Timing and control and data acquisition for a multi...

Details Timing and control and data acquisition for a multi... Timing and control and data acquisition for a multi...

1999-03-22

2002-12-31

Sotomayor, John B. (Department: 3662)

Communications: directive radio wave systems and devices (e.g.,

Transmission through media other than air or free space

C342S190000

Reexamination Certificate

active

06501413

ABSTRACT:

FIELD OF THE INVENTION
This invention relates in general to ground penetrating radar systems, and more particularly to the concurrent use of multiple transducers for a ground penetrating radar system (GPR).
BACKGROUND OF THE INVENTION
There is a growing demand for GPR systems that have the ability to acquire data with more than one transducer. The ability to run more than one transducer at a time is extremely complex given the nature of the problem. Systematic control of accurate timing in a distributed multitransducer network of GPR systems requires unique timing and logic elements.
In current practice, systems most often have one transmitter and one receiver transducer. Generally GPR systems obtain data along a measurement traverse line with the transmitter and receiver in a fixed geometrical configuration with respect to one another (prior art, FIG.
1
); the GPR system as a whole is moved over the ground or medium to be explored (Annan, A. P., Davis, J. L.,
Ground Penetrating Radar—Coming of Age at Last,
1997; Proceedings of the Fourth Decennial International Conference on Mineral Exploration (Exploration'97), Toronto Canada, Sep. 14 to Sep. 18, 1997).
References to the utilization of more than one transmitter or receiver are limited. Prior attempts have been made as described in U.S. Pat. No. 5,248,975 issued to Schutz, A. E., entitled “Ground Probing Radar with Multiple Antenna Capability”.
There are four major problems that have to be overcome.
The first problem is that the acquisition of ground penetrating radar traces in single transient waveform capture process, in digital form (or even analog form) is virtually impossible. Current commercially available analog to digital (A/D) converters are simply not fast enough nor do they have sufficient dynamic range to record the signals required for many of the GPR applications.
As a result, GPR systems resort to some sort of repetitive signal in order to capture the desired data. The most common approach is to use equivalent time sampling. Other approaches are to use a step frequency continuous sinusoidal wave technique that acquires data in the frequency domain by detecting the in-phase and quadrature response of the transfer function at a number of frequencies; the time domain signal is created by fourier transform.
A third approach is to use a fast A/D converter with few bits (i.e. limited dynamic range) and then stack the resultant signal for many repetitions in order that the resolution can be brought up. A fourth approach is to transmit some stream of random signals and use a correlation technique to extract the impulse response.
With all these approaches, considerable time is needed at each observation point to acquire data of a satisfactory nature. Combining such complex, individual signal capture processes with multiple spatially distributed transducers and simultaneously maintaining timing synchronization to very tight tolerances is a complicated task. The complexity arises is part arises in part because the transit time to transfer control signals between spatially separated transducers is both finite and are comparable or bigger than the measurement time lags.
The second major problem in trying to operate more than 1 unit is that multiple 2 transmitting sources operating at the same time can interfere with one another. If one wishes to operate two units, which are collecting independent information but operating at the same time then it is important that the signals from the transmitters do not get emitted at exactly the same time so that the two data sets can be acquired with high fidelity. In other words, a multiplexing process is required. In some instances it is desirable to have the transmitters operating simultaneously, but in this case one wants to make sure that the timing of the transmitters is synchronized in order to enhance the measurement process.
The third problem is that the transducers (or antennas) which create, emit and capture the electromagnetic signals which are transmitted into the ground are highly dependent on their immediate surroundings. When multiple transducers are placed in close proximity to one another, the transducers can interact in an almost unpredictable fashion and generate spurious signals.
The final problem is with the spatial distribution of the transducers. Since the signals that are being measured are radio waves that travel at the speed of light, all of the times involved in the measurement process are very short. Since the subsurface spatial dimensions may be similar to the separation distances between GPR components, the travel times on the inter connecting cabling or internal signal paths of the systems can become as large or larger than the travel times of the signals through the media being probed. As a result, it is important that any timing system be able to recognize these time differences and provide a means to measure and/or adjust times to eliminate the time delays associated with spatial distribution of the transducers.
FIGS. 2-5
, show the most commonly envisaged multi-unit systems.
FIG. 2
shows the use of multi transducer systems where the objective is to obtain data records from a variety of separations between the transducers. Many applications could benefit if data from a multiplicity of separations could be acquired simultaneously. Fisher, E., McMechan, G. A., and Annan, A. P.,
Acquisition and Processing of Wide
-
Aperture Ground Penetrating Radar Data;
1992; Geophysics, Vol. 57, p. 495-504, and Greaves, R. J. and Toksoz, M. N,
Applications of Multi
-
Offset Ground Penetrating Radar; Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems,
1994; (SAGEEP'94), p. 775-793 discuss the use of variable offset measurements and the enhancement of the data that can be achieved by coherent spatial stacking in the spatial dimension.
The acquisition of multiple separation data measurements made at each station along the transect line, is called multi-fold offset surveying. Multi-offset data available at every measurement point allows for the extraction of a velocity cross-section, an attenuation cross-section and an enhancement of data by determining an optimum spatial stacking velocity structure.
The second type of multi-channel system is depicted in FIG.
3
. In this case the objective is to cover a larger area more quickly. Many GPR applications require acquisition of data on a series of parallel lines in order that a large area can be covered to obtain a three dimensional volume view of the ground.
One way of improving such surveys is to have a number of measurement systems mounted side-by-side and have these transported over the ground simultaneously. In
FIG. 3
a
, a one channel system is shown sequentially measuring up and down 4 lines to acquire the same data that 4 transducers traversing once simultaneously over the four lines would achieve as shown in
FIG. 3
b.
It is useful to note in this application that the individual units can more or less operate independently. They do not require synchronous sampling times but it is desirable that the transmitter be set up to operate at different staggered times to eliminate any potential of interference between the units caused by simultaneous operation of the individual units.
FIG. 4
depicts still another type of application where multiple transducers or measurements are desirable. The bandwidth of GPR systems is limited by the intrinsic characteristics of antennas. For detailed study of the subsurface, a number of systems with different frequency bandwidths and corresponding different physical sizes may have to be traversed along the same line in order to achieve full coverage of the subsurface.
At present, this type of operation is achieved by surveying the line a number of times as depicted in
FIG. 4
, once with each transducer. The whole operation could be completed more quickly if all (three transducers in the example shown) transducers are moved simultaneously along the line at one time and the same data acquired. Coordination of spatial a

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