Geosciences Journal Vol. 14, No. 4, p. 393 − 401, December 2010 DOI 10.1007/s12303-010-0031-3 ⓒ The Association of Korean Geoscience Societies and Springer 2010 Fracture imaging using Image Point transform and midpoint imaging of RVSP data ABSTRACT: An Image Point (IP) transform integrates along a hyperbolic path and converts data to a point in the IP domain so that the reflection events are accumulated to the image point for given sources in the IP domain. In addition, the image point by definition is uniquely determined and the midpoints between sources and the image points always lie on the reflector surface. Exploiting these two facts, the image point transform technique for Vertical Seismic Profile (VSP) to reverse VSP (RVSP) data is expanded to suggest a new method called the midpoint imaging method for imaging reflectors such as fractures. Applying the transform with synthetic data for Two-dimensional (2-D) models with single reflector and two reflectors, we confirm and demonstrate that the reflection events are clearly identifiable in the IP domain. The midpoint imaging method can successfully image the reflector surfaces and the dip information even with only two shot records in a simple case. Key words: reverse VSP, Image Point transform, fracture imaging, midpoint imaging 1. INTRODUCTION Vertical Seismic Profile (VSP) and reverse VSP (RVSP) survey have offered more accurate imaging and information of subsurface structures than surface seismic survey (Gulati et al., 2004). Processing and imaging techniques for surface seismic data can be adopted and used to deal with VSP or RVSP data for imaging reflectors such as fractures. Amongst imaging methods, the concept of VSP common depth point (CDP) is widespread (Chen et al., 2000). Kirchhoff migration (Dillon, 1988; Bicquart, 1988) has also been used to image VSP data. Ashida et al. (1998) succeeded in imaging geological fractures in a horizontal tunnel using the concept of equal traveltime plane mapping. The weak amplitudes of the reflection events and the interferences between various waves often make it difficult to get good and reliable images of subsurface structures, especially fractures in crystalline rock. Cosma and Heikkinen (1996) and Cosma et al. (2001) showed good imaging results for inclined fractures in crystalline rock from offset VSP data. They introduced the concept of the image point (IP) transform that originated from the radon transform. First, they transformed offset VSP data into the IP domain and then applied processing techniques such as polarization analysis and a dip filter to enhance the signal. Imaging results were obtained with the data reconstructed by inverse IP transform. The IP transform integrates an event along its hyperbolic path and maps the integration result to a point in the IP domain so that a reflection event in the time domain is accumulated on an image point in an IP gather for a given source. In addition, an image point corresponding to a given source by definition is uniquely determined in a geometrical way, and the midpoint between the source and its image point always lies on the reflector surface. With these two characteristics of IP transforms, it can be suggested to provide a imaging method that inverse IP transform is not necessary; reflections can be identified and separated in the IP domain and the midpoints for multiple shots, in themselves, can provide information on a fracture such as its dip angle. Compared to VSP, RVSP surveys have an advantage in which a multi-offset data set can be obtained economically. Zimmerman and Chen (1993) noted that it is less expensive to record signals at many receivers on the surface using a source in a well than to repeatedly use surface sources with a limited number of receivers in the well. In addition, to exploit powerful, economical, nondestructive and repeat- able borehole sources such as a piezoelectric transducer (Balogh et al., 1998), a drill bit (Rector and Marion, 1991) and swept seismic sources (Kennedy et al., 1988) is another advantage of the survey. However, VSP data processing tool has been developed because wave separation in RVSP geometry is more diffi- cult than VSP geometry and RVSP data can be converted to VSP data with a simple change in source-receiver geome- try. But it is necessary that sources in RVSP are uniformly deployed to adapt a geometry change from RVSP to VSP. Changhyun Lee* Kwon Gyu Park Toshiyuki Matsuoka Toshifumi Matsuoka } Korea Institute of Geoscience and Mineral Resources (KIGAM), Geologic Environment Research Division, Daejeon 305-350, Republic of Korea Japan Atomic Energy Agency, Geological Isolation Research and Development Directorate, 1-64 Yamanouchi, Akeyo-cho, Mizunami-shi, Gifu 509-6132, Japan Department of Civil and Earth Resources Engineering, Kyoto University, Kyoto 606-8501, Japan *Corresponding author: jetlee71@gmail.com