Down-hole and Cross-hole Seismic tests

Down-hole/cross-hole seismic tests

1 Introduction

Dynamic geo-technical properties of soil and rock are determined by various in-situ test methods. Dynamic problems are related to earthquakes (liquefaction, ground response and slope stability), vibrating machinery, pile driving, dynamic compaction and other techniques for ground improvement and also from blast, wind, wave and ice loadings. Apart from these the traffic vibrations also cause dynamic problems in a great extent. Engineering interest is mostly concentrated on those methods that can measure the dynamic properties of soils/rocks in order to design the site specific problems (Campanella, 1994).

Various laboratory as well as in-situ methods are used to measure dynamic geotechnical properties of soil/rock. The importance of in-situ testing over laboratory is compared on the basis of testing accuracy and level of disturbances. The tests carried out in the field will provide minimum disturbances and hence the results are expected in high accuracy. However the level of cost is found opposite. Under the seismic tests both compression/pressure waves (P-waves) and shear waves (S-waves) are measured. P-waves are often limited to unsaturated soils, whereas S-waves are used to measure shear wave velocity (vs). S-waves travel through soil structure and are used to determine elastic shear modulus (Gs = .vs2) in low strain level. In addition to S-wave velocity, material damping (Ds) is also required to assess the dynamic ground response. Most of the dynamic field test methods are capable of measuring the low strain shear wave velocity profile. However, the considerable reliability can be obtained from down-hole, cross-hole and seismic piezocone tests which directly measure the shear wave velocity without requiring a soil model. Down-hole and cross-hole methods were developed in 1970s for dynamic testing. Following discussions will highlight testing principles of these methods. 2Down/up-hole Test

The objective of seismic down-hole/up-hole tests is to measure travel times of P and S-waves from the energy source to the receiver/s. These tests can be performed in single borehole. A hole is drilled to the required depth at the testing site and a vibrating source is created to determine shear wave velocity for various soil layers. In this case the waves will be travelling in vertical direction either down or up depending on the location of the source of impulse.

In down-hole test, the generated waves propagate through the soil layers in downward direction. A single wave source is located on the ground surface adjacent to the borehole. In side the hole, either a string of multiple receivers at known depths is fixed or a single receiver is moved to different depths as the test advances. All receivers are connected to high speed recording system and the output is measured as a function of time. The system with multiple receivers/transducers is given in Fig. 1. The shear wave velocity is obtained from the time verses depth curve which is shown in the same figure.

Fig 1: Down-hole test conducted by Larkins and Taylor (Das, 1984) In up-hole test, a movable energy source is located inside the borehole with a single receiver on the ground surface adjacent to the borehole. A schematic view of such test is given in Fig. 2 (b).

The propagating waves travel through the soil layers in the vertical direction in case of the down-hole test. Whereas the waves travel across the holes means in horizontal direction in cross-hole test.

Fig. 2: Principles for down-hole and up-hole tests

3 Cross-hole testSeismic cross-hole test uses two or more boreholes to measure the propagating waves along the horizontal path.

Fig. 3 shows the cross-hole test principles.The simplest Cross-hole test consists of 2 boreholes, one for an energy source and another for a receiver. By fixing both the source and the receiver at the same depth, the wave velocity through soil/rock material between the holes is measured for the depth. By testing at various depths a velocity profile against depth can be obtained (Kramer, 1996). Larkin and Taylors field and lab tests show the following relations:

vs lab = 0.25 vs filed + 83 (m/s)Sample disturbance factor, DF = Gfield/GlabFig. 3 Cross-hole test principle (Das, 1984)Trigger time measurement, casing, backfill effects and site isotropy are some of the sources of inaccuracy. The possible inaccuracies in velocity measurement due to trigger time measurement can be minimised by use of more than two boreholes as shown as in Fig. 4.

Fig. 4 Cross-hole test with 3 boreholesWave propagation velocities can then be calculated from the differences in arrival times at adjacent boreholes. If the length between boreholes, L and the recorded time, t then shear wave velocity, vs = L/t. The early arrival wave is identified as P-wave and the later wave is S-wave.Since the impulse source is located inside the borehole variation of P-wave is more difficult as compared with the surface based source. Various impulse sources are used to generate wave propagation. Most common sources are charging of explosives, vertical impact loading through a connected rod to borehole, torsional impact loading, etc. The best results are obtained from mechanical sources where the source can be reversed.Since the soil strata are deposited in nearly horizontal layers, the cross-hole test allows individual soil layers to be tested. It further helps to detect hidden soil layers that may not be seen by seismic refraction surveys.

Cross-hole test can provide reliable velocity profile to depths of 30-60 m with use of mechanical sources and for greater depths use of explosive sources is recommended (Kramer, 1996).

Amplitude attenuation is measured from cross-hole test with 3 or more boreholes as given in Fig 4. This is further used to compute the materials damping of soil layers. Accurately calibrated and oriented receivers are placed to the borehole wall and attenuation is measured. The material damping is then obtained after deducting the radiation damping which comes from the geometric attenuation. Simple site geometry and homogeneous soil conditions are the requirements for accurate measurements.

4 Conclusion

Wave velocities for P and S-waves are determined by down-hole or cross-hole tests. Wave travelling time from the energy source to the receiver/s is measured for a known distance. Down-hole and cross-hole tests measure the S-waves as SH-waves and SV-waves respectively. Generalised principle sketch for down-hole test is illustrated in Fig. 5. The shear wave velocity is computed as: vs = R/t, where the symbols are defined in the figure.

Fig. 5 Generalised sketch for down-hole testA plot of travel time verses depth can be generated from the test records. The slope of such plot represents the velocity of wave propagation through the ground. The time - depth plot given in Fig. 6 shows the P and S-waves velocities for different layers.

Fig. 6 P and S-waves from Down-hole test ( Kramer, 1996)References:

1. Kramer, S.L. (1996): Cross-hole and Down-hole tests. In Geotechnical Earthquake Engineering, pp. 205-207.

2. Campanella, R.G. (1994): Field methods for dynamic geotechnical Testing: An overview of capabilities and needs. In Dynamic Geotechnical Testing II, ASTM STP 1213, pp 3-23.3. Das, B.M. (1984): Down-hole and Cross-hole tests: Seismic wave Evaluation. In Fundamentals of Soil Dynamics, pp 115-117.

L

Receivers

Source

vs = dz/dt

Transd. 2

Layer 2

Layer 1

Transd. 1

S-wave arrival time, t

Surface source

Depth, z