On Monday morning, November 7. 2022, a demonstration of geophysical measurements was performed in front of the building of the Department of Geophysics (Horvatovac 95). Specifically, a demonstration of Multichannel Analysis of Surface Waves (MASW) and The Horizontal-to-Vertical Spectral Ratio (HVSR) measurements were performed. The demonstration was held by Asst. Prof. Mario Gazdek and Jakov Stanislav Uglešić mag. phys-geophys., researchers employed by the CRONOS project, for the students of the Department of Geophysics, the students from the Faculty of Geotechnical Engineering (Varaždin), as well as for the employees of the Department of Geophysics.
MASW is a non-destructive seismic method which is used to estimate the thickness of soil layers, shear wave velocity (1D or 2D), Poisson ratio and the density of the soil (Xia, Miller and Park, 1999). The method is quick and simple because simple seismic sources like impact hammer or explosion can cause strong enough surface waves with which a large depth range can be covered. The classic MASW measurement system consists of a 24-channel seismograph, 24 geophones (usually with a frequency of 4.5 Hz) and a data acquisition and storage device (Figure 4). The resolution of the recording will depend on the distance between the geophones (dx), while the measurement depth (Zmax) will depend on the distance between the source and the first nearest geophone (x), the distance between the first and last geophone, the strength of the source and the type of source. Surface waves are dispersive, i.e. the phase speed of the wave depends on the wavelength (frequency). This makes it possible to obtain a dispersion curve by measuring the travel time of individual wave phases. From the dispersion curve by inverse modelling, it is possible to obtain a 1D (Figure 5.) or 2D profile of the speed of shear waves (Foti et al., 2018).
HVSR is also a non-destructive seismic method that uses microseismic noise measurements to obtain the ground’s fundamental resonance frequency (f0), a key parameter used in earthquake engineering. HVSR is calculated as the ratio of the amplitude of the Fourier spectrum of the horizontal and vertical components of the microseismic disturbance (Nakamura, 1989). The microseismic disturbance is a constant shaking of the soil surface caused by natural (wind, sea waves, distant earthquakes…) and anthropogenic (industry, infrastructure…) sources. HVSR measurements are made with a three-component velocimeter/accelerometer firmly fixed to the ground at the measurement position. The frequency at which the maximum amplitude occurs, i.e. the frequency at which the greatest amplification occurs, is determined from the HVSR curves (Figure 6.). This frequency is called the fundamental response frequency of the ground (f0), f0 is used to classify the soil and to determine the depth to the bedrock (H800, depth to rocks with shear wave speeds (VS) greater than 800 m/s), as well as to determine the average shear wave speeds up to a depth of 30 m (VS30) using empirical relations (Stanko and Markušić, 2020) that connect H800 and VS30 with f0. Demonstration of HVSR measurements was performed on new Trominio instruments, acquired with funds from the CRONOS project.
Prepared by Mario Gazdek and Jakov Stanislav Uglešić
REFERENCES:
Foti, S. et al. (2018) ‘Guidelines for the good practice of surface wave analysis: a product of the InterPACIFIC project’, Bulletin of Earthquake Engineering, 16(6), pp. 2367–2420. Available at: https://doi.org/10.1007/s10518-017-0206-7.
Nakamura, Y. (1989) ‘A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface’, Railway Technical Research Institute, Quarterly Reports, 30(1).
Stanko, D. and Markušić, S. (2020) ‘An empirical relationship between resonance frequency, bedrock depth and VS30 for Croatia based on HVSR forward modelling’, Natural Hazards, 103(3), pp. 3715–3743. Available at: https://doi.org/10.1007/s11069-020-04152-z.
Strelec, S., Stanko, D. and Gazdek, M. (2016) ‘Empirical correlation between the shear-wave velocity and the dynamic probing heavy test : case study Varaždin, Croatia’, Acta Geotechnica Slovenica, 13(1), pp. 3–15.
Xia, J., Miller, R.D. and Park, C.B. (1999) ‘Estimation of near‐surface shear‐wave velocity by inversion of Rayleigh waves’, GEOPHYSICS, 64(3), pp. 691–700. Available at: https://doi.org/10.1190/1.1444578.
