Impact Shear Wave Generator for Dynamic Shallow Soil Characterisation

F. E. Almeida (1), R. Marques Moura (2)

(1) Department of Geosciences, University of Aveiro, 3800 Aveiro, Portugal

(2) Department of Geology, Faculty of Sciences, Oporto University, 4000 Oporto, Portugal

Introduction

P and S waves are used to determine Poisson ratios that are useful for micro zoning in seismic engineering studies as well S velocity. P waves are easily generated by many vertical impact sources. On the other hand, S waves are commonly obtained by a polarised sideways impact on a heavy beam perpendicular to the line of geophones (Mooney, 1973). Any S wave source generally produces some P wave component that can be extracted from the paired field data. Each pair consists of two opposite polarity records generated by inverted directional impacts. Recently developed sources are based on a vibration sweep pulse (Ghose et al, 1996) requiring dedicated hardware and processing software and thus making it impossible to use with standard engineering seismographs.

Previous work done by the Kansas Geological Survey compared eleven S wave seismic sources (Miller et al, 1992). Knowing that this particular source is in a early stage of the development, our aim is to present the basic principle behind the source and thus gather new directions for it's development within the meeting .

System and acquisition

We developed an alternative impulse source taking advantage of the initial torque of an electrical starter motor coupled to an inertial wheel (Fig.1). In Fig.1, notice the asymmetric placement of a removable lead mass on the wheel axis. The total system weighs approximately 30 kg and is coupled to the ground by it's own weight on two steel bars.

To improve a short and powerful current we use 24 Volts on a steady current motor. To prevent a long mechanical action, a trigger, connected in serial with a circuit breaker was used. The acquisition parameters consisted of two parallel lines of respectively P and S geophones spaced every 1 meter and using an offset of the same value. Three tests were made (Fig.2 from left to right) standard vertical hammer impulse (1stack), wooden beam/sideways impact (1stack) and inertial wheel impulse (24 stacks). This prototype was tested to obtain measurements of noise level and frequency bandwidth and compared with a standard lateral impact on a heavy beam (bottom of Fig 2). To show S wave polarity two sets of impulses were performed changing both the polarities of the impulse and of the acquisition seismograph. As can be seen in Fig. 2 S waves (channels 1-12) and P waves (channels 13-22) are superimposed. The frequency spectra are similar to those obtained by the Kansas Geological Survey S wave comparison tests (Miller et al, 1992) and the similarity remains between our tested S sources (Fig.2). The standard time-distance pick records (top of Fig. 2) were also performed and controlled with vertical hammer impact (left of Fig.2). A larger offset was chosen to complete the walkway noise test (Fig.3).

Results

After processing we were able to obtain picks of P and S wave first arrivals. These values were compared with data collected using a traditional shear wave source and, for the case depicted (Fig 2). The two compared S sources gave similar velocities and Poisson ratios (Table). Poisson ratios 0.43 to 0.45 were derived in accordance with the theoretical limits. The walkaway test shows some benefit in terms of lowering groundroll when horizontal geophones are used (Fig. 3). Thus, from our preliminary results, the system seems to function well, benefiting to some extent the human acquisition effort. With respect to the pulse signature, the inertial source generates higher frequencies throughout the entire record especially in the the first five to six geophones. Our system is constantly being improved and is still far from what we consider ideal and we intend to study it's radiation pattern.

Bibliography

Mooney, H. M., 1973, Handbook of engineering geophysics: Bison Instruments, Inc. Minneapolis.

Ghose, R., Brouwer, J., Nijhof, V., 1996, A portable S-wave vibrator for high resolution imaging of the shallow subsurface, Exp Abstr. EAGE.

Miller, R. D., Pullan S. E., Keiswetter D., Steeples, D., Hunter, J., Field, 1992, Comparison of Shallow S-wave seismic sources near Houston Texas, Open File Report#92-33, KGS