[IMAGEM_CAROUSEL]

This report refers to the GPR (Ground Penetrating Radar) surveys performed in some streets and avenues in the city of Belo Horizonte – MG, with the purpose of identifying the presence of interferences − pipes, ducts, cables, etc.

These elements supported an adequate planning for future implementation works of Telephone Company communication cables.

The execution of these services was foreseen by contract, in order to supplement cadastral surveys performed in a previous step, whose results were presented in specific documents, thus increasing the accuracy of location and depth of potential interferences that might affect the execution of the works, which, in this specific case, consisted of the execution of horizontal directed drilling (HDD).

These surveys were performed on May 2007, distributed according to a guideline provided by Telefônica, according to the Underground Ring Canalization Project – BH.

Methodology

Surveys with GPR (Ground Penetrating Radar) or GEORADAR, can be summarized as slowly dragging an antenna (transmitter and receiver) along the areas of interest.

The antenna is connected to a control unit through a coaxial cable, responsible for controlling all data acquisition and storage parameters.

In this study, data were obtained through continuous profiling, with monostatic arrangement, that is, the same antenna, of 200 or 400 MHz, works as transmitter and receiver of electromagnetic waves. In order to minimize the spherical divergence effect, characterized by the attenuation of waves in the propagated medium, a gain is applied during the acquisition from the analysis of trace amplitude in function of time.

Geographic position of profiles performed was referenced at remarkable points, based on project maps provided by Telefônica / Radiante. For this, during profile acquisition a survey wheel was used, coupled to the antenna that automatically creates “marks” at previously defined regular intervals, in a way that, knowing the starting and ending location of each section, it is possible to georeference each point surveyed by the GPR.

The signal submitted by the antenna is basically constituted of an impulse with duration of nanoseconds, and the return time of this signal varies according to the operator’s decision due to objectives and scopes of the study.

The choice regarding the antenna that will be used varies due to dimensions and depths of the reflective objects to be studied, as well as the survey objectives and electric parameters of the terrain (electric conductivity and magnetic permeability).

Data obtained during the survey are then stored in the central unit, which are subsequently transferred to a computer in order to be processed and interpreted.

Results Obtained

    • Nearly all profiles performed indicate the presence of interferences, these being represented by draining, cables, pipes networks, metallic structures etc.
    • The representation of these interferences in the profile performed was possible by interpreting the geometry of the various reflectors found. In general, these reflectors are presented as hyperboles, in box, parallel planes, etc.
    • In some profiles, the identification of interferences was affected due to the presence of boxes with metallic caps along the sidewalks, which, due to the high conductivity, makes it difficult or prevents the penetration of electromagnetic waves. In these cases, in executed profiles, the representation of these interferences is provided without depth value.
    • A similar situation can be observed in some profiles, due to the presence of noises. This type of unwanted signal is mainly related to the presence of emitting sources of electromagnetic energy, usually characterized by the presence of electric, radio transmitters, mobile phones, and other networks. In surveyed areas, since they are urban areas, it is common to see all of these sources.
    • Surveys performed enabled, in most profiles, the identification and consequently the confirmation of interferences previously known by Telefônica / Radiante technicians.
    • Profiles performed were executed under the guideline of the Underground Ring Canalization project, provided by Telefônica / Radiante.

Thus, it can be concluded that GPR was shown as a supplementary, high resolution tool, to map these interferences. It is worthy highlighting that, in addition to the accuracy of these surveys, this methodology is known as indirect or non-invasive, that is, there is no need to physically open ditches and/or trenches, which can directly impact more extensive deadlines and, consequently, adversely affect the work’s deadline.