Application of geological radar in water conservancy project quality inspection

1 Introduction

Geological radar has been used for more than ten years.
The attenuation coefficient is proportional to the square root of the conductivity ( σ ) and magnetic permeability ( μ ) , and inversely proportional to the square root of the dielectric constant ( ε ) .

The reflection coefficient of the interface is:

Where Z is the wave impedance, and its expression is:

Obviously, the wave impedance value of electromagnetic waves in the formation depends on the formation characteristic parameters and the frequency of the electromagnetic waves. It can be seen that the higher the frequency of the electromagnetic wave ( ω =2πf ) , the greater the wave impedance.

For the common frequency range of radar waves (25 ~ 1000MHz) , it is generally believed that σ << ωε , so the reflection coefficient r can be simplified as :

The above formula shows that the reflection coefficient r mainly depends on the difference in dielectric constants between the upper and lower layers.

The depth H of the target layer can be obtained by using the two-way reflection time recorded by the radar :

Where : t is the reflection time of the radar wave in the target layer; c is the propagation speed of the radar wave in a vacuum (0.3m/ns) ; εr is the average relative dielectric constant of the medium above the target layer.

⑴ Reflection layer picking

Based on the comparative analysis of the exploration borehole and radar images, the reflection wave group characteristics of various strata are established, and the signs for identifying the reflection wave group are phase similarity, similarity and waveform characteristics.

⑵ Interpretation of time profile

On the basis of fully mastering the regional geological data and understanding the geological structure background of the survey area , the characteristics of important wave groups and their mutual relations are studied, and the geological structure characteristics of important wave groups are mastered, among which the changing trend of the event axis of characteristic waves should be studied in particular. Characteristic waves refer to reflection waves with strong amplitude, long-distance continuous tracking and stable waveform. At the same time, common special waves on the time section ( such as diffraction waves and cross-section waves, etc. ) should also be analyzed to explain the reasons for the discontinuous bands of the event axis.

Fig. 1 Radar image (a) and geological interpretation map (b) of the slope foot of the left bank 9+638 ~ 9+721 dangerous section

According to the above interpretation principles, the geological interpretation of radar images is as follows:

Figure 1(a) is a radar test image of the slope foot of the left bank 9+638 ~ 9+721 dangerous section. This image is interpreted from shallow to deep as follows: ① The first phase axis (<4ns) is the initial signal of the radar wave; ② The second and third phase axes (<12ns , layer thickness of about 0.40m) show wide, strong amplitude, and continuously traceable horizontal layers. The phase axis is speculated to be the reflection of mortar masonry on the radar image. Especially when the third event axis sometimes has discontinuous sections or is missing or disorderly, it can be inferred that the quality of the mortar masonry here is poor or it is separated from the embankment soil to form overhead phenomena; ③ New artificial fill: the reflection layer is discontinuous, with large fluctuations and sometimes disorderly, reflecting that the fill layer is uneven and the layer is unstable, sometimes showing in the form of a lens. The thickness of this layer is about 2 to 4m ; ④ Old artificial fill: the reflection layer is continuous and stable, and the medium in the layer does not change much, reflecting that the fill layer is relatively uniform and has formed a relatively dense stratum. The thickness of this layer is about 1 to 3m ; ⑤ Natural stratum: that is, the bearing layer of the embankment foundation, with obvious reflection, stable stratum, and no sudden change or unevenness of the medium in the layer, reflecting that the deposition environment of the natural stratum is good and the density is relatively large. The top surface of this layer is buried at a depth of about 4 to 5m .

Figure 2 is a radar image of the foot of the slope protection at 32+960 on the left bank. The shallow part of the figure is similar to that of Figure 1. It mainly shows that the reflection layer below 0.4m is chaotic and extremely irregular in the section 6.0 to 12.0m , with poor continuous tracking, and many short reflection layers. The reflection of the lower part of the mortar masonry is also chaotic, indicating that the soil at the lower part of this section of the protection is loose, and the contact with the mortar masonry is poor. The upper and lower parts of the section 12.0 to 15.7m reflect more uniformly, with good horizontal stratification, indicating that the soil of this section of the embankment is dense and in good contact with the mortar masonry.

3 is the image of the known lower part of the mortar masonry when it is empty. The third reflection phase axis of this profile is disconnected from the profile point 9.4m , forming a "anticline"-shaped strong reflection layer. This phenomenon continues to the profile point 12.8m . This section of mortar masonry is separated from the lower soil and causes the overhead, and its range is consistent with the known situation.

Through geological interpretation of radar test results, a total of 73 mortar masonry sites were identified with varying degrees of hidden dangers or poor quality. The types of these hidden dangers are generally as follows: ① The thickness of the mortar masonry is relatively thin; ② The mortar masonry is separated from the underlying soil to form an overhead space; ③ The mortar masonry is poorly bonded or loose; ④ The mortar masonry has cracks and other undesirable phenomena.

Most of the sections with poor overall quality of masonry are in serious disrepair over the years. The masonry has poor contact with the soil of the lower embankment, and is often in a suspended state , resulting in common masonry fractures, collapses and other undesirable phenomena, which are often of a certain scale. After analysis, the author believes that the reasons for the above phenomena are that there are many gaps in the masonry surface, the mortar quality is poor, there is less mortar, and there is no anti-seepage protective layer at the bottom. The soil of the embankment is mostly composed of fine sand. After precipitation infiltration, the fine sand is partially washed away, forming a cavity between the masonry and the soil of the embankment, and there is a trend of continued expansion.

The geophysical exploration results were verified by excavation ( see Figure 4 - excavation photos ) and were completely consistent with objective reality, which was praised by Party A.

6 Conclusion

Geological radar can play an important role in the detection of dangerous projects with its high efficiency, speed and high precision, and has achieved good application results. It has a very broad application prospect in shallow or ultra-shallow engineering detection. However, the detection depth and accuracy of geological radar are closely related to the antenna frequency used. The lower the frequency of the antenna, the greater the detection depth, and the lower the accuracy; while the higher the frequency of the antenna, the shallower the detection depth, and the higher the accuracy. This time , an antenna with a center frequency of 250MHz is used for detection, and its depth and accuracy can meet the technical requirements of this survey.

Figure 4 Excavation verification results ( left bank - photo )