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A Comparison of MASW & Crosshole Test for Shear Wave Velocity Determination

A Comparison of MASW & Crosshole Test for Shear Wave Velocity Determination

Dr. Sanjay Rana, Director, PARSAN Overseas Pvt. Ltd.

New Delhi, India, sanjay@parsan.biz

 

Abstract

Shear wave velocity is an important parameter used in geotechnical engineering to understand the properties of soil and rock formations. There are different methods to measure shear wave velocity, such as the Multi-Channel Analysis of Surface Waves (MASW) and Crosshole Test. Both methods have their advantages and disadvantages, and it is important to understand the differences between them to determine which method is more accurate. In this paper, we will discuss the principles and procedures of both methods and compare their results to evaluate their accuracy.

Introduction

Shear wave velocity (Vs) is a fundamental parameter used in geotechnical engineering to determine the dynamic properties of soil and rock formations. It is an important parameter when designing structures that are sensitive to vibrations, such as high-rise buildings, bridges, and dams. The measurement of shear wave velocity is crucial in understanding the properties of the subsurface, and to evaluate the potential of soil liquefaction during earthquakes.

There are different methods to measure shear wave velocity, such as MASW and Crosshole Test. Both of these methods have been used extensively in geotechnical engineering and have their advantages and disadvantages. In this paper, we will discuss the theories and procedures of both methods and compare their results to evaluate the accuracy of each.

Principles and Procedures

Multi-Channel Analysis of Surface Waves (MASW)

The Multi-Channel Analysis of Surface Waves (MASW) is one of the most widely used methods to measure shear wave velocity. MASW involves the observation of surface wave propagation along the ground surface, and the use of this information to determine the shear wave velocity profile of the soil or rock formation. MASW utilizes a seismic source to generate surface waves, which propagate through the soil or rock formation and are detected by an array of geophones placed on the ground surface.

MASW uses a Fourier transform to obtain the dispersion curve from the recorded surface waves. The dispersion curve relates the wave velocity to frequency, and the slope of the curve is related to the shear wave velocity of the subsurface. The shear wave velocity profile can be obtained by inverting the dispersion curve using a non-linear optimization algorithm.

Crosshole Test

The Crosshole Test is another commonly used method to measure shear wave velocity. It involves drilling two or more boreholes at a known distance apart and inserting pairs of sensors in each of the boreholes. One sensor acts as the source of the seismic wave, while the other acts as the receiver. By measuring the time it takes for the seismic wave to travel between the sensors, the shear wave velocity of the subsurface can be determined.

The Crosshole Test is preferred in situations where high-quality data is required, or when there is a need to measure the velocity at specific depths. The test is usually performed in boreholes that are cased, so that the measurements are not influenced by the borehole wall.

Comparison of Results

The results obtained from both methods can vary depending on the properties of the subsurface and the accuracy of the measurement. In general, the MASW method is less expensive and less time-consuming than the Crosshole Test, making it more suitable for large-scale projects.

However, the Crosshole Test is considered to be more accurate and reliable than the MASW method, especially in situations where there is a need to measure the velocity at specific depths. The Crosshole Test is also better suited for situations where there are complex geologic formations or where the soil is heavily saturated.

Conclusion

In conclusion, both the MASW and Crosshole Test are widely used methods to measure shear wave velocity in geotechnical engineering. While the MASW method is less expensive and less time-consuming, the Crosshole Test is generally considered to be more accurate and reliable, especially in situations where there is a need to measure the velocity at specific depths. Both methods have their advantages and disadvantages, and the choice of method will depend on the nature of the project and the specific requirements of the project.