ST2424 24 CHANNEL 24 BIT SEISMOGRAPH

The unit is equipped with a capture card with 24-bit resolution. It is possible to serialize two units to reach 48 channels. Through the management software, which is installed on any PC or laptop connected to the acquisition unit, it is possible to configure all the parameters related to the type of seismic survey that you want to carry out with maximum simplicity and speed.

Technical Specifications

General

ADC technology: 24bit Delta-Sigma ADC

Geophone number: 24 + 1

Acquisition: Active and passive

Dimensions: 27x24x17 cm

Weight: 3.5 kg

Export compatibility: .seg2; .csv

In-line test: Geophone test, noise analysis

Case: IP67

Enviromental condition: -20°C / 80°C

Power supply: Internal Li-ion battery

Autonomy: > 7h

Methodologies

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.

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.

Re.Mi / ESAC

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 prospecting method is used to identify subsurface physical-geometric anomalies with a definitely higher resolution compared to the other seismic prospecting methods; gives the opportunity to create an image of the investigated object, where all the anomalies present will be reproduced (also the most particular ones, which could not be solved by any other method). In particular, the tomographic method makes it possible to reproduce the geometric distribution of the elements that make up a given section, based on the analysis of the behavior of the radiation that passes through it.

Tomography is a general technique that allows two-dimensional or three-dimensional objects to be reproduced through a defined number of one-dimensional and diversely oriented projections of said objects. Seismic tomography reproduces an image of the internal structure of the ground by measuring the crossing time (or amplitudes) of seismic waves propagating through a specific section.

The objective is to determine a detailed evolution of the distribution of physical properties such as the velocity or attenuation of seismic waves. A numerical simulation of the propagation phenomenon will identify the unknown velocity fields of the seismic waves and will thus allow, in this way, to calculate their crossing times with greater precision and, consequently, to carry out an effective discretization of the structure, which can then be transformed into the two-dimensional or three-dimensional image.

One of the simplest configurations of seismic tomography consists of a fan-shaped arrangement of the explosions (the seismic rays between the explosion and the geophones will define a fan-shaped geometry). This method allows you to quickly find systems and horizons that are strongly tilted, although they do not define a clear physical boundary. Assuming a circumscribed buried structure, refracted rays will travel immediately beneath the interface at a specific velocity, while direct rays will cross it at different velocities.

That is, if the buried object is at a higher speed, compared to the surrounding system, an earlier arrival will occur and vice versa.

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.

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)