The evaluation ahead of tunnel face using seismic while drilling Shunichiro Ito* (Suncoh Consultants, Japan, s.ito@suncoh.co.jp) Masahito Yamagami (TAISEI, Japan, ymgmsh01@pub.taisei.co.jp) Hiroshi Imai (TAISEI, Japan, hiroshi.imai@sakura.taisei.co.jp) Tomoyuki Aoki (TAISEI, Japan, aoki@sakura.taisei.co.jp ) Yoshiaki Yamanaka (Suncoh Consultants, Japan, y.yamanaka@suncoh.co.jp) Takao Aizawa (Suncoh Consultants, Japan, aizawa@suncoh.co.jp) Toru Takahashi (Fukada Geological Institute, Japan, takahashi@fgi.or.jp) Summary The seismic refraction method from the ground surface has routinely been applied to planning of tunneling as a preliminary survey. A tunnel support pattern is designed based on elastic wave velocity of the mountain obtained by seismic refraction survey. Where overburden on a tunnel is thick, the resolution in velocity structure obtained by this technique becomes poor. A survey ahead of the tunnel from the tunnel face while boring is considered to be effective compared with a ground seismic survey. Two methods are generally used in the survey ahead of the tunnel from tunnel face: one by seismic reflection such as the Tunnel Seismic Prediction (TSP) and Horizontal Seismic Profiling (HSP) methods and the other by drilling data called the Drilling Survey System (DRISS). The authors applied a new exploration method by the use of drilling vibration data called TSPD (Tunnel Seismic Probe Drilling) to estimate elastic wave velocity distribution ahead of the tunnel face. Unlike Seismic While Drilling (SWD) commonly used for ahead of drilling in the petroleum exploration, this method investigates the nature of the rocks between the bit of pilot drilling and the cutting edge of the tunnel. The field tests of TSPD were carried out at a road tunnel. The first test helped to identify and correct the problems in data acquisition1)2). The second field test confirmed improvement of S/N ratio of the waveform, and evaluated the elastic wave velocity structure ahead of the tunnel face. SWD and TSPD In the exploration for or exploitation of petroleum resources, SWD using vibration generated by drilling a well is used to investigate distribution of elastic wave velocity of the drilled section by the direct wave, and to image around the well by reflected waves. The principle of SWD by a direct wave is shown in Figure 1. Vibration generated by drill bit (Direct wave in Figure 1) is measured by sensors attached to the swivel of a drill on the well (pilot sensor) and planted on the ground away from the drill (receiver sensor). A conventional seismic trace (which was shot at point of drill bit and arrived to receiver sensor on the ground) can

be extracted from the vibration by cross-correlating waveforms recorded by the pilot sensor and the receiver sensor3). In Figure 1 T2 is the travel time from the drill bit to the receiver sensor on the ground surface, Tr is the travel time picked on cross-correlated waveform, and T1 is the travel time picked on the auto-correlated waveform at pilot sensor. The velocity of elastic wave corresponding to every depth of drill bit is estimated by arrival time using this technique. TSPD is based on the same principle applied to the elastic wave velocity ahead of tunnel face. A field test was carried out to demonstrate TSPD at a road tunnel.

Field Test 1) Site overview The field test was carried out in a road tunnel about 2,100m long and about 11m wide. Maximum thickness of overburden is 150m. The geological feature includes sandstone, shale and alternating sandstone and shale of the Mesozoic age. The sandstone is mainly distributed over the survey area. In this tunnel, some low velocity zones were recognized by a refraction survey. But it is not present in this test area (Fig. 2).

Figure 1: Sequence of obtaining arrival time in SWD. Waveform propagated from drill bit to receiver is extracted by cross-correlation using pilot trace.

The evaluation ahead of tunnel face using seismic while drilling

2) Method of the data acquisition A schematic diagram of the TSPD is shown in Figure 3, and a site photograph in Picture 1. In the first test, the data could not be processed over 28m of the 55m test because of strong noise. Therefore, improving S/N ratio is an important scope of this field test. The method of drilling as a source was changed to the water hammer drill (a percussion drill which strikes near the tip of the drill bit) from the drill jumbo (which strikes on the spindle where the pilot sensor is installed). It was expected to reduce the noise of the pilot sensor by planting the pilot sensor away from the striking position.

Hydrophones had been used in the previous test. In the records, surface waves and sonic waves generated by the drill were noted as strong noise. In order to reduce the noise, the receivers were placed in the boreholes drilled on the left and right side walls near the cutting face. The length of borehole is 4.0m. 3-component geophones were placed at a 1.0m interval in each borehole. In addition, the receivers were planted at five points outside the borehole to

compare with the receivers in the borehole. Layout of the receivers in this test is shown in Figure 3. The depth of forward drilling (= exploratory depth) was 88m, the data processing was performed for each 1.0m of progress of drilling. At the same time, a DRISS was carried out to acquire the data of specific energy, pressure, drilling depth, etc.

Figure 2: Longitudinal geological profile of the test tunnel.

Picture 1: Tunnel face and drill machine.

Figure 3: Schematic diagram of the drill and receiver (plane view).

Water hammer drill (drilling type to strike at the tip of the drill bit)

Drill pipe

Strike direction

Depth L=88m(m)

Other data Refraction survey Observation of the faceSpecific energy

Left sideL=4.0m

(D=2m,3m,3.8m)

Right side L=4.0m (D=2m,3m,3.8m)

Swivel Pilot sensor

Recorder

Receiver in the borehole

Receiver out of the borehole

No3 No4 No5 No9 No10 No11

No8 No7 No6

No1 No2

The evaluation ahead of tunnel face using seismic while drilling

3) Result of the field test Raw traces of all receivers before and while drilling are shown in Figure 4. The seismic traces from the borehole receivers show the differences of the amplitude between before and while drilling more clearly than the other receivers on the tunnel face and tunnel walls. Especially, the borehole receivers in the left wall have high S/N ratio. The pilot sensor also recorded the increase of amplitude. Figure 5 shows auto-correlated traces of pilot sensor to which bandpass filter (100-200Hz) has been applied. A multiple reflection coming from the drill bit to pilot sensor through the pipe, which travelled 3 times between the tip of the drill pipe and pilot sensor. The velocity of the seismic waves propagating along the drill pipe was estimated to be 5.0 km/s.

The seismic traces from the receivers in the borehole drilled on the left wall were cross-correlated with the pilot

trace. A relatively clear P-wave first break is seen in the tunnel axial component (z) of the receiver No.4 (depth 3.0m). Figure 6 shows a picking of the first break of the tunnel axial component of the receiver No.4. Travel time is calculated from the first break time which was corrected by drilling pipe propagating velocity, and the curve is shown in the top graph of Figure 7. Although the traces are disturbed by noise between the distances18 and 30m from the tunnel face, the other part of the graph shows a linear travel time curve. The test results were compared with the observation of the face and refraction survey results in invert (roadbed) measured after the tunnel excavation.

Discussion Figure 7 shows the results of TSPD and some geological information. The P-wave velocity estimated by TSPD is 1.6 km/s at the distance 0-8m, 2.6 km/s at the distance 8-18m, 4.5 km/s at the distance 30-88m. We carried out a refraction survey in invert measured after the tunnel excavation and found that the P-wave velocity is 3.2 km/s at the distance 0-22m, 4.0 km/s at the distance 22-88m. These results show that we could predict the boundary between low velocity (below 3.2km/s) and high velocity (above 4.0km/s) zones at about 18-30m. An observation of the tunnel face found the soft rock (Face evaluation point less than 20) at the distance 0-25m and hard rock (Face evaluation point more than 20) at the distance 25-70m. This observation agrees with the result of TSPD. On the other hand, drilling energy data do not coincide with the other results due to its vast variation. As the drilling energy is considered to respond to the hardness localized to the tunnel face. We plan to devise an evaluation method to complement each other. We consider that the disturbance of the cross-correlated waveform at a distance 18-30m (Figure 6, top of Figure 7)

Figure 4: Raw traces at start of drilling. Those traces were acquired by borehole receivers and the receivers on the tunnel face and tunnel walls. The pilot sensor was attached to the swivel of a drill.

Figure 5: Auto-correlated traces of pilot sensor.Blue line is a signal position coming from the drill bit through the pipe. Red line is a multiple reflection travelled 3 times between the tip and pilot sensor.

Figure 6: Cross-correlated traces (the tunnel axial component of the receiver No.4). Green dots are picked as the first break of P-wave.

The evaluation ahead of tunnel face using seismic while drilling

resulted from intermittent drilling due to a trouble of the drilling machine. It is demonstrated that TSPD is effective in estimating the elastic wave velocity ahead of the tunnel to 90 meters from the tunnel face and can be used for designing the tunnel support.

Conclusions To improve the S/N ratio of the data, the drilling method was changed and the receivers were replaced by 3-component geophones. This resulted in high S/N ratio in the drilling vibration records of both pilot sensor and receiver sensors, and the seismic signal that travelled between drilling bit and swivel became clear in the auto-correlated data of pilot sensor. The estimated elastic seismic velocity ahead of tunnel face using TSPD agreed with the data from the seismic refraction survey and observation of tunnel face. Acknowledgments Authors would like to acknowledge Ministry of Land, Infrastructure, Transport and Tourism (MILT) for offering the field and providing the geological data.

Figure 7: Results of the tests were performed at the tunnel.