Seismic Refraction

Knowing the travel time that P waves use in crossing the distance that separates the source and the receivers, can be determined the speed of propagation of means located between both. From the values of speed it is possible to be obtained a model of the subsoil made up of layers of constant speed and variable thickness.

This method is very useful to determine, in a fast way, the structure of the subsoil. Their more frequent applications are the detection of the rocky substrate, studies on rippability, stability of slopes, etc.

Seismic Tomography

The seismic tomography is a geophysical method similar to traditional refraction. The main advantage of this method is the degree of detail of the models and the presence of layers of low speed, forts lateral gradients or high dips are not limitations.

The seismic tomography is based on the investment of residues (difference between observed travel times and theoretical P waves travel times). The investment process is an iterative process, in which an initial model is updated until giving rise to the final model.

Another advantage of the seismic tomography is the possibility of determining, of quantitative form (value of the minimum of the residues) and qualitative (through the space distribution of rays within the model), the degree of certainty of the reached solutions, which is very useful at the time of realising the interpretation of the model.

Vertical Seismic Reflection

When a seismic wave affects a discontinuity that separates two means, a fraction of the incident energy propagates to the average second and the reflected rest returns to the first one. The amount of energy that is transmitted and is reflected depends on the resistance of acoustic impedance between both means and on the angle of incidence.

If the angle of incidence is minimum (perpendicular incidence to the horizontal discontinuity) most of the incident energy it returned to the first one. This behavior is the foundation of the vertical reflection in seismic methods.

After a careful processing of the signals acquired in field an assembly is obtained in which the main discontinuities can be observed that define the structure of the subsoil. This method commonly is used in the hydrocarbon exploration (for the detection of deposits) and in civil engineering (detection of faults).

REMI

The method REMI obtains a distribution of the wave velocity of shears with the depth by means of the spectral analysis of registries of natural vibration of the land. For this reason, it is a method especially apt for urban atmospheres, where the presence of vibrations is elevated.

This method is based on the physical principle of the dispersion of the waves in the land. In reality, all the means are, in major or measured minor, dispersive, and therefore, the different frequencies that compose a certain wave packet propagate at different speed. As the wave packet moves in means, the individual frequencies are separated of the other, since the respective speeds of propagation are different. Studying the speeds at which the different frequencies propagate it can obtain a curve of variation of the speed of propagation of waves S with the depth.

After a careful processing of the signals acquired in field an assembly is obtained in which the main discontinuities can be observed that define the structure of the subsoil. This method commonly is used in the hydrocarbon exploration (for the detection of deposits) and in civil engineering (detection of faults).

MASW

Within the techniques based on the waves dispersion, we emphasize the MASW (Multichannel Analysis of Surface Waves). It is a method developed at the end of years 90 in the United States. Their main advantages are that it can use (and combine if it is necessary) generated waves Rayleigh of active way (by means of mace, vibrator or explosive source) or passive (traffic, wind, tides, etc).

Another advantage of this method is, besides analyzing the fundamental way of dispersion of the Rayleigh waves, it can incorporate originating information of the ways vibration superiors, generating in this way speed models with a greater resolution. At the moment, it is the geophysical method based on waves dispersion more popular anywhere in the world, given to his reliability and versatility.

In addition, Geopisica Consultores incorporate the use of land-streamer for the acquisition of the field data, reducing the times of acquisition and the associated economic costs. With a separation between geophones of 1,0m and a system of optimized displacement models of wave velocity S of hi-res are obtained in few minutes. This system has been used with total success in zones where the time of registry is reduced (airports, railcars, railway lines of high speed, etc).

Down-Hole

With the down-hole test the elastic modules (dynamic) of the land are obtained.

For the accomplishment of a down-hole test is necessary to realise a sounding in which a three-dimensional geophone is introduced. In a point next to the sounding, P and S waves are generated, and they are registered in the geophone ino a certain depth. The test is repeated at regular intervals of depth and, in this way, the variation of the speeds propagation of P and S waves is calculated, and in an indirect way, the elastic modules of the different layers that compose the subsoil.

Cross-Hole

A Cross-Hole test allows to consider elastic modules (dynamic), coefficient of Poisson, Young's modulus, module of shears, etc, of the land.

These parameters are very important at the time of determining the degree of rigidity, compression and deformation of the land and are used in geotechnical studies.

The method is based on determining the propagation speed of the shears and pressure waves (p) (s) at different levels of depth. For the execution of the test, a minimum of two soundings is necessary. The technique consists in generating seismic pulses within one of the soundings (emitting sounding) and determining the passed time since the pulse is generated until it is received, by the three components geophone in the other sounding (receiving sounding).

Knowing the distance that separates both soundings, the speed of transmission of the seismic waves can be obtained. In addition, it is necessary to calculate, of independent way, the density of the extracted materials of the soundings.

Cross-Hole Tomography

The seismic tomography tests in well are used in terrains where it is desired to accurately know the P waves distribution velocity.

Its main advantage respect to other analogous methods from the surface (seismic of refraction) is that the presence of low speed layers does not suppose an impediment for its accomplishment. Another advantage is the distribution of sources and sensors allows to obtain a homogenous readings, which reduces the uncertainty degree of the obtained models.

In addition, the data acquisition in depth can be combined with the installation of geophones in surface between soundings to obtain readings in surface which allows to increase the resolution in the most superficial levels.

Parallel Seismic

The Parallel Seismic method is used to determine the foundations elements length (piles, walls screen, etc.) and it is based on determining the depth which an abrupt change in the P waves propagation speed is observed, since the seismic waves propagation speed in the materials that compose the foundations elements is, generally, far beyond the one of the means that surround the foundations layings.

In order to realise the test, a series of sensors (hydrophones) is introduced in a sounding next to the foundations laying that is tried to analyze. By means of a mace is struck in the foundations superior part and the vibration is registered in the hydrophones located inside the sounding. The seismic waves propagate downwards throughout the foundations laying with an approximately constant speed until the moment of arriving at the base. In this point the seismic waves propagate with a inferior speed, the corresponding one to the means that surround to the foundations layings. This abrupt change of speed is detected analyzing the time of arrival of the seismic waves to each one of the hydrophones and time-distance is pronounced by means of a slope change in the diagrams.

In-Seam Seismics

The in-seam seismics is a method used in the coal exploration. The coal layers habitually are between denser and with faster seismic speeds materials (arenaceous, slate, etc.).

Producing a mechanical disturbance (by means of explosives) within the coal layer guided waves are generated (waves Rayleigh and Love waves) that propagates in a very effective way through the coal layer. Analyzing the guided waves undergone dispersion in different coal layer points it is possible to determine parameters as the thickness of the coal layer in each one of the passages crossed by the guided waves and to locate the presence of structural alterations (faults, abrupt diminutions of the power of the coal layer, etc.). This information is very important for the extraction workings since they allow to optimize the production and to diminish risks.