Basic knowledge-classification and characteristics of geophysical exploration methods

(1) Understand the basic concepts and essence of geophysical exploration;

(2) Learn the classification methods of geophysical exploration and geophysical exploration methods mainly used in environment and engineering geology;

(3) Understand the application of geophysical exploration in environment and engineering geology.

An exploration method to solve geological problems by observing and studying the changes of various geophysical fields is called geophysical exploration, or geophysical exploration for short. The so-called geophysical field refers to the physical space that exists in and around the earth and has physical effects. For example, the gravitational material space inside and around the earth is called gravity field; The natural or artificial material space with electric (magnetic) force is called electric (magnetic) field; The material space of particle vibration propagation is called elastic wave field. Due to the differences in the earth's crust, there are often differences in density, elasticity, electricity, magnetism, radioactivity and thermal conductivity, and these differences will cause local changes in the corresponding geophysical field in space (or time), which is called geophysical anomaly. Geophysical exploration is to observe these geophysical anomalies through special instruments, obtain geophysical data such as their distribution and morphology, and then analyze and study them with known geological data, infer underground geological structures, or determine the properties of geotechnical media, so as to solve geological problems.

Environmental and engineering geophysical exploration is a set of geophysical exploration methods for engineering geology, hydrogeology and environmental geology.

Geophysical prospecting method was applied earlier in hydrogeological exploration, and its main task is to find groundwater. With the strengthening of people's awareness of environmental protection and the rapid development of environmental protection work, the research and application of environmental geophysics are in a stage of vigorous development. Geophysical methods play an important role in environmental protection, environmental monitoring and governance, and their application will be more and more extensive. It is also an important task of environmental geophysical exploration to find groundwater that can be used for industrial and agricultural production and life.

Engineering geophysical exploration, that is, geophysical exploration methods used in engineering. Because modernization needs to explore geological conditions, geophysical exploration has the advantages of being faster and more economical than other direct exploration methods such as drilling, and some exploration needs nondestructive testing, which makes geophysical exploration methods widely used in engineering construction and gradually forms a set of geophysical exploration methods.

There are many similarities and overlaps between environmental geophysical exploration and engineering geophysical exploration in the application of methods, so this book discusses environmental and engineering geophysical exploration together.

Environmental and engineering geophysical exploration is a detection method based on the differences of properties (density, electricity, magnetism, elasticity, radioactivity, thermal conductivity, etc.). ), rock and soil, building materials and other physical media, and through observing and studying the temporal and spatial distribution laws of various natural and artificial geophysical fields, to solve environmental and engineering problems.

There are many geophysical methods for environment and engineering. Elastic wave method and electrical method are mainly used in exploration, and nuclear geophysical methods are also used.

Electrical method is the most colorful method. In addition to the traditional electrical sounding and electrical profiling, the high-density electrical method has made great progress in theory and application in recent years. It uses a number of electrodes with small spacing on the profile to collect quickly. Through data sorting, it can be transformed into various equipment forms and mapped to different depths or planes on the section, which improves the resolution. Some new equipments have increased their exploration ability, such as five-level vertical axis sounding method for exploring the depth of the top and bottom of caves, and differential electrical sounding which can detect thin layers with a thickness of several tens of centimeters in a depth of several tens of meters. The seepage electric field method is effective in detecting the leakage of reservoir dam and the direction of groundwater flow, but it has been rarely used in recent years, so it is necessary to mention it again. Induced polarization method is a special method to detect groundwater. At present, the time domain method is commonly used, and the second time difference method is developed by using the characteristics that different aquifers have different polarization ability under different excitation currents.

In addition to very low frequency method, audio magnetotelluric method, frequency sounding method and multi-frequency ground electromagnetic method, transient electromagnetic method has also begun to appear. It is a widely used and effective method to use borehole radio wave perspective as the apparent absorption coefficient profile of rock mass between holes to detect karst. In recent years, more and more attention has been paid to ground penetrating radar, which has a good application prospect in shallow karst detection. In addition, many instruments and methods for underground pipe network exploration are also based on electromagnetic method. Frequency-domain induced polarization method also has a good example in groundwater exploration, such as dual-frequency or multi-frequency induced polarization method, which shows a good prospect.

Refraction wave method and reflection wave method are still the main methods in elastic wave method, and high resolution methods such as shallow shear wave reflection method are paid attention to. In order to improve the resolution and submit the results in time, the land minimum offset high bandwidth reflection continuous profiling method (land sonar) has appeared in recent years. Electro-acoustic methods, such as shallow stratigraphic profiling, have played a role in exploring the accumulation and siltation of ports, docks, bridge sites, seabed and river bottoms. Acoustic wave method has been widely used in rock exploration, determination of geotechnical physical and mechanical parameters and quality inspection of concrete structures. Surface wave exploration, borehole transmission CT and cross-hole method are also effective and promising methods. Among them, steady-state and transient surface wave exploration, using Rayleigh wave dispersion characteristics and the correlation between surface wave wavelength and depth, has achieved good results in shallow exploration and cave exploration, and is being favored by people; In the aspect of pile foundation detection, various special methods are formed by using the dynamic and kinematic characteristics of reflected waves; It should be pointed out that it is more and more necessary to use the dynamic characteristics of waves in engineering geophysical exploration, and there are successful experiences in judging the water content of the measured rock strata, the grouting quality of the mortar rubble retaining wall, and the pile testing of concrete quality. It is also one of the contents of architectural engineering exploration to measure the vibration spectrum and dominant period of site foundation by using natural microseisms.

Nuclear geophysical exploration is also a rich method, and radioactive measurement methods are often used in environmental and engineering geological work. In geological exploration, γ survey, α card method, α cup method and radon survey are commonly used in geological mapping and fault structure finding. It is used to absorb or scatter gamma quantum and thermal neutron, reflecting the quality, water content and hydrogen content of the substance, so it is one of the good methods for engineering quality inspection, and is widely used in nondestructive inspection of concrete quality, filling and rolling quality inspection, asphalt pavement quality inspection and so on.

Other methods, such as high-precision magnetic survey and high-precision gravity survey, have been successfully applied to geological mapping, finding structures and caves, artificial burial objects and archaeology. Geothermal survey is widely used in underground hot water exploration; Remote sensing is an effective means for engineering site selection, building search and dynamic monitoring of adverse geology such as landslide and debris flow. Using far-infrared photography and video camera to find loose rocks, cracks and water accumulation points in structures has attracted the attention of engineering circles in recent years. Logging and the combination of static penetration and geophysical logging technology should be an important application content in environmental and engineering geophysical exploration, but the application and development in this field in China is not enough and should be paid attention to.

Environmental and engineering geophysical prospecting work usually has the following characteristics:

Most objects are shallow and small. The exploration depth varies from tens of centimeters to tens of meters, which requires high exploration resolution and high quantitative interpretation accuracy.

2) It is required to find out not only the distribution law of exploration objects, but also the spatial position of a single object (such as a cave).

3) Closely combine with environmental engineering geological work. Exploration data are often used for design or construction, and the time is closely related, which often makes the exploration conclusions can be verified and fed back in time, which requires higher work conclusions.

4) The exploration object is complex. The laws of shallow small targets are complex, the geological conditions near the surface are uneven, the anisotropy along the horizontal and vertical directions is serious, and even the physical parameters gradually change continuously, which brings many difficulties to the qualitative and quantitative interpretation of data.

5) Most of the environmental problems to be solved are concentrated in industrial centers and big cities, so they are often disturbed by man-made noise. Such as magnetic interference, electromagnetic interference and industrial traffic vibration caused by underground pipelines (wires), cables, high-voltage lines and railways, so it is necessary to take corresponding effective measures to suppress all kinds of interference. In addition, the field work space (scope) in environmental investigation is usually small, which requires geophysical exploration methods to have anti-interference and flexibility.

At present, environmental and engineering geophysical prospecting is mainly used in the following aspects:

1) regional geological survey. Its purpose is to provide first-hand information for regional (urban) planning. Its contents include finding out the main fault structures, the distribution of main rock and soil layers, the weathering of bedrock, the distribution of bearing layers of building foundation, buried depth, thickness and seismic zoning.

2) Investigation of engineering geological environment. Provide basic engineering geological data for site selection and engineering design (including structure, rock distribution, geotechnical mechanical parameters, etc. ); Geological disasters that may occur around the project (including landslide, karst collapse, debris flow, underground engineering water inrush and collapse, etc.). ) are all predictive.

3) Advance forecast in the process of engineering construction or roadway excavation. For example, underground engineering construction such as subway, exploration of sand layer and soft soil layer in deep foundation pit excavation, exploration of soft interlayer in dam foundation excavation, exploration of structural cracks and unloading cracks in high slope, and bad geological forecast in front of tunnel face.

4) Detection of engineering construction quality and engineering status. Such as pile foundation detection, tunnel lining quality detection, concrete quality detection, anchor saturation detection, underground pipeline exploration, tunnel lining state evaluation, dam and reservoir leakage detection, etc.

5) Environmental geology. Including the investigation and prediction of urban groundwater pollution, land subsidence, seawater intrusion, radioactive pollution and other issues.

6) Water resources investigation. Geothermal and site heat source investigation.

7) Archaeological and cultural relics protection investigation.

Physical characteristic parameters (i.e. electrical characteristics, elasticity, magnetism, density, radioactivity, etc.). Geophysical methods are used to solve environmental and engineering geological problems, and their applicable scope and conditions can be roughly summarized as Table 0- 1.

Table 0- 1 Common physical parameters, applicable scope and applicable conditions of environmental and engineering geophysical prospecting methods

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