Geophysical methods

Geophysics is a science that deals with all aspects concerning land, focused on the physical
study. It is an experimental discipline that seeks to understand the evolution of the earth and
the physical condition. That said, we highlight the study directly linked to the phenomena
affecting infrastructure.
There are numerous geophysical methods through experiments or tests that bring out the soil
conditions and are extremely necessary to know the characteristics or nature of the land; it
should be mentioned that the seabed is included when speaking of the earth.
Geophysical surveys are closely related to geology, mineralogy, geotechnics, among other
disciplines that are used in the field of engineering.
There are several geophysical methods that are very important because through them you can
conduct research to assess the geotechnical characteristics of the land on which it is intended
to work.
Before any project, it is necessary to survey the field since geophysical methods are part of
the techniques of geotechnical investigation aimed at determining the depth of the water
table, risks of leaks, volume excavations, problems of instability, among other countless
aspects to be considered when carrying out any type of work. These methods should be used
as supplements to surveys conducted in the field.
Although there are several geophysical methods, this research will emphasize three of them,
which are:
 Seismic method
 Electrical Resistivity method
 Geoelectric method Tomographies
These methods should be applied by trained and qualified personal to perform the various
tasks and procedures that compose staff. This is important for the effectiveness of the results.
 The seismic method
There are many methods to study the behavior of the terrestrial structure. One of them is the
seismic method, which is the one that provides the most information, based on the study of
earthquakes and the propagation of seismic waves.
Earthquakes are caused by the fracture of large masses of rocks found in the earth's crust,
these fractures are caused by the internal movement of the earth and cause great vibrations
on the earth's surface.
When these movements occurs, it is obvious that they generates large energy releases, which
in this case are in the form of waves (seismic waves).
Seismic waves can be of various types.
 P waves or primary: They propagate to solid and fluid media, due to the propagation
of their compression waves through the same trajectory of the wave. They are the
waves that reach the highest speed when they originate.
 S waves or secondary: They are transverse waves that propagate the particles
perpendicular to the direction of propagation. They are transmitted only to solid
media and their speed is lower than P waves.
 Surface waves: They are nothing more than the waves generated when the P and S
waves reach the surface.

When the earthquake occur, the seismograph (apparatus that measures and record the
magnitudes of earthquakes) allow to know the different limits between the materials that
make up the earth.
With the constant evolution in technology and with the need to know our planet, man has
already been able to create forms, methods and processes to study our environment.
The seismic refraction method consisted of generating a situation like an earthquake, using
impulses that can be impacted or small colloquial explosions generally on the surface.
This method is complemented using geophones that are transducers of displacement and
speed that convert the movement of the ground into an electrical signal. These devices are
arranged at specific distances around the point of impact or explosion and their main function,
in this method, is to measure when the seismic waves arrives and measures the transmission
speed.
Links
 https://es.wikipedia.org/wiki/M%C3%A9todos_s%C3%ADsmicos
 http://e-
ducativa.catedu.es/44700165/aula/archivos/repositorio/750/975/html/3_el_m
etodo_sismico.html

 Electrical resistivity method

The electrical method is the oldest geophysical method used in civil and geological
engineering, which consists in performing tests to determine the geotechnical characteristics
that a terrain can have. The soil materials resist the flow of electrical current and this is what
is called resistivity. Also, it can be defined that resistivity is an intrinsic property of rocks
and needs water content, internal structure and degree of salinity.
The electrical resistivity method is used to determine the lateral or vertical variations of the
conductivity in the subsoil, it is frequently used to know the different depths in which the
firm rock is found and the unconsolidated materials.
The technique consists of placing 4 aligned electrodes and knowing the distance between
them, in other words, the measurement is made by introducing a certain intensity of electrical
current (I) in the ground, through two stainless steel electrodes (A and B); the flow of the
current in the subsoil produces an electrical potential or voltage, which can be verified by
two other stainless-steel electrodes (M and N) that will be introduced into the ground (figure
1).

Figure 1: Tetra electronic device for the measurement of soil resistivity

Resistivity can be expressed as:

R: resistivity
V: voltage
I: current
The test can also be carried out in the form of electrical probing, verifying the variation of
resistivity with depth. For this, tests are carried out and the distances "d" between the
electrodes must be changed and the center of the alignment of the four electrodes must be
kept at a fixed point.
When extending the path, the depth reached by the current lines increases, therefore, the
ground depth also increases. If the resistivity rises, it can be concluded that there is a deep
layer with greater resistivity, therefore, if the resistivity decreases with increasing separation.
The maximum depth that can reach will be about 20 meters.

 Geoelectric method Tomographies

It is a geoelectric field scan method to discover the existence of geological deposits. It is
widely used in studies applied to civil engineering. This test is used to restore the geometry
of complicated structures. It is difficult to perform these studies evident through surface
measurements; so, it is necessary to use distribution model’s resistivity that can regulate the
desired surface measurements. By the electrical potential this method seeks to determine the
resistivity distribution in the subsurface.
Goals:
This method allows a pseudosection that is only resistivity profiles forming a mesh in two
dimensions (2D, length depth). These profiles show variations resistivity qualitatively
studied section.
Electrical scans are based on modeling two-dimensional finite element or finite difference.
However, at present modeling it is implemented in three dimensions.
CT scans have been so successful as data fully automatically thus reducing the time was
employed in conventional methods that were displaced by CT scans are obtained. This allows
taking many measures in different directions (length and depth), it can be mentioned that in
approximately one and a half can be obtained 500 measures 2D. One of its main advantages
is that by applying numerical techniques provides an efficient tool to manage large body of
data.
Scan performed on hillside, in a study of the lithological structure. Source: Center for Studies and
Experimentation of Public Works of the Government of Spain.

Study of a raft. On the right-side drawer and concrete it sits under your seat leaks are detected.
Source: Center for Studies and Experimentation of Public Works of the Government of Spain.

Fields of application
 Characterization of landfill
 Cavity detection
 Location of water table
 Stratigraphy Land
 Location of geological structures
 Studies aggregate reserves
 Reservoir studies
 Subsurface resistivity
 Among many others.
Functioning
The theoretical basis of this method is grounded in the conventional methods only using four
electrodes and inserted in the soil current field through two electrodes that are connected to
a multiamperímetro and the other two electrodes are connected to a millivoltmeter, thereby
determining the midpoint and to a depth indicated the electrical potential which can calculate
the resistivity.
We can say that the difference between the conventional method and the geoelectric scans is
that the calculations are automated through the device which measures the resistivity, without
moving the electros manually and making all combinations assigned.

Synthesis of the scans numbered from 1 to 7 along several profiles made in David, Chiriquí
Province. Source: Application of Geophysical Prospecting Techniques by Alexis Mojica
and Eduardo Silva.