金曜日セミナー(7月10日)仲田典弘 氏 (MIT)

Towards utilization of full wavefields in ambient noise correlation
~ Field application at Groningen, the Netherlands ~


Nori Nakata (MIT)

Great success of ambient-noise correlation and tomography is founded mainly by the usage of fundamental-mode Rayleigh waves. These types of wavefields have been used for regional, crustal, and more local scale imaging. Although fundamental-mode Rayleigh waves might be one of the most coherent waves we can extract from ambient noise, and hence easy for us to use, ambient noise contains many other types of waves, such as higher-mode surface waves, P and S body waves, reflected and refracted waves, and multiply scattered waves. Extracting and using these waves have great potential to enlarge our imaging areas, enhance spatial/temporal resolution, and improve our interpretation of elastic media. Here I discuss the possibilities and difficulties to use such wavefields. To demonstrate, I use ambient noise data recorded at the Groningen gas field, the Netherlands.  The Groningen field is the largest gas field in the Europe, and is also seismologically well-instrumented area (35 x 45 km2). The Groningen field has about 70 shallow boreholes, temporary node arrays with three components, and deep borehole arrays. Here I focus on shallow boreholes (G-array) and temporary node arrays (Loppersum array), especially continuous ambient-field recording parts, to extract 3D and 4D structural information from the surface to the reservoir.