Measuring instruments, geophysical instruments


Since 1956, PASI has been manufacturing seismographs for "active seismic" surveys: these are instruments capable of measuring the seismic perturbations produced artificially in the ground by a sledge hammer or an explosive charge and measured by a type of sensor known as a geophone. A certain number of geophones (min. 3, more often 12, 24 or more) are driven into the ground, arranged along a rectilinear profile (seismic spreading) with a certain spacing (e.g. 12 geophones at 10m spacing covering a profile that is 110 m long). The most common acquisition methods include:


This geophysical method is based on measuring the arrival times of seismic waves refracted by the interfaces between layers of ground, characterised by different propagation speeds. The energy source is represented by an impact on the surface. The energy radiates from the "shot point" - travelling both directly in the uppermost layer (direct arrivals), and deep down and laterally along layers at a higher speed (refracted arrivals) - then returning to the surface, where it is measured through the spreading of geophones geophone spreading (10 Hz frequency). Energising in different positions on the surface, it will be possible to deduce information about the geometry of the deep refractor layer, in many cases coincident with the bedrock.


This geophysical survey is based on measuring the outbound/inbound travel times of the seismic waves transmitted from the surface and reflected towards the surface of geological horizons with different characteristics. The energy transmitted is only reflected when there is a contrast of acoustic impedance (the product of the speed x the density of the material) between two superimposed layers. The scale of the contrast in the acoustic impedance between the two layers determines the amplitude of the reflected signal, which is measured on the surface thanks to a spreading of high frequency geophones (40 Hz, 100 Hz). As in the case of seismic refraction, the energy is produced by a ¨shot¨ or impact on the surface. For surface applications, this involves the use of a sledge hammer and a striking plate, a dropping weight, a seismic energiser or an explosive charge.


In this type of seismic survey, the source and/or geophones are located in a hole prepared especially in the ground. One of the most common methods involves down hole testing (the source is on the surface, the sensor on the other hand is a 3D borehole geophone (P- & S-waves) or a chain of hydrophones (P-waves only, in a hole filled with water or drilling fluid). The down hole test aims to determine the profiles of the seismic compression waves (P-waves) and shear waves (S-waves) with the depth. It consists of producing a perturbation on the ground surface by means of a mechanical source and measuring the arrival time of the P- and S-waves at various depths in the hole prepared for the purpose. This technique is also used for calculating the Vs30 as an alternative to surface methods (e.g. MASW). Cross hole seismic tests, on the other hand, involve measuring the speed of the seismic waves between two survey holes, one for energisation (normally made with a borehole energiser or explosive) and the other for measurements (with a three-dimensional borehole geophone clamped at a certain depth). For each acquisition, the energisation depth and measuring sensor depth in the two holes should be the same. In this case, therefore, there must be two separate survey holes whose reciprocal distance from all the measurement levels must be known.


MASW is the acronym for Multichannel Analysis of Surface Waves. This indicates that the phenomenon being analysed is the propagation of surface waves. More specifically, the analysis focuses on the dispersion of surface waves (i.e. the fact that different frequencies – with different wavelengths - travel at different speeds). The basic principle is quite simple: the various components (frequencies) of the seismic signal that is being propagated travel at a speed that depends on the characteristics of the medium. More specifically: the larger wavelengths (i.e. the lower frequencies) are influenced by the deepest layers, while the small wavelengths (the highest frequencies) depend on the characteristics of the layers nearest the surface. As, typically, the speed of the seismic waves increases with depth, this will be reflected in the fact that the lowest frequencies of the surface waves will travel at a higher speed than the higher frequencies. MASW is traditionally performed by analysing Rayleigh waves, which are recorded using common 4.5Hz vertical component geophones - those used also for refraction in compressional waves - and considering a very common source with vertical impact, i.e. the classic sledge hammer. This occurs for at least two reasons: 1. these geophones (and this acquisition method) are by far the simplest and most common. 2. the propagation and dispersion of Rayleigh waves occurs without any problems even in low speed channels (speed reversals) which, as we know, are invisible for refraction. On the other hand, exploiting the dispersion of Love waves (together with that of Rayleigh waves) is an exciting new frontier for MASW analysis (see the winMASW manual for more information) (please note that the use of Love waves is only possible with the MASW technique and not with the ReMi – Refraction Microtremors - technique). In summary: as the dispersion of the surface waves depends on the characteristics of the sub-soil (mainly on its vertical variations), by determining the dispersion curves, it is possible to deduce the characteristics of the medium (the essential parameters are the speed of the shear waves and the thickness of the layers) and all the parameter requested by the new seismic regulations introduced in most Countries all over the world.


In light of the new seismic legislation, the measurement of environmental seismic vibrations or seismic noise has acquired considerable importance. The analysis of seismic noise measurements can be conducted using three methods: Fourier spectra Spectral ratios H/V spectral ratios The latter, which provides the most reliable results, is also known as the HVSR (Horizontal to Vertical Spectral Ratio) method or the Nakamura method. The H/V spectral ratio technique consists of calculating the ratio of the Fourier spectra of the noise in the horizontal plane H (generally the spectrum H is calculated as the average of the Fourier spectra of the horizontal components NS and EW) and the vertical component V. The acquisition of HVSR data, obtained using low frequency triaxial (3D) geophones, make it possible to determine with accuracy the characteristic frequency of resonance of the site, an essential parameter for the correct dimensioning of earthquake-resistant buildings. During the design, it is important to build structures with different resonant frequencies to the ground, thereby preventing the ¨double resonance¨ effect which is extremely dangerous for structural stability. Seismic microzonation studies using the HVSR measurement method have therefore become an integral, essential instrument in the design of earthquake-resistant buildings.