10:15 AM - 10:30 AM
[SSS14-06] Extraction of fault structures in land-sea boundary areas using airborne gravity gradiometry data
Keywords:borders of land and sea, airborne gravity gradiometry (AGG), fault investigations, three-dimensional analysis, density model, horizontal derivative
Gravity surveys used for fault investigations are mainly ground-based and shipboard gravity surveys, and there are measurement gaps in the borders of land and sea (e.g., AIST, 2013). This results in lower analytical resolution and reliability in the borders of land and sea, making it difficult to extract fault structures. To solve this problem, airborne gravity surveys have been conducted on the borders of land and sea, and have been successful (e.g., Komazawa et al., 2009; Segawa, 2014). Furthermore, to improve the accuracy of observations in the borders of land and sea, Sato et al. (2017) conducted airborne gravity gradiometry (AGG) in the northern Ibaraki Prefecture, where faults are estimated to extend from the sea to the land, with the aim of examining the possibility of extracting fault structures in the land-sea boundary area. The results suggested that faults, which had previously been assumed to be separate in the land and sea areas, have continuity from the land to the sea.
AGG was first introduced in Japan in FY2012 by the Japan Organization for Metals and Energy Security (JOGMEC) for the purpose of geothermal resource exploration. Since then, it has been used in 21 regions nationwide by FY2024. AGG combines the characteristics of gravity gradiometry (emphasis on density structure at a depth from 1 to 2 km, which is shallower than conventional ground gravity survey, and acquisition of the six gravity gradient tenor components allowing more accurate analyses) and those of airborne geophysical exploration (dense survey stations and uniformly acquired data regardless of such topography as land and sea areas).
In conventional fault structure surveys using gravity exploration, portions with abrupt changes in gravity anomaly are determined by filter analyses, including vertical and horizontal derivatives, and are interpreted as faults. However, such items of information as dip direction or angle, or depths of the structure concerned are not delivered.
In contrast, by using a density model obtained by three-dimensional analysis of the gravity gradient components, it is possible to interpret the dip direction and angle of faults, as well as their continuity in the depth direction, in 3D. Ninomiya and Shirota (2024) computed the horizontal derivative of the density distribution in the density model, traced a ridge of the high horizontal derivative zone that reflects the rapid density change, and interpreted the trace as fault with density contrast. The trace is called High Horizontal Derivative Line or HHDL. In geothermal resource exploration, HHDLs are interpreted as faults and associated fracture zones containing geothermal fluid, and they have been used as drilling targets, delivering good results.
In this study, we analyzed the density structure in 3D using AGG data obtained by Sato et al. (2017) in northern Ibaraki Prefecture and extracted the geological structure using HHDLs. The HHDL extracted in the mountainous area in the western study area reflects the lithological boundary among granites and metamorphic rocks in the Abukuma belt. In addition, several continuous high-density mounds in the NW-SE direction were observed from the land area to the sea area, and HHDLs in the same direction were extracted on their margins. Those HHDLs are thought to be similar structures to the normal faults distributed in the north of the study area (Yunodake fault, Futatsuya fault, etc.; AIST, 2024). In the sea area, we also extracted HHDLs that have a direction consistent with NS-oriented faults found by sonic exploration (Japan Atomic Power Company, 2015).
As described above, the faults distributed across the borders of land and sea were extracted from the density model generated by 3D analysis of AGG data. Although the AGG's depth of investigation is rather shallow compared with seismic source depths, we believe that by combining it with other exploration methods, AGG can be extremely useful for fault investigations in the borders of land and sea, such as the source area of the Noto Peninsula earthquake.
AGG was first introduced in Japan in FY2012 by the Japan Organization for Metals and Energy Security (JOGMEC) for the purpose of geothermal resource exploration. Since then, it has been used in 21 regions nationwide by FY2024. AGG combines the characteristics of gravity gradiometry (emphasis on density structure at a depth from 1 to 2 km, which is shallower than conventional ground gravity survey, and acquisition of the six gravity gradient tenor components allowing more accurate analyses) and those of airborne geophysical exploration (dense survey stations and uniformly acquired data regardless of such topography as land and sea areas).
In conventional fault structure surveys using gravity exploration, portions with abrupt changes in gravity anomaly are determined by filter analyses, including vertical and horizontal derivatives, and are interpreted as faults. However, such items of information as dip direction or angle, or depths of the structure concerned are not delivered.
In contrast, by using a density model obtained by three-dimensional analysis of the gravity gradient components, it is possible to interpret the dip direction and angle of faults, as well as their continuity in the depth direction, in 3D. Ninomiya and Shirota (2024) computed the horizontal derivative of the density distribution in the density model, traced a ridge of the high horizontal derivative zone that reflects the rapid density change, and interpreted the trace as fault with density contrast. The trace is called High Horizontal Derivative Line or HHDL. In geothermal resource exploration, HHDLs are interpreted as faults and associated fracture zones containing geothermal fluid, and they have been used as drilling targets, delivering good results.
In this study, we analyzed the density structure in 3D using AGG data obtained by Sato et al. (2017) in northern Ibaraki Prefecture and extracted the geological structure using HHDLs. The HHDL extracted in the mountainous area in the western study area reflects the lithological boundary among granites and metamorphic rocks in the Abukuma belt. In addition, several continuous high-density mounds in the NW-SE direction were observed from the land area to the sea area, and HHDLs in the same direction were extracted on their margins. Those HHDLs are thought to be similar structures to the normal faults distributed in the north of the study area (Yunodake fault, Futatsuya fault, etc.; AIST, 2024). In the sea area, we also extracted HHDLs that have a direction consistent with NS-oriented faults found by sonic exploration (Japan Atomic Power Company, 2015).
As described above, the faults distributed across the borders of land and sea were extracted from the density model generated by 3D analysis of AGG data. Although the AGG's depth of investigation is rather shallow compared with seismic source depths, we believe that by combining it with other exploration methods, AGG can be extremely useful for fault investigations in the borders of land and sea, such as the source area of the Noto Peninsula earthquake.