Russian geophysicists have reached good success in the field of Ultra Wide Band Electromagnetic Sounding because of unique elaboration of high capacity generators with nanoseconds impulse and of unique mathematics model of electromagnetic impulse spreading through solid matter. Therefore, now it is possible to receive high quality signal from the depth about 100 m and make its interpretation.
From 12 years of experience and researches the representative database of different rocks parameters was collected which allows recognize different layers parameters in new areas.
Small sizes and mobility of equipment make the researches available in different remote areas and conditions.
Thousands of explorations were made in Russia and abroad.
METHOD OF ULTRA-WIDE BAND IMPULSE SOUNDING
The method of ultra-wide band (UWB) impulse sounding is based on synthesizing a map of the structure of a geological section or engineering construction from reflected signals of electromagnetic pulses of nanosecond (10-9) duration. As an analog of this method it is possible to consider seismic sounding by acoustical waves. The UWB sounding method has been developed worldwide, in such countries as the US, France, Japan, Switzerland, and others in the last 20 - 25 years. The efforts of numerous scientific teams and business concerns has made it possible to create instruments capable of probing objects to a depth of several meters. However, attempts to increase the depth of sounding even up to tens of meters were unsuccessful, because it was not possible to generate pulses of the necessary power and, even more importantly, because it was impossible to mathematically process the received signal.
A qualitative leap in the efficiency of UWB sounding was achieved by the creation of a special mathematical pattern of the distribution of electromagnetic pulses through solid environments, and by the development of a1 uniquely powerful nanosecond pulse generator.
This new method of UWB sounding allows geologic structures up to a depth of more than 100 meters to be revealed. The identification of layers occurring at these depths is possible: sand, clay, chalk, saturated layers, petroleum, etc. The accuracy of determining the position of layers is within about 5% or less of the actual depth. It is also possible to define the outlines of engineering structures in the ground, such as foundations, supports, piles, tunnels, headers, etc. In addition, UWB sounding enables the detection of defects in such structures. The scope of problems which can be resolved by UWB sounding is permanently extended.
TECHNOLOGICAL BASIS OF THE UWB METHOD
Electromagnetic waves, being distributed throughout the environment, experience absorption and reflection. These two processes are affected by many parameters of the environment, such as dielectric factor, conductivity, homogeneity, damp, polarization, the relaxation time of natural oscillations, and so on.
Any environment has a particular set of such characteristics. Theoretically, it is possible to conduct an identification of the environment by measuring characteristics of absorption and reflection. Thus, if the time and speed of a signal from an object to the receiver is known, the distance to the object is easily calculated.
However, in real life at any point of sounding there are alternations of geologic soils. Thus, the borders of layers or sections are arbitrary and can have impregnations of other soils or cavities. As a result, the received signal presents a superposition of convolutions of many reflections and fading, and the process of identifying and measuring distances to layers becomes extremely complex.
In such conditions, achieving results is defined by possible solutions to several problems: the generation of an electromagnetic pulse with an optimal frequency band and sufficient power; the quality of the receiving - transmitting antenna; the volume range of the receiving instrumentation actually defining the depth of sounding; and the processing logic of the received signal.
The method of UWB sounding combines in itself advanced achievements in the field of generation of nanosecond pulses of high power, as well as the emission, reception and processing of broadband signals.
Small-sized nanosecond pulse generators, with a peak power up to several megawatts, have been designed in Russia incorporating new semiconductor switches, which exceed the perfor-mance of foreign semiconductor switching devices of similar classes. Using them, a series of pulse generators providing a frequency band, indispensable in UWB sounding, from 108 to 1010 GHz has been developed.
The measuring system is comprised also by wideband antennas having good coordination with transmitting paths and a sharp response to the received signal.
A primary condition of receiving a large depth of sound with a high accuracy of identification of layers is the special mathematical processing program combined with a large experimental database, accrued over several years' experience with the system. Use of the experimental database, producing a correlation between the nature of a received signal and a specific type of geologic environment, substantially facilitates receiving high-quality results.
INSTRUMENTATION AND TECHIQUE OF UWB SOUNDING
The system of field instrumentation consists of several components:
• Nanosecond pulse generators producing pulses with durations of 1 - 10 nanoseconds, peak power as high as 106 watts, and a clock rate of following pulses of 10 KHz.
• Receiving - transmitting antennas with a band of 100 KHz - 1GHz.
• A reception - recording block with high noise immunity, providing a record of received signals in field conditions.
In the field, all components of the system can be operated with constant power of 12 watts. When connected to mains power, they will operate at 220VAC, 50Hz. Consumed power is no more than several hundred watts.
The weight of the entire system is about 10 kg.
The entire process of sounding one point requires several seconds. From one point of sounding it is possible to construct a view of a section of the sub-soil environment in a given place. To construct a view of an extended area it is necessary to make soundings at several points. The number of sounding points required is defined by the following conditions: the specific nature of the area, the kind of object (geologic environment, foundation, etc.), and the depth of sounding.
For example, to produce a profile of a buried aquifer 1000 meters in length at a depth of about 100 meters, it is necessary to make soundings every 20 - 50 meters. If on a structure 1000 meters long at a depth of about 100 meters it is necessary to define the exact disposition of a layer of soil most favorable to laying a tunnel or header, sounding every 10 - 20 meters is required. To define the structure of a foundation or the presence of piles under a foundation at a depth of about 10 meters, sounding every 1 - 2 meters is necessary. Thus, the number of required points of sounding is determined by the problem to be decided. Sounding through a layer of water is possible.
SIGNAL PROCESSING AND CONSTRUCTION OF A MAP OF AN OBJECT
Signal processing and the construction of a map of an object takes place, as a rule, in the laboratory under fixed conditions, and is the most complex and laborious procedure of UWB sounding.
The reflected signal which has been recorded in the field is mathematically processed by a specially-designed program on a powerful computer. Processing time for one point depends on the depth of sounding and can be as long as one hour. The speed of processing can be increased by further improvement of the program and/or by using a more powerful computer. The processed signal is compared with the available database of different environments and provides preliminary identification of the components which are included in a given section.
The final identification of the environment and the preparation of a map of a section is done by geophysicists who have experience with the complex database of known environments. After signal processing, the construction of a linear or volumetric map of the object is prepared. It is possible to isolate features constituting the object.
An essential acceleration and improvement of accuracy of the operation takes place if there is an actual gauge, or example in the database, similar to the object being researched. It allows the system to be calibrated with an allowance for the features of the soils and the characteristics of the given terrain.
BASIC APPLICATIONS OF THE UWB METHOD
• Performance of geophysical research for completion of civil engineering, the structure of tunnels, bridge transitions, headers, etc.
• Revealing defects in complex hydraulic engineering structures, piles, foundations and other concrete structures.
• Determination of the disposition of underground engineering structures and communications.
• Archeological operations.
• Determination of the disposition of mineral deposits (solid, liquid or gaseous).
• Determination of sub-surface environmental pollution (from petroleum, etc.).
• Determination of sub-surface aquifers and layers of water.