subcontractor to NMC.
MT is the only practical technique for sensing such deep resistivity changes, because it "sees" deeper than other EM techniques. MT costs less than controlled-source EM because MT requires no man-made energy source.
After feasibility studies by NMC were completed, Phoenix designed and installed a 30-channel MT monitoring system with 13 Phoenix System 2000 MTU data acquisition units.
Of these, six units (18 channels) in the main steam production zone are fully automated, with centralized power supply and continuous recording. Data is automatically transferred to a central site and processed. Various MT parameters and plots are computed and updated each day.
A fixed, continuous-recording system eliminates the extra cost and inaccuracies arising from repeated surveys. It provides data redundancy, and it can measure possible short-term changes which may occur between separate surveys.
Managing geothermal reservoirs
Knowledge of subsurface temperature variation due to movement of formation waters is important in managing geothermal reservoirs. The underground temperature regime must be maintained so as to permit continuous steam (or hot water) production for several decades, until an acceptable return on investment has been obtained.
As hot water and steam are withdrawn from the reservoir, adjacent groundwater (both hot and cool) moves into the reservoir to replace the withdrawn fluids. This is called hydrologic recharge. The generating plant separates the steam from the hot water; steam passes through the electricity-generating turbines, then cools and condenses to water. The cool water is re-injected into the geothermal reservoir at some suitable distance from the steam-producing wells.
The re-injected fluids and hydrologic recharge fluids move through the subsurface along faults, fractures and permeable beds. It is important to keep low-temperature fluids from invading the hot area around steam-producing wells. If the temperature is lowered, less steam is produced, lowering productivity. Geothermal wells are costly to drill (as much as $1 million), so one objective of geothermal reservoir monitoring is to maintain the quality of existing wells and reduce the need to drill new ones.