- New
Reference: 10HS ECHO - 5Mts
Reference: 10HS ECHO - 5Mts
Reference: BASE-U-4
Reference: UA-002-64
Brand: ONSET
Reference: U20L-02-KIT
Brand: ONSET
Reference: DS1922E-F5
Brand: iBUTTON MAXIM INTEGRATED
Description
Top-range seismograph equipped with 24-48-72-96 embedded channels for active and passive seismic surveys. X824S basic configuration integrates 24 channels and can be expanded with 24 channels modules, without external boxes, up to maximum 96 integrated channels. Instrument has a PC with 10” monitor and operative system Windows 10. Management of all settings, acquisition parameters, recording and visualization of recorded data, are performed sequentially and in an intuitive manner by means of the operative system installed on the instrument, which includes also a guide with step by step instructions for the realization of the most common surveys.
Methodologies
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.
Reflection Seismic
The seismic exploration of reflection type, which is extensively used in oil explorations, is also used nowadays to obtain detailed information on surface soils.
Due to the high resolution of the survey, it is used to define the development of geological structures in the sub-soil, defining the shapes, sizes and positions.
The prospecting is carried out by placing high frequency geophones in line and close to each other, sending seismic pulses through energy (also at high frequency) and by measuring the travelling times of the waves that, once penetrated in the ground, are reflected by the uneven surfaces that delimit the lithological passages with net impedance contrast.
M.A.S.W. (Multichannel Analysis of Surface Waves)
The MASW (Multichannel Analysis of Surface Waves) technique has the objective to identify variation profiles with the depth of the speeds of volume waves (Vp and Vs). The method is based on the known relations between these speeds and the dispersion of surface (or Rayleigh) waves observed when propagating through a stratified elastic medium. The analysis can be based on signals produced with a borehole on site by acquisition device (with a ram or explosion), or on the recording of the vibrations produced by far away sources (rives, industrial activities, traffic, etc).
In the first case, we are talking about active MASW, with which it is possible to explore a few tens of metres of sub-soil, and in the second case, we are talking about passive MASW, that allows to reach greater depths, in particular conditions.
Passive MA.S.W. is used with the purpose to obtain a speed profile 1D of the elastic waves of cut S. The technique consists in the recording of the “seismic noise” in temporal windows and following study of the signal processed. It is carried out by arranging a bidimensional geophonic chain with low resonant frequency in line or in “array” (circular and irregular geometries) and measuring the environmental noise. From the F-K analysis (frequency-space) of the wave-trains, it is possible to obtain a dispersion curve of surface waves that leads to the calculation of the speed profile of the shear waves and estimate of a coverage in relation to the semispace.
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.
Seismic vibration monitoring
The survey methodologies based on passive seismic may be of "on time" type, where the instrument performs a constant recording of the signals coming from the sensor connected to it, or of "trigger" type.
TRIGGER BASED SEISMIC MONITORING
The seismic vibrations monitoring called "trigger", or threshold, is one of the methodologies the non-destructive testing (CND) surveys are made up of and it is widely used within structural monitoring and dynamic surveys on the structures. In the trigger acquisitions such "stand alone" seismographs are used; these are specifically designed for the continuous seismic monitoring, where even if they are constantly listening, they are ready to start recording as soon as an event exceeding the trigger threshold (time 0 , start of recording) set in the instrument at the programming time. The sensors used are usually the tri-axial low frequency 1, 2 Hz type. The use of trigger seismic vibrations allows:
to assess the consequences of seismic vibrations generated by vehicular traffic or temporary construction sites on buildings, infrastructure and surrounding houses, in order to assess their potential dangerousness on them.
to plan eventual corrective actions in the traffic circulation, in the works progress modalities, in order to avoid breaks, irreparable structural damages or buildings collapses in case of construction sites.
to constantly monitor (at low cost) the state of monuments and buildings of particular value
to allow (at the design beginning of works and infrastructures) avoiding any accident during the work, coming from specific problems of the site that cannot be detected by any other methods of surveys.
TIME BASED SEISMIC MONITORING
This technique is used to obtain information concerning possible dynamic amplification effects of seismic waves in "emersion". It is based on the recording of the background noise in the time domain and following elaboration of the signal’s frequencies in the domain. It is carried out by placing a tridimensional geophone on the ground with low frequency response and by recording the seismic noise in different temporal windows. Later on, the study of the spectrums obtained from the convolution of the frequency of the signal recorded in the domain for the three components of the ground motion and the application of techniques on spectrum analysis, allows to define and measure possible local seismic amplifications and the seismic frequency of the site. The measurements of the microearthquake can also be taken in linear “arrays” for the localization of faults.
Seismic monitoring is carried out in areas subject to risks related to a seismogenic activity, by acquiring the seisms with time and recording the seismograms. Seismic stations are used able to record in threshold or continuous type, and low frequency geophones or seismic accelerometers. The recording in the long period, of the earthquakes relative to a site or fairly large area allows to configure the seismic scenario of an area and evaluate the risk and vulnerability conditions. If the monitoring activity is supported by specific knowledge of geological and geo-technicaltype, we are talking about seismic Micro-zoning.
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.
Re.Mi / ESAC
RE.MI. technique (REfraction MIcrotremors)
RE.MI. technique (REfraction MIcrotremors) belongs to passive seismic surveys methodologies. Created by the University of Reno in Nevada (USA), it is similar to the MASW, as far as, the operating simplicity and the idea of using the analysis of surfeys waves to go back to the stratigraphic model, is concerned; but it is different because it uses the recording of vibrations coming from sources which are distant from the site to be investigated.
The distance of the sources allows to analize more in detail the low-frequency components of the surface waves and then to reach greater depths of investigation. Furthermore this methodology is more appropriate (compared to the MASW) to the use within the urban sector, where the seismic noise is inevitable and represents a disadvantage for the active technicques, while it is advantageous for the RE.MI.
A limitation of this technique is the necessity of a omnidirectional origin of the microtremors. The instrument needed is basically the same used for refraction seismic and active MASW, eventually with lower frequency
geophones.
Seismic Tomography
This survey method is used to identify physical-geometrical anomalies of the sub-soil with a definitely higher resolution compared to the other seismic prospecting methods; it gives the opportunity to create an image of the object investigated, where all the anomalies present will be reproduced (also the most particular ones, which could not be solved through any other method). In particular, the tomographic method allows to reproduce the geometric distribution of the elements that constitute a specific section, starting from the analysis of the behavior of the radiations that cross it.
The tomography is a general technique that allows to reproduce two-dimensional or three-dimensional objects through a defined number of one-dimensional and variously oriented projections of those objects. The seismic tomography reproduces an image of the internal structure of the ground by measuring the crossing time (or amplitudes) of seismic waves which propagate through a specific section.
The purpose is to determine a detailed distribution progress of such physical properties as the speed or extenuation of the seismic waves. A numerical simulation of the propagating phenomenon will identify the unknown fields of seismic waves velocity and will allow, in that way, to calculate more accurately their crossing times and, consequently, to make an effective discretization of the structure, which can be then transformed into the two-or-three-dimensional image.
One of the simplest configurations of seismic tomography consists of a fan-shaped disposition of the explosions (seismic rays between the explosion and the geophones will define a fan-shaped geometry). This method allows you to quickly find systems and horizons which are strongly inclined, although they don’t define a neat physical boundary. If assuming a circumscribed buried structure, the refracted rays will travel immediately below the interface at a specific speed, while the direct-ones will cross it at different speeds.
That is, if the buried object is at a higher speed, compared to the surrounding system, an earlier arrival and vice versa will occur.
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)
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