Continuous Full-wave Sonic Logging
The FWS tool performed logging through the tube filled with water. The FWS tool (OYO co.) is
2.84 meters long, and separated into two sections, the telemetry unit and the transducer unit. The former is a fully digital sonic tool, performing 12-bit A/D at one micro-second sampling with 1,024 point samples, at frequencies ranging from 10 to 20 kHz. The latter has two monopole transmitters and two receivers, spaced 60 centimeters apart.
To acquire sonic signals at narrow intervals of five centimeters, the sampling rate was set to one
microsecond, and the record length to 1.024 millisecond. Figure 2 shows the iso-offset trace of the monopole FWS record in the tube. The velocity obtained from the first arrivals of the P-wave of FWS varied from 1.6 to 3.8 km/sec. But the s/n ratio was extremely low at shallower than 60 meters in depth. On the other hand, that of the conventional downhole measurement logged at intervals of 0.5 meters ranged from 0.3 to 3.2 km/sec. The velocity of FWS was indicated much more precisely than that of the conventional downhole measurement.
Figure 3.: Results of Continuous Full-wave Sonic logging (FWS) in the synthetic rubber tube.
Discussion
Compared with the conventional downhole measurement, continuous full-wave sonic logging
successively provides a larger quantity of higher resolution velocity data that could be used to evaluate the physical properties of fractures. The velocity was obtained successively, while the S/N ratio was extremely low at shallower than 60 meters in depth due to incomplete coupling between the tube and the dry hole wall. The rubber tube was considered not to contact firmly to the hole wall. The rubber tube was made of a flexible, inflatable material, so the weight of water stretched the bottom of the tube. The tube material needs to be more plastic for stretching. On the other hand, the bottom part was not sealed so completely that the continuous FWS could not cover all along the dry hole. The bottom sealing should also be improved.
The velocity obtained from FWS was more than 1.5 km/sec in this study. Low velocities part of
less than 1.5 km/sec could not be measured due to the structure of the FWS which measures in the tube filled with water.
Conclusions
Continuous Full-wave Sonic logging (FWS) successfully collected sonic signals as full waveforms through the tube in a dry hole. FWS successively provided a larger quantity of higher resolution data than conventional downhole measurement, enabling sonic signals to be acquired more precisely and effectively than the conventional downhole technique under the dry condition. Individual fractures in bedrock in all hazards could be identified. However, the method could not be used for measurement at the low velocity part of less than 1.5 km/sec in water due to structure of FWS in the tube filled with water.
References
1. Inazaki, Tomio and Fumio Watanabe (1998), “Continuous Sonic Logging in a Dry Well (Part
1); Basic Concept and the System Design”, Proceedings of the 100th SEGJ Conference,
pp.39-41. (in Japanese with English Abstract)
2. Inazaki, Tomio, Xinglin Lei, Toshiyuki Kurahashi, and Shiro Watanabe (2001), “Delineation
and Geotechnical characterization of fractures in bedrock by high-resolution rock surface
seismic and full waveform sonic measurements, Proceedings of the 5th SEGJ International
Symposium, pp.361-368.
3. Inazaki, Tomio, Toshiyuki Kurahashi, and Shiro Watanabe (1998), “Comparison of Sonic Log
Data for 4 types of Acoustic Tools”, Proceedings of the Symposium of the Application of
Geophysics to Environmental and Engineering Problems (SAGEEP 1998), EEGS, pp.305-314. |
