Geophysical Surveys

In the field of geophysical prospecting Aureli Soil Srl provides different types of exploration through non-invasive active and passive seismic techniques, geoelectric and electromagnetic methods useful in designing projects and infrastructure.
The numerous investigations carried out have allowed us to choose high-tech equipment and refine the acquisition method, improving the quality of the results achieved.

SEISMIC REFRACTION AND SEISMIC REFLECTION

Through the use of this methodology it is possible to evaluate the geometry and the thickness of the subsoil, to define the velocities of the seismostrates allowing an elasto-dynamic parametrization of the subsoil through the surface recording of the P and S waves.
Seismic reflection investigations are carried out in high-resolution, able to reach great depths rebuild the stratigraphic and structural aspects of the deepest geological elements.
It is different from seismic refraction because of it is not only measuring of the elastic waves firts arrival timeto the individual geophones, but also carrying out careful analysis of the wave trains received, through wich it may succeed in recognizing the signals coming from the soil layers characterized by seismic velocities.
The final result will be a graph wich highlights the reflection progress of the underground surface (the seismic reflectors) that will signal the presence of the various discontinuities encountered, the depositional geometry, the stratification, the discordance surfaces, the faults, and the thrust faults, etc…

SEISMIC TOMOGRAPHY

From the experience gained between the various methods of interpretation of seismic refraction surveys, the one that allows the restitution of a seismic tomography is preferred.
Compared to conventional refractive seismic, this technique limits the interpretative problems due to velocity inversion layers (ghost horizon) or reduced thicknesses.
This type of return proves to be an instrument of high precision and excellent for the identification of buried structures, cavities, reconstruct and delimit the volume of landslide bodies.
The processing of the imaging is done with the Rayfract software that performs the inversion of seismic data acquired with the 2D WET method Wavepath Eikonal Traveltime which represents the evolution of the G.R.M. (Generalized Reciprocal Method).
The Eikonal process models the multiple paths of the signal propagation that contribute to a first arrival and also models the diffraction, in addition to the refraction.

SEISMIC IN BOREHOLES: DOWN HOLE - CROSS HOLE

The seismic hole tests have as their purpose the determination of the propagation velocity of the compression waves “P” and the shear waves “S”.
For these investigations it is necessary to prepare a suitably equipped barehole.
The Down-hole method provides the energy source on the surface and the sensors inside the barehole. Particularly assembled five-component geophones are used to be lowered and fixed at increasing depths against the wall of an appropriately conditioned perforation; by energizing the ground on the surface and measuring the arrival times of the first impulse to the geophone, it is possible to determine the speed and elasto-dynamic modules of the lithotypes encountered in the drilling.
The Cross-hole test is performed by practicing at least two probing holes, in one of which the triaxial geophone is housed while in the other it is energized. The energy source must be able to generate high frequency and energy-rich elastic waves, with directional waveforms, ie with the possibility of mainly obtaining polarized compression and / or cutting waves on vertical planes.
They are also performed for the realization of seismic tomographs between the probing holes and are useful in the localization of fractured zones (mechanically degraded) and more generally to identify zones of speed anomaly in the area of ​​investigated land.

SEISMIC SURFACE: MASW e Re.Mi.

The geophysical exploration with the surface waves makes it possible to obtain, with simplicity and cheapness, the propagation velocity of the S waves in the subsoil.
The MASW (Multichannel Analysis of Surface Waves) method allows the determination of the Vs30 speed profile.
It overcomes some of the limitations present in the refraction technique such as speed reversal.
It does not require long times in the acquisition phase and good results even in the presence of noise.
Rayleigh waves, in a stratified medium, are dispersive by propagating with different phase velocities and group velocities. They are recorded along the layout of geophones artificially energizing and are subsequently analyzed through complex computational techniques based on a multi-layered terrain recognition approach. Observing the frequency spectrum it is possible to highlight that the S wave propagates at a variable speed according to its frequency, as a result of the dispersion phenomenon. Once the pick-up on the f-K spectrum or on the dispersion curve obtained from the campaign data was carried out, by means of inversion processes, the velocity profile is obtained with the depth that allows to define the parameter of Vs30.
Re.Mi. analysis (Refraction Microtremor) is a passive seismic technique that records microtremors, due to natural sources (wind, sea, anthropic activity), through multi-geophonic dispositions. Also with this methodology the dispersive properties of Rayleigh waves are studied.
During the analysis, the velocity spectra are determined for various time windows of the recording made of which one chooses the most clear for the identification of the curve to be inverted in a similar way to the MASW analysis, however, unlike the latter, it is a method chosen for surveys in particularly noisy areas and has a higher resolution in determining the deeper layers.

HVSR

Passive seismic survey able to identify the characteristic resonance frequencies of the land. Useful in the presence of strong impedance contrasts in estimating the depth of the seismic bedrock, it is particularly used in seismic microzonation studies.
The versatility of this type of investigation also allows to obtain the resonance frequencies of a building, to then compare them with those of the ground and avoid, in the design phase, the problems arising from the Double Resonance.

ELECTRICAL 2D AND 3D TOMOGRAPHY

Aureli Soil Srl has been dealing with geoelectric tomography for years; this non-invasive investigation allows to obtain vertical sections (2D) or three-dimensional (3D) models of the subsoil constructed on the basis of the electrical resistance parameter.
The presence of fluids in the subsoil causes rocks and soils, traversed by the current, to behave relatively like good conductors of electricity; on the contrary, structures with a low content of fluids such as non-fractured dry rocks and natural or anthropogenic cavities behave like bad conductors, if not as insulators.
Therefore the buried geometries respond to the flow of artificial current, introduced in different ways, according to the physical parameter that regulates this behavior: the electrical resistivity ρ (Ohm • m).
This sinvestigation, if conducted on a landfill site, allows to highlight any suspended stratas of percolate.

GEORADAR

Ground Probing Radar (GPR) is a high resolution indirect investigation technique that uses electromagnetic waves of variable frequency from 40 MHz to 2 GHz.
The use of the methodology is able to investigate the land and the materials with considerable detail; the limit is the depth of investigation, which is inversely proportional to the frequency.
The georadar surveys are used for the research and the geometric reconstruction of networks of underground utilities, reservoirs, underground structures, archaeological remains and structures in general.
The principle is based on the propagation of electromagnetic waves from the surface by means of a transmitting antenna and on their reflection on the interfaces present in the subsoil; in order for reflection to take place, there must be a difference in terms of dielectric permeability and electrical conductivity between the buried object and the surrounding matrix. In particular, the georadar methodology allows to identify, with good precision and detail, any type of anomaly present in the first meters of subsoil of the site in question, guaranteeing, at the same time, low costs and rapidity of intervention. In this way it is possible to investigate areas of considerable size and to identify the areas in which to concentrate any subsequent direct investigations (excavations, surveys and the like).

The georadar surveys are used for:
• Localization and reconstruction of underground utilities, to avoid damage in case of excavations or drilling of polls.
• Location of underground tanks and drums.
• Location of archaeological remains (walls, cavities, tombstones, etc.).
• Characterization of reinforced concrete or masonry structures (location of irons or reinforcement bars, thickness of walls or concrete slabs, presence of voids on the back of the structure, degree of deterioration, etc.).
• Characterization of the geometry of buried objects (foundational works, presence of poles / micropiles, etc.) with antenna hole.