Comparison of Shear Wave Velocity Derived from PS ...

Bangladesh Journal of Geology Vol. 26, p.84-97, 2013

Comparison of Shear Wave Velocity Derived from PS Logging and MASW –A Case Study of Mymensingh Pourashava, Bangladesh. DM Enamul Haquea A.S.M Maksud Kamalb A.S.M Woobaid Ullaha Md. Badrul Alama a

Department of Geology, University of Dhaka Department of Disaster Science and Management

b

Abstract The paper highlights the outcome of fifteen down hole tests and twenty two MASW tests conducted at Mymensingh Pourashava, Bangladesh. These two tests are being widely used to determine shear wave velocity in many countries of the world. From the comparison of performed tests for this study, the MASW can give a close estimate to of AVS 30 and therefore considered as a cost and time effective alternative over the down hole seismic test. But in terms of greater accuracy PS logging (down hole seismic) is more preferable. It is found that, the average shear wave velocity of upper 30m layer of different sites of Mymensingh Purashava varies 125-255m/sec derived from PS logging, 161-269 m/sec from MASW (active) and 172-374 m/sec from MASW (passive).

Introduction Now a days, PS (Primary Secondary wave) logging and MASW (Multi-channel Analysis of surface wave) are widely used tools to calculate shear wave velocity in various countries of the world. PS logging is one of the most accurate tools to determine AVS 30 (average shear wave velocity of upper 30m layer). In comparison to the conventional seismic survey methods such as cross-hole and down-hole, the MASW proves to be less expensive and less time consuming (Jumrik Taipodia, 2012) and it provides the benefit of precision and swiftness to estimate the subsurface shear wave velocity profile over a large area. Fifteen PS logging and twenty two MASW tests have been performed at various locations in Mymensingh Pourashava to determine shear wave velocity profile & model and compressional wave velocity. Some other engineering geological parameters (Poisson’s ratio, shear modulus, constrained modulus and Young modulus) can also determined from the derived shear wave and compressional wave velocity. But in the present study the authors are concerned with shear wave velocity only. PS logging consists of three sections: cross hole, down hole and up hole; named after the positions of source and receiver (Figure 1). For this study only down hole test has been performed as down hole test requires one bore hole and more economical than cross hole which needs at least two bore holes. Seismic down hole test is a direct measurement method for obtaining shear wave velocity profile of the soil stratum. This test aims to measure the travelling time of elastic wave from the ground surface to some arbitrary depths. The seismic wave is generated by striking a wooden plank by a sledge hammer at surface and recorded by geophones kept in bore holes at various depths. The schematic diagram of down hole seismic is shown in figure 2. The wooden plank is placed on the ground surface at 3m in horizontal distance from the borehole. The plank is hit separately on both ends to generate shear wave energy in opposite directions and is polarized in the direction parallel to the plank. The shear wave is detected by a tri-axial geophone. The

Comparison of Shear Wave Velocity Derived from PS Logging and MASW

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geophone is lowered below the ground surface at a specified depth and attached to the bore pipe by inflating an air bladder.

Cross Hole Test

Down Hole Test

Up Hole Test Figure 1: Types of PS Logging The OYO PS Logging System uses a 7-meter probe, containing a source and two receivers spaced 1 meter apart, suspended by a cable. The performed process is also done at interval (mostly 1m is used) to get velocity at different depth. In down hole seismic, an accelerometer is mounted to a wooden plank source is used to trigger the data collection. Figure 2 also shows the calculation of shear wave velocity by down hole method.

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Geophone

Figure 2: Calculation of shear wave velocity by down hole seismic test. R1= the distance from the source to top Geophone and R2= the distance from the source to bottom Geophone. The multichannel analysis of surface wave (MASW) method deals with surface waves in the lower frequencies (e.g., 1-30 Hz) and uses a much shallower depth range of investigation (e.g., a few to a few tens of meters). It provides shear-wave velocity (VS) information of near-surface materials in a highly cost-effective manner. There are two ways of surface wave generation. “Active source” means that seismic energy is intentionally generated at a specific location and recording begins when the source energy is imparted into the ground. This is in contrast to “passive source” or “micro-tremor” surveying where there is no time break and motion from passive, ambient energy generated by cultural noise, traffic, factories, wind, wave motion, etc. is recorded. The investigation depth is usually shallower than 30 m with the active method, whereas it can reach a few hundred meters with the passive method. Surface wave energy decays exponentially with depth beneath the surface. The energy or amplitude of any particular frequency is dependent on the ratio of depth to wavelength. Thus, for each frequency, the amplitude decreases by the same factor when the depth increases by a wavelength. Higher frequency wave corresponds to the velocity of shallower layer, and lower frequency wave corresponds to the velocity of deeper layer, in general. (Figure 3). Sounding by surface wave

(Wave of different wave length relates the different range in depth. Wave of shorter wavelength is relating to shallower ground condition).

Figure 3: Sounding by surface wave.


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Dispersion, or change in phase velocity with frequency, is the fundamental property utilized in surface wave methods. The sampling depth of a particular frequency component of surface waves is in direct proportion to its wavelength, and this property makes the surface wave velocity frequency dependent, i.e., dispersive. Surface waves show different phase velocity at different frequencies. This phase velocity curve in frequency domain is called dispersion curve. Shear wave velocity can be derived by inverting the dispersive phase velocity of surface waves. Surface wave dispersion can be significant in the presence of velocity layering, which is common in the near-surface environment (upper 100 meters). The multichannel analysis of surface waves (MASW) method tries to utilize this dispersion property of surface waves for the purpose of VS profiling in 1D (depth) or 2D (depth and surface location) format. Basically it is an engineering seismic method dealing with frequencies in a few to a few tens of Hz (e.g., 3–30 Hz) recorded by using a multichannel (24 or more channels usually) recording system and a receiver array deployed over a few to a few hundred meters of distance (e.g., 2–200 m). A general specification of conducted MASW survey is given in Table 1. Table 1: General specifications of surface wave survey. Survey

Source

Target

Array

Target

Item

Type

Frequencies

Length

Depth

5~30 Hz Geophone

30m

~15m

5~30 Hz Geophone

60m

20~60m

(Sensor) MASW

Active

(Active)

(Hammerring)

MASW

Passive

(Passive)

(Microtremor)

For the present study, 12 channels (linear array) with 3m interval, 6 m source (sledge hammer) offset, 0.125 ms sample interval, 2 seconds record length and auto trigger option were used. At every station one data was acquired by stacking (6 times hammer hit) to enhance the data quality. But the geophone interval was kept 4m in Station 28 and 90 (two out of twenty two stations). In terms of Passive MASW data acquisition, L-shaped array configuration (total array length 60m), by keeping receivers at 6m interval with 8 ms sample interval and auto trigger option were used. Since the angles of propagation are unknown, if a linear array is used the calculated phase velocity may be higher than the actual phase velocity. For this reason it is important to record with a 2D array.

Analysis of PS Logging and MASW Data

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In the down hole seismic test the generated wave on the ground surface travels through the multilayered profiles and the ray path would be refracted based on stiffness difference between layers. For the analysis of down hole seismic data, conventional methods including the direct method, the interval method and pseudo-interval method are currently used in practice. (Krammer 1996; Martin and Mayne 1997; Stewart and Campanelaa 1993; Campanella and Stewart 1992). To improve the quality of the wave velocity profiles evaluated by the down hole seismic method new data analysis techniques have been developed (Bang,2001; Batsila 1995; Joh and Mok,1998; Mok1987), including modified interval method based on straight ray paths and the inversion method based on Snell’s law ray path. In the present study, conventional interval method has been used. P-wave travel time is calculated by the first arrival of either peak or trough in the seismic trace and P-wave is characterized by higher frequency and lower amplitude. On the other hand, shear wave is characterized by lower frequency but high amplitude. (Figure 4).

Figure 4: P wave and S wave in the Computer Window. S wave travel time is calculated from the first cross as we hit in both direction of the wooden plank so there generate opposite phase shear waves in radial and transverse direction and cross at some points (Figure 5).

Arrival of S wave Figure 5: S wave arrival in computer window.

After calculating the shear wave velocity for each one meter depth increment the average shear wave velocity is calculated by using the following equation. T30 =∑ (Hi/Vi) AVS 30= 30/ T30


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Where, Hi: Thickness of the i th layer and 30=∑Hi; Vi= S- wave velocity of the i th layer A data sheet of shear wave velocity calculation from PS logging is shown in table 2. Table 2: PS logging calculation sheet for bore hole no 41 (one out of fifteen boreholes).

MASW utilizes the frequency dependent property of surface wave velocity, or the dispersion property, for Vs profiling. It analyses frequency content in the data recorded

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from a geophone array deployed over a moderate distance. It transforms the shot gather into phase velocity image. First, shot gather data is converted to phase and amplitude by Fourier transform. And then, applying phase shift and stacking technique, we can get a phase velocity image. And the maximum amplitude value of each frequency is the dispersion curve. This procedure is illustrated in the figure below (Figure 6)

Waveform f (x,t)

Fourier Transform

F   

1 2



 f t  e

it



dt

Phase, Amplitude F (x,ω)

Phase Shift + Stack 

F c,     F x,    e 

i

x c

dx

Dispersion Curve F(c,ω).

Figure 6: Transformation from shot gathers to phase velocity image.

In the phase velocity analysis, SPAC (Spatial Autocorrelation) method (Okada, 2003) is employed. Okada (2003) shows Spatial autocorrelation function 𝜌 𝜔, 𝑟 is expressed by Bessel function. ρ(ω,r)=J0(ωr/c(ω))……….(1) Where, r is the distance between receivers, ω is the angular frequency, c (ω) is the phase velocity of the waves, J0 is the first kind of Bessel function. The phase velocity can be obtained at each frequency using equation (1). Figure 7 shows an example of dispersion curve of the survey, the frequency range between 5 and 15 Hz.


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Figure 7: Dispersion Curve for Active MASW.

A one-dimensional inversion using a non-linear least square method has been applied to the phase velocity curves. In the inversion, the following relationship between P-wave velocity (Vp) and Vs (Kitsunezaki et. Al.., 1990): Vp=1.29+1.11 Vs ………………… (2) Where Vp and Vs are the P-wave velocity and S-wave velocity respectively in (km/sec). But, here Vs is utilized only. These calculations are carried out along the measuring line, and the S-wave velocity distribution section was analyzed, then summarized to one dimensional structure; SeisImager software can also give a 2-D velocity model (for active), a sample of which is shown in Figure 8.

Figure 8: One dimensional Velocity Structure and 2 D velocity Model

Figure 9 shows an example of dispersion curve for passive MASW and phase velocity versus frequency as a sample. A one dimensional inversion using a non-linear least square method has been applied to the phase velocity curves and one dimensional S-wave velocity structures down (Figure 10).

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Figure 9: Dispersion Curve for Passive MASW

Figure 10: One dimensional velocity structure for Passive MASW In addition, the AVS30 from MASW can be calculated as follows: T30

=

∑(Hi/Vi)

and

AVS 30= (30/ T30 )

th

Where, Hi= Thickness of the i layer and ∑Hi= 30; Vi= S wave velocity of the Ith layer. Survey Result of PS logging and MASW PS logging and MASW are very effective tools to investigate shallow subsurface and are widely used in various countries of the world. For the present study, shear wave velocity found from PS logging test conducted in fifteen bore holes and MASW performed in twenty two different locations of Mymensingh Pourashava are utilized. The average shear wave velocity results of all performed MASW test are shown in Figure 11 and a


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comparison among the results of down hole seismic and MASW is given in Figure 12. Figures show similar results found from mentioned measurements except MASW (Passive) as Passive MASW give velocity for more than 50m depth in most cases. Others, discrepancy in some local zone is not unexpected because the down hole test is a direct measurement while the MASW employs inversion process which cannot be able to match every aspect in the measured signal. Nonetheless, the algorithm used in MASW tried to match with the global trend as seen from recorded data, the overall velocity, or in other words, the Vs30, from both tests are more or less similar. Moreover, AVS 30 maps are shown in figure 13 and 14 which are constructed by GIS tools by using IDW (Inverse Distance Weighted Method) on the basis of down hole seismic and MASW results. These two figures also exhibit identical result.

Figure 11: Comparison of AVS 30 obtained from active and passive MASW.

Figure 12: Comparison of AVS 30 obtained from PS logging and MASW at common locations. It is found that, the average shear wave velocity of upper 30m layer of different sites of Mymensingh Purashava varies 125-255m/sec obtained from PS logging and 161-269 m/sec from MASW (active). The following two figures are constructed on the basis of these two measurements. And it shows that many common wards of Mymensingh Pourashava are located in similar velocity zone.

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Fig 13: AVS 30 Map (PS Logging). The red numerical values are indicating ward numbers of Mymensingh Pourashava.


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Fig 14: AVS 30 Map (MASW). The red numerical values are indicating ward numbers of Mymensingh Pourashava. Conclusion

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To determine the Vs (Shear Wave) profile of the ground in Mymensingh Pourashava, down hole seismic tests were conducted at fifteen sites and compared with the result from MASW tests performed at twenty two sites of which twelve sites are common. From the comparison it can be said that, the MASW can give a close estimate to of AVS 30 and therefore considered as a cost and time effective alternative over the down hole seismic test. But in terms of greater accuracy PS logging is more preferable. It is found that, the average shear wave velocity of upper 30m layer of different sites of Mymensingh Purashava varies 125-255m/sec derived from PS logging, 161-269 m/sec from MASW (active) and 172-374 m/sec from MASW (passive). Acknowledgement The authors are grateful to CDMP (Comprehensive Disaster Management Programme) of Bangladesh Government for giving opportunity to work in the project titled “EARTHQUAKE RISK AND DAMAGE ASSESSMENT AND SUBSEQUENT DEVELOPMENT OF SCENARIO- BASED CONTINGENCY PLANNING OF MYMENSINGH POURASHAVA” and utilizing the acquired data in this research paper.

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2.

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