GPR

A new underground imaging system has been developed and tested in Russia during the past five years, and the first prototype system is now being used commercially. The UIS has provided imaging with excellent resolution for a variety of civil engineering tasks including subway tunnel design, bridge support analysis, locating mineral deposits, analysis of locations containing underground storage tanks, and river-bottom exploration. Additional applications are expected to include: location of buried munitions, borehole-to-borehole imaging, and oil and gas exploration. This system allows solution of many geotechnical engineering tasks including detailed display of geological structures. The system is accurate to +/- 1% of the depth. Exploration depths of more than 200 meters have been achieved in complex soil conditions, including strata of clay, with good resolution.

The success of this system is based primarily on the mathematical methods which are used in the signal processing. A second integral element of this system is a new methodology of data interpretation. A nanosecond pulse generator is the third key component.

The UIS is composed of several pieces of equipment that are portable (the total weight of the equipment is approximately 10 kg) for easy use in the field:

Mathematical processing of return signal data is presently performed in the laboratory on a Pentium Pro -- 150 MHz computer. The time required for processing the data from a single point of exploration at a depth of 100 meters is approximately 3 hours. (Real time results on site would require comparably faster computational capability, e.g. a workstation.)

The data processing methodology is based on the solution of inverse interference problems in the form of Green's stationary function, which defines the spatial-frequency distribution of the field for stationary monochromatic tasks, or in the form of Green's non-stationary function which is a solution of non-stationary tasks with excited delta-function. Green's stationary and non-stationary functions are correlated to each other by Fourier transformation.

Simulation of the geological/geophysical structure defines the depth and identification of objects. The steps of the simulation process are:

Characterization of the geostructure reflection and definition of the type and magnitude of criteria is accomplished by comparing return signals with bore-hole control data. This enables the user to build a database for improving the interpretation of the resolved data.

The UIS was designed and tested for various tasks: to create an image of a given geological structure, to identify and locate water-bearing strata, to show defects in concrete structures, to determine the location of oil, to locate utilities, and other tasks. The system can operate through layers of ice, water, and clay and can be used at angles other than vertical, e.g., for horizontal mapping of strata in mines or tunnels.

In the upcoming months, additional research and development will be conducted with a goal of achieving penetration depths greater than 200 meters.

The UIS can be customized for solving specific technical problems, providing simpler and faster processing and thus faster final results.

The primary goal of the system is to receive a return signal from a large depth. It is a fact that the attenuation of a signal through soil is great and that low frequencies fade less than high frequencies. The UIS system is designed to work with -- not against -- those laws of physics and still produce images for deep strata or objects.

The primary pulse has a frequency in the range of 10⁷ - 10⁹ Hz. Two receive antennas are employed - one for high frequency and the other for low frequency. During operation of the UIS, signals were lost at a depth of 70 m on the high frequency antenna, but were successfully recorded at greater than 100 m on the low frequency antenna.

The signal goes first from the pulse generator to the transmitting antenna (with a starting voltage of 1,000 V) which is the first critical element in the system. It determines, to a large degree, the signal loss. The transmit antenna incorporates unique "know-how" and a special design. The receiving antennas also employ certain "know-how" and provide a return signal which is much less attenuated than one would expect given the depth and variety of strata through which the signal passes.

The signal is then transferred to the oscilloscope. The power of the received pulse is about 10^-6 watt and maximum power of the generator is about 10⁵ watt. The entire dynamic range of this device is approximately 10¹¹, and this parameter is a key factor in enabling deep imaging. It is important to note that at every stage of a system (generator-to-antenna, antenna-to-cable, cable-to-oscilloscope, etc.) there is a significant possibility of signal loss, the inventors have paid close attention to this problem and have specially adapted their cables and connectors for reliable coupling of signal and data at each point in the system.

Registration of the signal takes only a few seconds, the time required to save the data on the notebook computer. However signal processing takes 2.5 - 3 hours on a Pentium computer for imaging a depth of approximately 100 m. This is a major disadvantage in the UIS compared with GPR systems some of which provide real time data. However, this system was designed specifically for tasks requiring imaging at deep depths and a high level of detail of the strata. Time has been sacrificed for accuracy. The main goal of imaging at deep depth with good resolution has been achieved and now work is in progress to improve other aspects of the system's operation, primarily data processing speed and expansion of the data base for correlating processed signals to strata composition.

Date	Location	No. of Measurements	Results of Investigation
Aug. 1997	St. Petersburg, Muzhestva Square (near the Aurora and Red October factories)	228	Investigated the geological conditions in this area to a depth of 90m.
Aug. 1997	Yartsevo in Smolenskaya Oblast', bridges on the Moscow-Minsk Highway (340.6 and 334 km)	182	Determined the technical condition of the bridges and the parameters of the underground structures.
Jun. 1997	Moscow, "Smolenskaya" subway station	84	Determined the parameters of voids behind the false ceiling in a passageway of the station.
May 1997	St. Petersburg, Sennaya Square, vestibule of the subway station	22	At a site where soil had been previously frozen, determined the presence of residual rock to a depth of 23 - 25m.
May 1997	St. Petersburg, Kazanskii Bridge	58	Determined the construction of littoral bridge abutments and their technical condition.
Apr. 1997	Yartsevo in Smolenskaya Oblast', bridges on the Moscow-Minsk Highway (373.9 and 386.6 km)	174	Determined the construction of littoral bridge abutments and their technical condition.
Oct. 1996	Novokuibyshevsk in Samarskaya Oblast', an oil processing plant	371	Approval of a method of ecological monitoring at oil processing plants.
Oct. 1996	Moscow, a bridge to the village of Besedy on the MK Highway	385	Determined the technical condition of reconstructed supports and their underground structures; investigated the geological formations of some cross sections to a depth of 35 - 40m near the construction of supports for a new bridge.
May-Jun. 1996	Moscovskaya Oblast', agricultural area	108	Located water-bearing layers for supplying drinking water.
Mar. 1996	Kashira, a bridge over the Oka River	26	Determined the technical condition of the underground bridge structure.
Oct. 1995	Moscow, a vehicle tunnel at Dobryninskaya Square	160	Determined the position of a blockage in the drainage system of the tunnel under the road bed.
Oct. 1995	Tara in Omskaya Oblast', Tarskoye zirconium-titanium deposits	108	From the surface, mapped excavation chambers located within the general excavation site at a depth of 50 - 55m. Mapped specific geological layers.
Aug. 1995	Moscow, Leningradskiy Bridge across the Moscow Canal and the Saykinskiy Overpass	94	Determined the depth of asphalt pavement.
Jul. 1995	Novgorodskaya Oblast', bridges across the Meta (in the town of Bronintsa), Shelon' and Mshaga (in the town of Shimsk)	270	Determined the construction and parameters of concealed (underground) elements of the bridge structures. Studied their technological condition and the presence of zones of dispersion of the soils below the supports.
Jun. 1995	Moscow, Nagatinskiy and Bol'shoi Krasnokholmskiy bridges, bridge at Kutuzovskiy Prospect	107	Determined the depth of asphalt pavement.
Jun. 1995	Moscow Oblast', Molodil'naya River bridges	55	Determined the parameters of underground parts of the bridge support and the presence of a pile foundation. Studied their physical condition.

Task A geophysical examination had been previously conducted at several construction sites of the Leningrad Metropolitan State Institute for Transportation Design and Planning (LENMETROGIPROTRANS) using the Subsurface Pulse Sounding (SPS) method. Subsequently an independent investigation of the same areas was carried out by means of an ultra wide-band (UWB) underground imaging system located at the ground surface.

Results The results of the investigation were processed on a Pentium 120MHz computer using a specially designed software package based on particular mathematical algorithms. The main output product of the computer processing was a graphical illustration of the UWB signal (see Fig. 1,2, and 3), separated by depth at the point of sounding, and of the reflected geophysical character of the geological structures. The interpreted data was compared with geological cross sections provided by LENMETROGIPROTRANS geological service, they matched completely. The interpretation was confirmed by the geophysical measurements carried out previously at the site and furthermore, more correctly identified the mass of the rock in given points.

Task To conduct underground imaging of the foundation and underlying strata of an underground pedestrian walkway to identify and locate physical and hydro-geological dislocations which might have led to buckling in the engineering structure (longitudinal cracks in the bulkheads and dislocation of the level of the floor).

Results A geophysical investigation was conducted at depths of 9.6 - 11.9 meters using the underground imaging system. The distance between surface measurement points was 9 m. The results of the data interpretation are provided in five sectional views contained in Figures I-I, II-II, III-III, IV-IV, and V-V. The interpretation showed that water-saturated areas of soil exist throughout the area of investigation (as illustrated by diagonal lines in Figures I-I and III-III). Certain other areas were characterized by the dynamics of the ground water located within them (as illustrated in Figure III-III, point 8 at 5.4 - 7.3 m and Figure V-V, point 13 at 6.7 - 8.6 m). Physical-mechanical disturbances were noted in the strata which was investigated and in the concrete structure located there, in particular, the presence of fractured zones in the concrete foundation of one of the tunnel bulkheads.

MHE - Underground Imaging System