High frequency measurement of P and S wave velocities on crystalline rock massif surface methodology of measurement
High frequency measurement of P‐ and S‐wave velocities on crystalline rock massif surface – methodology of measurement Jan Vilhelm – Charles Univers...
High frequency measurement of P‐ and S‐wave velocities on crystalline rock massif surface – methodology of measurement Jan Vilhelm – Charles University in Prague Lubomír Slavík – Technical University of Liberec The Czech Republic
Monitoring of the behavior of the rock massif joint systems by means of geophysical methods. Project no. TA03020408
• G Impuls Ltd., Prague
• Charles University in Prague, Dept. of Hydrogeology, Engineering Geology and Applied Geophysics
• Technical University of Liberec, Institute of Systems Control and Reliability Management
The measurement at rock surface • The possibility for monitoring of voids and fractures – Presence of fracture, formation of a new crack – Fracture stiffness – Possible changes in time
• Use of seismic methods – monitoring of mechanical properties
Measurement in tunnel • Underground gallery made by TBM (Tunnel Boring Machine) • Granite, measurement across macroscopically visible void • Registration of waveforms
Seismic measurement at rock massif surface • Directly at rock surface
• • • • • •
Time of flight measurement Measurement base about 1 m Frequency of seismic waves ~ 40 kHz Piezoceramic accelerometers P‐ and S‐waves Recording of waveforms
Surface seismic measurement • Increase of velocity with increasing depth – refracted waves (“diving waves”)
• Measurement base 100 m
Seismic measurement near to source P‐transducer
S‐transducer
• Increase of velocity is low – measuring base ~1m • For P‐wave measurement S‐transducers are more suitable than P‐transducers
Seismic measurement: P‐ and S‐ transducers S‐transducers
P‐transducers time s
time s Time (msec)
Time (msec)
300
350
400
450
500
550
600
650
250
300
350
400
450
500
550
0.5 m 0.5 m
distance
0.5 m
250
P‐wave velocity ~5300 m/s
280 s
• P‐wave in the first arrivals marked • Measurement at surface: for P‐wave measurement S‐ transducers are more suitable than P‐transducers
600
650
Measuring system – transducer excitation • • • •
Selection of suitable pulse generator Wideband frequency range x narrowband Olympus 5072 (damped spike pulse) Olympus 5077 (square pulse) 5072
5077
Measuring system – transducer excitation
• Exponentially damped pulse, duration t=2ns (~f=50MHz), selectable damping slope • Wide frequency band • Advantage: short duration of wideband impulse
• Square pulse • Selectable frequency 20 MHz – 0.1 MHz • Advantage: at sensor resonant frequency the amplitude is about 12 dB higher
Comparison of signals from spike and rectangle high voltage source
• • • •
Source and receiver transducers 100 kHz S‐sensor, measuring base 100 cm, across fracture Blue ‐ square pulse 100 kHz (duration 10s) Red ‐ spike source, maximum damping Recorded waveforms are similar, square source pulse generator – significantly higher amplitude
Comparison of frequency contents of recorded waveforms for spike and square source
• Selected parts of signals (150 s, P‐wave arrival) used for frequency spectra comparison • Velocity of P‐waves approx. 5300 m/s
Comparison of frequency contents of recorded waveforms for spike and square source
• Normalized frequency spectra • The frequency content is similar, no higher frequencies for spike source (only higher noise due to lower amplitude) • For 40 kHz the wavelength is 13 cm (measuring base 1 m)
Frequency dependent attenuation
• • • •
Normalized frequency spectra Source: 100 kHz and 1 MHz transducer Registration: 100 kHz and 1 MHz transducer There is no increase in high frequency content for 1 MHz transducers ‐ the frequency content in the distance 0.5 m is determined by HF attenuation in the rock material (1 MHz ‐ higher HF noise due to lower amplitude)
Frequency dependent attenuation
• • • •
Normalized frequency spectra Source: 100 kHz rectangular pulse, 100 kHz S‐transducer Registration: 100 kHz S‐transducer There is only slight decrease of high frequency content with increasing distance (150 cm ‐ higher HF noise due to lower amplitude)
S‐wave measurement S‐transducers
P‐transducers
Time (msec) 300
350
400
450
500
550
time s 600
650
700
time s
Time (msec) 750
800
850
250
300
350
400
450
500
550
600
650
700
750
800
0.5 m 0.5 m
distance
0.5 m
250
S‐wave velocity ~3000 m/s
500 s
• For S‐wave measurement P‐wave transducers are more suitable
850
Conclusions • Measurement on rock surface, measuring base ~1 – 2 m: • S‐transducers are suitable for P‐wave velocity measurement • P‐transducers are suitable for S‐wave velocity measurement
• Due to frequency dependent absorption the use of frequency ~40 kHz is recommended • Accuracy of repeated P‐wave velocity measurements can be better then 1% in the distance 0.5 m