Characteristics of physical exploration methods

Physical exploration relative to other methods of investigation has the following characteristics.

1) the application of the method has "more than three". First, multi-parameter (find water, find ore, etc. can be applied to several or even more than a dozen parameters, such as electrical measurements in the resistivity, polarization rate, excitation ratio, half decay time, decay time, the degree of deviation, water-bearing factors, etc.). The second is multi-functional (not only can find minerals, but also can find water, find oil, but also can be used for engineering testing, geotechnical determination, etc.). The third is multi-series (for different detection targets, there are different combinations of methods series. (Such as water search for bedrock fissure water, pressurized water, brackish water, etc., respectively, using different combinations of physical exploration methods program).

2) instrumentation on the "three high". First, high sensitivity (observation sensitivity, mostly hundreds of times.). Such as electric measurement technology measurement from millivolt level to microvolt level development, magnetic measurement technology observation has been raised from nano units to milli-nano units, etc.). Second, high resolution (measurement resolution are greatly improved, including seismic technology, the resolution of meters, ground-penetrating radar centimeters, etc.). Third, high precision (measurement accuracy are from low precision and medium precision to high precision, has begun to promote high-precision magnetic measurement, high-precision gravity measurement technology, etc.).

3) There are "three excellences" in the data results. The first is the optimization of information (after the observation data are included, they can be corrected, extracted and pre-processed to optimize the useful information). The second is optimization of inference (for the interpretation of inference with the data processing software, including forward calculation and inverse calculation of the software has been compiled hundreds of kinds of flexible selection for different practical situations). Third, optimization of display (the results of the graphic display is crucial, graphic image processing system will be gradually promoted, three-dimensional images will also be applied in the results of physical exploration technology).

All kinds of physical exploration methods have the advantages of perspective, high efficiency, low cost, but also has the disadvantages of limitations, conditions, interpretation of the results of multiple solutions. In practice, the use of comprehensive physical exploration methods maximize the advantages of each method, to overcome their shortcomings, can improve the geological effect of physical exploration work, for hydrogeological investigation, groundwater detection to provide objective reflection of the geological structure of the reliable information.

1) perspective. Physical exploration method is through the observation of underground geological body in the ground generated by the physical field spatial distribution pattern, to infer the geological situation, to achieve the purpose of geological exploration. Relative to the naked eye observation and drilling means to understand the deep geological structure, it obviously has a similar perspective nature of the characteristics.

In the hydrological physical exploration, the commonly used natural electric field method, acoustic frequency geodetic electric field method, geothermal method and magnetic method are all passive source methods of measuring natural field. The electric current field of the deep structure is detected through the medium action of the material covering the surface layer; the magnetic field of the underground magnetic body of the deep structure can pass through the surface layer and be observed by the physical exploration instruments on the ground. In addition to these natural field methods, a large number of methods of detection by artificially excited physical fields (active sources) are used. In hydrological physical exploration, the main use of electrical exploration, which can be from different directions, different ranges, from far and near, from shallow to deep on the underground existence of electrical differences in the exploration of geological bodies, so as to obtain the effect of the underground geological structure and groundwater distribution mapping, and thus guide the layout of the exploration works, minimizing the expensive exploration workload. At the same time, it can directly lay out the location of drilling holes for water source wells, which effectively improves the accuracy of water searching and well formation.

2) High efficiency. Conventional means of geological exploration, in all types of scale hydrogeological census, survey and exploration, always according to the corresponding scale and a certain degree of mesh drilling exploration; in addition to labor-intensive, high-cost, but also very time-consuming, a borehole construction, less than a few days, or a month or even longer. According to the corresponding scale and a certain net degree of physical exploration work, each physical observation point generally only a few seconds to a few minutes, that is, the electric bathymetry point at most only a few minutes observation; and installation of lightweight, mobility, low cost. It can be seen, high efficiency, low-cost physical exploration methods in the hydrogeological census, survey and water exploration in a wide range of applications, is to achieve the modernization of hydrogeological work is an important aspect.

3) Conditionality. The application of various types of physical exploration methods must have sufficient prerequisites and conditions. Otherwise, it will cause a waste of manpower, material and financial resources. The conditions are mainly as follows.

a. Physical difference. The detected object and the surrounding rock must have obvious physical differences; no physical differences, can not produce geophysical anomalies; no geophysical anomalies, can not carry out physical exploration work.

b. Exploration depth. The ratio of the size of the explored object to the depth of burial cannot be too small. Even if the explored geological body and the surrounding rock have great physical differences, its scale relative to the depth of burial is too small, the field strength with the increase in distance and attenuation, so that the field strength measured on the ground may not be detected by the physical exploration instrument.

c. Interference factors. The target geological body produces anomalies, other different geological bodies with the same physical characteristics, will also produce similar anomalies detected and formed interference, so that the analysis and interpretation of the physical exploration data complicate. This also belongs to the "multiple solutions".

From the above three aspects of the physical exploration conditions, physical exploration methods do have great limitations. In the complex geological conditions of groundwater exploration, we should try to understand the geological situation of the work area, physical characteristics, interference level, in the conditions should boldly apply a variety of methods of physical exploration; on the other hand, we should strengthen the development of high resolution, detection depth, anti-interference ability, adapt to a wide range of new types of detection instruments.

4) Multi-solution.

a. The geological body that produces physical field anomalies is not unique. As stated in the aforementioned "interference factors", different geological bodies, due to the same physical (electrical) characteristics, reflected as the same anomalous physical (electrical) layer; the same geological layer, due to humidity, particles, and other factors reflecting several different anomalies in physical (electrical) layer, which will bring great difficulty to the data analysis and interpretation. In groundwater detection and geothermal exploration in mountainous areas, the geological conditions are often complex, coupled with the influence of poor terrain conditions and grounding conditions, it is difficult to distinguish anomalies with a single physical exploration method for many geological tasks. Only the use of comprehensive physical exploration methods, comprehensive study of all aspects of information, to a certain extent to solve this problem.

b. The quantitative inference of anomaly parameters is not unique. For example, the phenomenon of equivalence in the quantitative interpretation of the conventional resistivity method of electrical bathymetry three-layer section. When the thickness of the middle layer is too large, there exists the phenomenon of equivalence of longitudinal conductance S2 (S2 = h2/ρ2) for H-type and A-type curves, and there exists the phenomenon of equivalence of transverse resistance T2 (T2 = h2ρ2) for K-type and Q-type curves, which makes the quantitative interpretation of the quantitative interpretation not get the uniquely solved value of h2. Only under the condition of mastering the intermediate layer resistivity ρ2, the single value of h2 can be answered. This so-called definite answer is also relative due to the fact that the physical data carries a certain error. Sometimes the error of quantitative interpretation of electric bathymetry data reaches 20% to 50%.

The ultimate goal of the physical exploration work is to analyze and interpret the information on the physical properties of the underground material bodies obtained as geological conclusions and geological results. Physical exploration data qualitative analysis and quantitative interpretation of accuracy or not, on the one hand, rely on advanced, scientific analysis and interpretation methods; on the other hand, rely to a large extent on the experience of the analyst. There are generally two kinds of experience: the analyst's own "direct experience" summarized by long-term accumulation in practical work, and the "indirect experience" summarized by others in their work. Therefore, in the groundwater exploration under complex geological conditions, it is very important to get the correct "single solution" from "multiple solutions" and summarize the rules of physical exploration for water under different geological conditions. The application of the half decay time and deviation of excitation to find water, the application of comprehensive physical exploration method, the application of mathematical and statistical regression analysis method and other new parameters, in order to improve the qualitative analysis of data, quantitative interpretation of the credibility and accuracy of an important role.

Because of the physical exploration methods have the disadvantages of "conditionality" and "multiple solutions", the following issues should be noted in practice:

a) the application of various methods of physical exploration must have a certain geophysical premise --There is a certain physical difference between the exploration object and the surrounding rock. The greater the difference, the more obvious physical exploration anomalies reflect, the greater the reliability of the results of interpretation.

b) the object to be detected relative to the depth of burial should have a certain scale, so that it caused by the geophysical field changes are more obvious, in order to use the physical exploration instrument observed on the surface, and can be from a variety of interfering factors to the useful anomalies identified.

c) due to the instrument, geoelectric conditions, interpretation methods and other aspects of the limitations of a single method of physical exploration often has certain limitations, and therefore we should pay attention to the use of comprehensive means of physical exploration to obtain a variety of parameters, compared with each other, each other to verify, in order to obtain a more accurate geological conclusions.

d) Explanation of the physical exploration data, we must adhere to the qualitative to quantitative, from the known to the unknown, repeated interpretation of the principle of repeated understanding, so as to achieve a better explanation of the results.