DESCRIPTION
Seismograph for active and passive seismic, equipped with 6 channels, 24 bit resolution, extremely compact and easy to use. Instrument is managed through an intuitive application which must be installed on a PC or tablet with Windows operative system connected to the instrument through USB interface. Ideal for stand-alone passive seismic methodologies such as seismic vibration at time or trigger threshold, non destructive test, surveys on buildings with three-dimensional sensors and HVSR investigations. Management software can be installed on every notebook or netbook and allows to interface with instrument for the management and configuration of recording parameters for all the different typology of seismic investigations. Connection between notebook – tablet and ST6 is made through an USB interface present on instrument panel. Instrument extremely compact dimensions, its reduced weight and quite competitive price, make it particularly ideal to the use in specific situations which require a rugged, reliable and at same time small instrument, easy to transport even in logistically complicated situations. It is possible to connect to ST6 seismograph both 1D (6 channels) and 3D sensors (12 channels), therefore it is particularly recommended also for structural vibrations monitoring.
METHODOLOGIES
SEISMIC DOWN-HOLE/CROSS-HOLE
Down-Hole Seismic Exploration
This type of survey is performed for the mechanical characterisations of grounds crossed during the probing phase. The technique consists in the measurement of the travelling times of the elastic waves between the seismic source on the surface and the geophones located inside the probing hole, properly conditioned with PVC pipe or geo-technical pipe. The seismic exploring activity in the down-hole takes place by placing one or more triplets of sensors (horizontal and vertical) inside one of the probing holes and at various depths, aimed at receiving the seismic signals generated through ram on anchored plate. Energy will be supplied in phase inversion in order to polarise phases S on a horizontal plane H, according to an orientation of 180°. Through seismic speeds Vp and Vs, it is possible to obtain information, such as elastic modules and geo-seismic parameters. Vs 30 can be measured on probing holes up to 30 metres of depth (O.P.C.M 3274/2003).
Cross-hole Seismic Exploring
This type of survey is performed through the physical –dynamic characterisation of the portion of ground between the two probing holes. The technique consists in the measurement of the travelling times of the elastic waves between the source located in a hole and the geophone/s located in another hole/s at the same depth. The cross-hole is made by introducing the borehole in one of the holes and the tridimensional geophone (or geophones) in another hole/s aimed at receiving the seismic signal incoming from the source at the same level. The elastic modules and mitigations of the medium between the holes can be obtained from this test.
Nakamura Method, HVSR, H/V
A significant part of the damages observed in destructive earthquakes all over the world is associated with the amplification of seismic waves due to the effects of the local site. The analysis of the site response is therefore essential in the evaluation of the seismic risk in areas subject to earthquakes. In order to evaluate the effects of the local site, a series of surveys must be carried out. Among the empiric methods, the method of spectrum analyses H/V on environmental vibrations is one of the most common. The method, also called “Nakamura” technique (Nakamura, 1989), was introduced by Nogoshi and Igarashi (1971) based on the initial studies of Kanai and Tanaka (1961). Since then, many researchers worldwide performed a large number of applications.
An important requirement to carry out the H/ V method consists in a fairly good knowledge of seismology combined with basic information on local geological conditions supported by geo-physical and geo-technical data. The method is generally applied in micro-zoning studies and in the analysis of the local response of specific sites.
M.A.A.M
The analysis of the Rayleighwavesdispersion(velocityphase) can be performed according to the Miniature Array Analysis of Microtremors (MAAM)passive technique. Thisisa methodologythat in many ways issimilar to the ESACone, which enables to delineate the Rayleigh waves dispersion curve by using 3/4 geophones (together with a three-component triad, which is also useful to perform HVSR acquisitions). The strength of this approach is in its effectiveness, considering the few meters space available, and therefore making it particularly interesting for urban applicaton suses. The technique involves arranging the geophones according to triangle or pentagon geometries,with a radius that typically ranges between 0.5 and 5m. This defines the dispersion curve in a frequency range that is proportional to the action radius itself. It goes without saying that, especially when working in urban areas with limited room for maneuver, the MAAM approach represents the onlyeffectiveusefulsolutionto define the Rayleighwavedispersioncurvesin passive mode.
MAAM acquisition parameters
sampling rate: 4ms (Nyquist frequency 125 Hz
acquisition length 30 min
radius: 2 + 5m
sensors: four vertical 4.5 Hz geophones and 1 tricomponent sensor
Dal Moro G., 2014 Surface Wave Analysis for Near Surface Applications Publisher: Elsevier
Less is More (Dal Moro et al., 2015) – GNGTS 17-19 November 2015 – Trieste (Italy)
REFRACTION SEISMIC
The seismic exploration of refraction type is among the most diffused and used active seismic methods.
This type of survey has the purpose to determine the thickness of the overburdens (aerated) above a rigid sub-layer and reconstruct a seismic stratigraphic sequence in terms of apparent longitudinal speed. If carried out according to more sophisticated calculation methods, it can be used to intercept, measure and characterize geo-structural profiles.
Seismic exploration of refraction type is carried out by placing equidistant geophones in line on the ground, and generating seismic pulses through mechanical “inputs”.
Then the travelling times of the pulses that once penetrated in the ground are refracted nearby the lithological passages at different density, will be measured.
Technical Specifications
GENERAL
ADC technology: 24bit Delta-Sigma ADC
Geophone number: 6
Acquisition: Active and passive
Dimensions: 21x17x9 cm
Weight: 0.5 kg
Export compatibility: .seg2; .csv
In-line test: Geophone test, noise analysis
Case: IP67
Enviromental condition: -20°C / 80°C
Power supply: External source
ACQUISITION
Dynamic range: 144 dB (dynamic); 109 dB (at 1ms sampling)
Broadband: -3dB at 3500 Hz
Frequency: from 250Hz to 12kHz active; 251Hz, 502Hz, 1008Hz passive
Gain: from 0 to 36dB
Samples per event: 6255 pretrigger; 29127 acquisition
Anti aliasing: -3dB, 80% Nyquist’s frequency, -80dB
Common mode rejection: 110 dB at 50Hz
Input signal: ±2.5V
Trigger type: Single-ended trigger, external channel
