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PIEDRA OFFERS THE FOLLOWING SERVICES
Shallow and deep geological explorationapplied to civil engineering, tunnelling
and mining.
Hydrogeophysicalexplorationto evaluate the water potential, wells sitting and
hydrogeologicalmodels.
Environmental geophysics.
Detection of underground structures and natural or artificialcavities.
Diagnosisof civil engineering.
Seismicclassificationof soils and seismic microzonation.
GEOPHYSICSUNIT
Luigi Benente, M.Sc., Geólogo
GiorgioLombardi,M.Sc, Geólogo
Refractionseismic - P and S waves
High resolution seismic reflexion
Hybrid seismic
Multichannelanalysis of surface waves MASW 1D-2D-3D
Down-Hole,Cross-Hole y VSP (Vertical SeismicProfile )
Multielectrodesgeoelectricalsurvey
Seismicrefraction and geo-electricintegrated surveys
AUDIOMAGNETOTELLURICUNIT
Mario Naldi, Ph.D.
Audiomagnetotellurics- AMT
Time domain electromagnetic– TDEM
Hydrogeophysics
PIEDRA
A multidisciplinarycompany
operating in applied geophysics,
that offers technical solutions
for civil engineering, mining
exploration, geotechnical
characterization and seismic
risk.
High-tech instruments are
employed in the geophysics and
audio-magnetotellurics units
allowing us to:
 solve engineering problems
 provide our clients with data
on geological exploration
 supply data to the analysis of
seismicrisk
 achieve excellent results
CONTACTS
 info@piedra-consulting.com
 benente.luigi@gmail.com
 +39 338 2122646
 www.piedra-consulting.com
“Piedra employs a multidisciplinary approach in
order to achieve maximum results at minimum
costs. "

3. GEOELECTRICAL METHODS
The electrical tomography is the measurement of electrical rock and soil
parameters, which through an accurate selection of details, obtains a deep
geological model showing the structures among 2D sections. Such a technique
not only gives information at the first sub-surface level but also defines deep
geologicalstructures.
The acquisition is performed by different automatic devices according to the
distinct configurations of traditional electrodes (i.e. Schlumberger, Wenner,
dipole-dipole,etc…).
The innovation, taking into account the vertical electrical soundings profiles
(VES), consists in the collection of a large number of measures in a short period
of time (over 1.000 measurements in 30 minutes !), and in the performance of
tomographicdata processing.
This technique can be implemented in the presence of irregular topography
and strong heterogeneity of soils and rocks: the reachable depth depends on
the length of the line. In general, we can say that if L is the length of the line,
the maximum depth is L/6 configurations Schlumberger, Wenner and Wenner-
Schlumbergerand L/4 pole-pole and pole-dipole.
Figure 1 – Carriage of geoelectrical instrument and
cables in an inacessible area.
APPLICATIONS
The geoelectrical tomography
offers a great potential for
applications in the field of
geological,geotechnical and
hydrogeology exploration, such
as:
Detection and characterization
of faults determining its area of
influence, direction, dip and
depth extension.
Detection of contacts among
lithologicalunits of different
nature, determining the
morphology and precise
location of such discontinuities.
Detection and characterization
of cavities and undercuts, such
as accidents karst, pipes, tanks,
clay fillers, etc.
Determination of aquifer units,
water tables, saltwater
intrusion, etc.
The study was conducted with 96 channels Syscal Pro, with electrodes
separated by 10 meters and a total length of 950 meters. The purpose of
this study was the construction of an hydroelectric tunnel and we reached
over 200 meters of depth. Please note the correspondence between
stratigraphy and RQD from borehole: high values of RQD match with high
values of resistivity ; in the same way low RQD match with low resistivity.
This piece of information is critical because it highlights the presence of
weathered and altered zones or bad geotechnical characteristics.
Figure 2 – Rocks resistivity values.
The study was conducted with 72 electrodes (Syscal Pro 72 channels), spaced
by 5 meters with a total length of 355 meters, reaching a depth of 50 meters.
The blue areas are wet productive layers with a resistivity of less than 200
Ohm * m.
Example [2]
Depth geoelectricalinvestigationfor a hydroelectricproject in Ecuador..
PZ-02
Example [1]
Geoelectricinvestigationfor well siting for water use in mountainous areas.
page -2-

4. Seismic
layer
Description
Velocity of P waves
[m/s]
Thickness [m]
1
Debris coverage and
weathered soil
500 < Vp < 1500
1 meters to 5-6
meters
2
Altered conglomerate
substrate
1500 < Vp > 2500 3 meters a 10 meters
3
Compact conglomerate
substrate
Vp > 2500 -
SEISMIC METHODS
Piedra-consulting offers a wide range of seismic methods that can meet the
market demand in geotechnical exploration and mining, such as : seismic
refraction, high-resolution reflection, 1D and 2D MASW ( multichannel
analysis of surface waves). These methods allow us to collect information
from shallow to high depths and at different level of resolution.
Please find a brief comparison of the different methods depending on the
objectives in the table below.
TARGET REFRACTION REFLEXION MASW 2D
Bedrock depth up to 50 m perfect good perfect
Bedrock depth over 50 m perfect good good
Soils and rocks quality good good perfect
Seismic classification - - perfect
The study was conducted with a 48 - channels - seismograph, 40 Hz
geophones, sensors spaced by 5 meters and 13 shot points. The purpose of
the study was to define the thickness of the debris cover and the altered
substrate.
Example [3]
Seismicrefraction investigation within the project of road construction in a
mountainarea.
APPLICATIONS
The refraction of seismic waves,
reflection and surface waves
allow:
Location of bedrock
Areas of location of faults,
fractures and channels
Detection of cavities
Stratigraphic correlation
Mining investigation
Wells sitting for geotechnical
purposes
Select suitable areas for
construction sectors
Get important petrophysical
information:
 elastic modulus
 density
 attenuation
 porosity
 speed of pressure waves (P)
and shear (S)
 anisotropy
 excavability
Figurae 3 – Geoelectrical team in Thailand.
page -3-

5. EXAMPLE [5]
Hybrid seismic reflection and refraction for the construction of a railway
tunnel.
APPLICATIONS
Shallow seismic refraction and
high resolution 2D MASW
possible to acquire detailed
stratigraphic characteristics for
civil work.
The hybrid seismic survey
allows to obtain fundamental
data for feasibility studies on
projects of deep excavation.
The study was carried out with a 72 channels seismograph, and 30 Hz
geophones, separated by 2.5 m between each other and energized by
dynamite.
The study purpose was to define the thickness of debris cover, altered
bedrock and fault zones.
Depth target
[m]
Geophones spacing
[m]
< 25 0,5
25-50 1
50-100 2,5
100-250 5
> 250 10
HIGTHMEDIUMLOW
HAZARD CLASS
Example [4]
2D seismic refractionand 2D MASW surveys within the execution of an
undergroundtunnel.
Túnel
As shown above, seismic refraction can give only partial information in
the context characterized by significant speed inversion. The refraction
method lacks of information on low velocity layers under fast velocity
layers whereas the 2D MASW technique can be used in noisy
environments. Both sections were acquired with geophones located 2 m
far from each other and with shot point every 3 meters.
Vp [m/s] Vs [m/s] Rippability
< 600 < 300 Loose soils
600-1300
300 - 600 Easily rippable
1300-1700 600 - 800
Normal
rippability
1700-1900 800 - 900
Hard to be
ripped
> 1900
> 900 Non rippable
Figure 4 – Geophysical team in Nigeria.
page -4-

6. APPLICATIONS
The MASW (Multichannel
Analysis of Surface Waves)
study is aimed at characterizing
and determining the
lithostratigraphicVs30 seismic
parameter, for soils
classificationof:
EXAMPLES [6]
MASW survey for soil classification - Vs30
In the figure above the experimental scattering of Rayleigh waves is
shown.
The numerical inversion of the curve, according to an iterative process of
least-squaresfitting, produces a shear waves velocity profile.
The seismic MASW data acquisition is managed with a 24 channel
seismograph, geophones with frequency resonance of 4.5 Hz and
energizationobtained with a 10 Kg hammer.
MASW survey is very fast, 30-40 minutes, allowing to get important
information about compaction, soil stratigraphy and susceptibility to
liquefaction.
Soil Vs30 (m/s)
A >800
B 360÷800
C 180÷360
D <180
E —
S1 <100
S2 —
Shear velocity Vs profile in
meters/second
Ground
type
Description of stratigraphic profile Vs30 (m/s)
A
Rock or other rock-like geological formation with Vs30 higher than 800 m/s,
including at most 3 m of weaker
material at the surface.
>800
B
Deposits of very dense sand, gravel, or very stiff clay, at least several tens of
meters in thickness, characterized by a gradual increase of mechanical
properties with depth ( or Nspt > 50 and Cu> 250 kPa).
360÷800
C
Deep deposits of dense or medium-dense sand, gravel or stiff clay with
thickness from several tens to many hundreds of meters
(15 < Nspt < 50, 70 < Cu < 250 kPa)
180÷360
D
Deposits of loose-to-medium cohesionless soil (with or without some soft
cohesive layers), or of predominantly soft-to-firm cohesive soil.
(Nspt < 15, Cu < 70 kPa)
<180
E
A soil profile consisting of a surface alluvium layer with Vs values of type C or D
and thickness varying between about 5 m and 20 m, underlain by stiffer material
with Vs > 800 m/s.
—
S1
Deposits consisting, or containing a layer at least 10 m thick, of soft clays/silts
with a high plasticity index (PI > 40) and high water content
(10 < Cu < 20 kPa)
<100
S2
Deposits of liquefiable soils, of sensitive clays, or any other soil profile not
included in types A – E or S1
—
Dispersion of Rayleigh waves
AVERAGE Vs velocity
till 30 meters depth
page -5-

7. The study was performed using a
24-channel seismograph, a triaxial
borehole geophone and a
piezoelectricsparker energizer,
made by Geotomographie
http://www.geotomographie.de
The figure on the right-hand side is
an example of tomographic down
hole, also called VSP (vertical
seismicprofile).
EXAMPLE [7]
Seismiccross-hole test: example of surveys before and after the execution of
soil consolidation under the pillarsof an historic building, using jet-grouting
technique.
APPLICATIONS
The seismic Downhole and
Crosshole method allows to
obtain the following types:
The stratigraphy of soils and
rocks
Location of fractured zones
Detection of cavity
Seismicmicrozonation
Level of densification of a
foundation
Verification of interventions
consolidation (jet grouting)
Getting important petrophysical
information, such as:
 elastic modulus
 density
 attenuation
 porosity
 speed of pressure waves (P)
and shear (S)
 anisotropy
Figure 5 – Instruments for acquisition of cross-hole
data.
DISTANCE [m] DISTANCE [m]
DEPTH[m]
DEPTH[m]
SHOTS RECEIVERS SHOTS RECEIVERS
Compression waves
velocity (P) –m/s
page -6-
Shot points
B
Another example of applicationof the cross-hole tomography technique follows
below.In this case the objective was to characterizeaccurately the physical and
mechanicalconditionof the soil and rock masses between two boreholes.

8. The acquisition of data is carried out by placing the sensors (geophones at
100 Hz) and the points of energization opposite the axis of the structure to
be investigated.
The tomographic reconstruction is made with the code GEO-TOM CG, which
uses an iterative algorithm called SIRT (Simultaneous Iterative
ReconstructionTechnique).
Starting from an initial velocity model, the algorithm reduces gradually the
difference between the periods measured along different paths and the
periods calculated on the velocity model, which is determined in the
preceding iteration and defines the pattern of wave velocity Vp.
EXAMPLE [8]
The seismic tomography surveys performed by transparency, allow to obtain
the distribution of the compression waves velocity in structures, such as
dams, bridge piers and walls.
APPLICATIONS
Transparency seismic
tomography allows to
characterize anthropogenic
structures (i.e. dams, bridge
piers, walls, columns of jet-
grouting and land, rock masses
between tunnels and
topographic surfaces) in terms
of the seismic waves
propagation.
The information obtained in
terms of petrophysical
characteristics are:
elastic modulus
density
attenuation
porosity
speed of pressure waves (P)
anisotropy
Figure 7 – Top of the dam where geophysicalsurveys
were performed.
33195 33200 33205 33210 33215 33220 33225 33230 33235 33240
Longitudine Nord
668
670
672
674
676
678
680
682
684
686
688
690
692
694
696
698
700
Quota(m.s.l)
Area characterizedby low
velocitiesof propagation of the P
waves – Deformationsector
Figure 8 – 100 Hz geophones installed in tunnel at the
bottom of the dam.
Figure 6 – Climber for the installation of sensors
and energization.
page -7-

9. Schist quartziferous
Shists
AUDIO-MAGNETOTELLURIC EXPLORATION
The audio-magnetotellurictechnique from natural source (AMT) and artificial
source (CSAMT) measures the fluctuations of the magnetic and electric fields
of the Earth. As to the natural source, these variations occur in the
ionosphere thanks to the solar activity whereas for the artificialsource, they
are generated by an instrument emitting a controlled electromagneticsignal.
Figure 9 – Audiomagnetotelluric crew.
EXAMPLE [9]
Audio-magnetotelluricinvestigationfor geothermal application, data were
acquiredwith an AMT system model MTU-5A manufacturedby Phoenix
GeophysicsLtd.
The acquisition point grid was 50x50 m. As a result we had a three
dimensionalrepresentationevaluating the subsurface resistivity distribution
and the presence of discontinuityup to 800 meters deep.
EXAMPLE [8]
Audio-magnetotelluricsurvey for the feasibilitystudy of an hydroelectric
project in Ecuador.
The data acquisition was performed by carrying out an AMT profile every 50-
100 meters for a total of 360 profiles.
The investigated depth was around 600 to 800 m whereas the high density
of AMT points was necessary to direct investigations due to the
inaccessibilityof sites.
The instrument used was a MTU-5A produced by Phoenix Geophysics -
http://www.phoenix-geophysics.com
Below the 2D resistivity section is shown.
APPLICATIONS
The audio-magnetotelluric
method offers great potential
for applications in the field of
deep geological prospecting
applications at once.
Here are some examples:
Mining Exploration determining
its area of influence, direction,
dip and depth extension.
Engineering applications and
environmental assessments as
performing deep tunnels, site
characterization for the storage
of nuclear waste, CO2 storage.
Exploration for oil and gas
Exploring aquifer units, aquifers
and saline intrusion.
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