66 Geophysics in Geothermal Exploration (a) (b) Figure 2.8 Gravity Gradiometry method. (a) The full tensor gravitational field, (b) Tensor Display for the Vinton Dome Air-FTG™ survey. Gzz clearly images the cap rock on the salt dome (outline in white). The cap rock’s response in each of the independent components is also circled (after Murphy and Mumaw, 2004). Time-lapse microgravity is used to identify the mass deficit that occurs in reservoirs in petroleum and geothermal fields because of mass extraction carried out during exploitation. The study, conducted by Pasaribu et al. (2024), explores the application of the time-lapse microgravity method at the Awibengkok geothermal field in Indonesia since the commencement of production. This method utilizes gravimeter equipment with a precision of up to 0.001 miligal (mgal) to monitor mass changes resulting from fluid extraction. Gravity measurements since 1994 reveal a significant annual average decrease in gravity acceleration (–9.2 microgals per year), indicating ongoing mass depletion in the reservoir. The approach includes digital leveling for gravity data correction and subsidence risk assessment. Gravity data modeling employs inversion methods to visualize density changes beneath the surface, demonstrating notable density decreases in production areas and localized increases near injection wells, suggesting potential dynamics of fluid recharge. Gravity changes that occurred from 1998 to 2008 and 1998 to 2017 are depicted in Figure 2.9. Decreasing gravity acceleration from1998 to 2010 was –230 μgal and increased in 2017 to over –500 μgal. The largest decrease in gravity acceleration occurs in the middle of proven area (blue to purple). The gravity and gravity-gradiometry methods are particularly suitable for evaluating depth to basement and mapping basin 3D structures and basement features such as lineament, faults, etc. For more information about the gravity method, we recommend reading the book written by Fairhead (2015).
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