With the expansion and deepening of the exploration field, the encountered geological conditions become more and more complex, geophysical exploration will face a variety of problems. The main problems can be summarized in the following three aspects, the future development will also be carried out around overcoming these problems.
1. Improve the acquisition and processing level of weak geophysical signals
Geophysical exploration technology is based on the analysis of observed geophysical field data to achieve the purpose of detection. Therefore, data acquisition is the foundation of geophysical work. Historical development fully demonstrates that the improvement of data acquisition accuracy makes the application effect and application range of geophysical exploration expand continuously. For example, the accuracy of the gravimeter has been improved from (0.2-0.4) × 10-5m/s2 in the 1950s to (0.01-0.03) × 10-5m/s2 at present, which has greatly strengthened and broadened the capability and scope of application of gravity exploration. The progress of geophysical methods and theories requires the progress of data acquisition technology as a guarantee in order to be realized. All the countries in the world with developed geophysical technology are backed up by strong instrument research and manufacturing industries. In order to make China's geophysical work in the development of the world's advanced level, it is also necessary to strengthen the development of instruments.
Including: ① high-performance detection transducer development, such as new geophones and nuclear ray detectors, etc.; ② high-performance artificial sources of development, in geophysical methods, in addition to the observation of gravity and magnetic fields and other natural field methods, there are many of them with the help of artificial field excited by physical field, such as seismic exploration and most of the electrical exploration, in order to obtain more geological information, the field source often In order to obtain more geological information, the field source often plays a big role, therefore, a variety of field source research, will also be an important aspect of future development, such as high-performance seismic source, high-power power supply, high yield ray source, etc.; ③ high-performance data logging system development, with the advancement of the method, the increase in the amount of data, the requirement of recording system has higher performance, such as three-dimensional seismic and high-density electrical method, are required to increase the number of channels of the instrument. In order to improve the resolution of detection, the recording system is required to increase the bandwidth and dynamic range.
The purpose of geophysical data processing is to eliminate various interfering factors and highlight the required geological information. These interfering factors include influences related to measurement techniques, environmental influences, and influences of other geologic factors that are not the target of the study. Different geophysical methods are affected by various factors to different degrees, and thus the focus and methods of treatment are different. Take seismic exploration as an example, in order to improve the accuracy of the data, need to eliminate the influence of near-surface factors on the consistency; in order to effectively improve the resolution, need to improve the signal-to-noise ratio processing; in order to reduce the spatial false frequency in order to reduce the reflection inclination is relatively large, need to carry out the channel interpolation processing; in order to improve the accuracy of interpretation, need to carry out the improvement of the seismic data fidelity processing and so on.
2. Detection and description of non-uniform geologic body
Mineral resources with simple geometry, uniform distribution of physical properties, shallow depth of burial and easy to find will become less and less in the future, and the personnel of the physical exploration will be faced with the exploration object with uneven lithology, complex structure and tectonics, and large variations of physical properties both vertically and horizontally, as well as deeper burial and complex geological conditions. In order to identify the spatially non-uniform changes in the object, it is necessary to obtain enough parameters that can characterize the internal structure and nature of the underground, so that it is possible to outline the complex characteristics of the object in a more detailed manner. By sufficient parameters is meant, firstly, the type of parameters and, secondly, the number of each parameter. In order to clearly show the spatial characteristics of the research object, the imaging research of various physical fields has made great progress in the last 20 years, including seismic wave imaging, electromagnetic wave imaging and bit field imaging.
Seismic wave imaging can be performed at the surface, between wells and between wells and ground. Geometric structure imaging can be performed when the velocity is known, or physical structure imaging when the geometry is known. Seismic wave imaging has achieved some practical results in oil and gas exploration, of which prominent examples are the use of pre-stack depth offsets to clearly obtain the insides of ancient submarine mountains (Yang Changchun et al., 1996), but at present, the actual observation of seismic exploration is still mainly the vertical component of longitudinal waves, and the observation and application of multi-wave and multi-component research is just the beginning. In addition, the actual subsurface medium has not only longitudinal and transverse inhomogeneity, but also longitudinal and transverse anisotropy. Only by fully utilizing the multiple information of seismic waves can we have a more accurate understanding of the lithological changes, the development of fissures and the nature of fluids in the pores. Well-oriented seismic wave stratigraphic imaging has a higher resolution than surface seismic, and with the development of downhole equipment, it will become an important tool for developing seismic. Single-well seismic wave imaging that maintains the advantages of downhole seismic waves not interfered with by the surface layer, while not subject to the limitations of the need for two wells, has the potential for greater development. Ultrasonic well wall imaging is another important application of imaging technology in oilfield exploration, it can be divided into fracture development layer section, so as to effectively circle the fracture reservoir, at present, its resolution is still relatively low, quantitative interpretation technology needs to be developed.
Electromagnetic wave imaging includes low-frequency electromagnetic induction method and geomagnetic bathymetry, as well as high-frequency ground-penetrating radar imaging. Electromagnetic wave imaging can also be carried out on the ground, downhole, between wells or between wells and ground. Compared with seismic wave imaging, the method theory and technology of electromagnetic wave imaging are still in the initial stage of development, and many places follow the method technology of seismic wave imaging. However, because the equation describing the electromagnetic wave propagation process contains a diffusion term and its propagation constant is complex, the seismic wave imaging methods and techniques used to deal with electromagnetic wave imaging problems often do not get the desired results. At present, the application of low-frequency electromagnetic wave imaging is still in the embryonic stage (He Jishan 1997), therefore, the further development of electromagnetic wave imaging must explore new ways according to its own characteristics.
Since the high-frequency electromagnetic wave equation can be simplified to be similar to the fluctuation equation of elastic wave, the data processing and interpretation of ground-penetrating radar mostly adopts the method technique of reflection seismic, and the main modification lies in the scale calibration and parameter selection. High-frequency electromagnetic wave imaging across holes, when the distance between wells is not large, has achieved some successful examples in the direction of detecting highly conductive metal ore bodies and cavities. In order to improve the resolution of the high-frequency electromagnetic wave method for geometrical structures, the development of processing techniques for their dynamical characteristics is inevitable (Wang Miaoyue et al., 1998).
With the improvement of data acquisition technology, the DC resistivity method imaging method has also made some progress in recent years. Theoretically, DC resistivity method imaging is different from seismic and electromagnetic wave imaging methods in that the DC electric field is described by the Laplace equation. Due to the DC resistivity method observation equipment and field operation method is simple, detection depth is larger, so in the oil and gas exploration, metal ore exploration and engineering exploration in the application of broader prospects.
Geophysical detection of complex objects, is driven by the rapid development of computer technology to realize. Imaging technology is characterized by a large number of unknowns, a large amount of observation data, only observation information on each unknown covered a sufficient number of times, in order to make the solution of the unknown is more reliable. Similarly, the need to visualize the results of geophysical exploration has driven the advancement of computer technology, and computers will play a major role in the calculation of geophysical data in the future.
3. Comprehensive use of a variety of information to reduce the geophysical inverse problem of multiple solutions
Geophysical exploration is through the surface, air or downhole local geophysical field observation results, to analyze and deduce the nature of the underground can not be directly observed part of the nature of the material and morphology. Because of changes in the morphology and nature of the material on the geophysical field of the effect of the phenomenon of equivalence, so that the answer to the anti-question is not unique. The problem is further complicated when the effects of factors such as observation errors and disturbances are taken into account, as well as the unrefined nature of the mathematical expressions and computational methods used to describe the physical field. In a sense, geophysical exploration technology has moved forward around the goal of reducing the effects of multiplicity of solutions and giving more reliable geological answers. It will continue to move in this direction in the future.
The more complex the geophysical object, the more variables characterize its nature, structure, and configuration. In addition, different geologic objects may have some of the same physical properties. Therefore, in order to accurately describe a complex exploration object, or to distinguish between different research objects, should be comprehensive use of a variety of information, which has become the majority of researchers *** knowledge. For example, in oil and gas exploration, in addition to seismic, well-logging data synthesis, the integrated use of other exploration data, such as heavy magnetic exploration and electro-magnetic exploration data, in dealing with complex geological conditions, is also very important. With the progress of the integrated application of multiple information, the research idea of oil and gas exploration is also changing. The development of oil storage geophysics is a good illustration (Liu Guangding et al., 1998). It can be expected that with the continuous emergence of complex exploration objects, it will promote the further development of the integrated information search method. At the same time, the following aspects of research will be pushed forward.
1)Exploration of new methods and new parameters: geophysical exploration theory and method, driven by objective needs, is always in the continuous improvement of existing methods and exploration of new methods in two simultaneous forward. The application of new physical parameters will reduce the impact of multi-resolution, for example, when seismic waves are utilized, through the comprehensive utilization of longitudinal and transverse waves, greatly reducing the uncertainty of lithology judgment. The study of multiple waves and multiple components in seismic exploration, the study of geoelectrochemical methods and electromagnetic missiles in electrical exploration, and the study of seismic-electric and seismic-magnetic effects are all part of the effort to explore new methods and parameters. When the geophysical data do not contain enough geological information, only relying on data processing will not achieve the purpose, and new physical parameters must be added to supplement and enrich the geological information carried in the geophysical data, and then only through appropriate data processing methods can reliable geological conclusions be obtained.
2) Combination of "direct" and "indirect" searching (Sun Wenke, 1991; Zhao Wenjin, 1991): "Direct" searching is based on the geophysical data generated by an ore body or a group of ore bodies. The "direct" search is based on the geophysical field anomalies generated by the ore body or group of ore bodies to directly point out the attributes, specific location or other relevant information of the ore body or group of ore bodies. "Indirect" prospecting is to point out the possible distribution area of the deposit according to the direct ore-controlling factors of the deposit and the anomalies caused by the surrounding rocks close to the deposit. In order to correctly determine whether the task of physical exploration is "direct" or "indirect" mineral exploration, it is necessary to correctly understand the geological and geophysical characteristics of the exploration object, and to establish a geological-geophysical model of the target object. The purpose of geophysical exploration is to make a fine portrayal of the geological units, so the model is firstly based on the geological model. The modeling will result in the best combination of exploration work procedures and methods, i.e., the exploration work model, as well as markers for identifying targets, i.e., guidelines for predicting targets (Sun Wenke, 1988, 1991). A predictive guideline is a combination or system of valid sign information that can indicate or circle the presence of a mineral resource object. In this system, if both "direct" and "indirect" information is included, the effect of non-uniqueness of the solution will be greatly reduced. Through the study of deposit genesis model, people can have a clearer understanding of the genesis of different types of deposits and the conditions of deposits under different geological backgrounds of mineralization. Therefore, with the help of the ore deposit genesis model, people can get a clear idea of the search for minerals and the direction of the search for minerals. Geophysical workers in the deposit genesis model on the basis of the combination of geophysical field characterization, and gradually formed a more complete comprehensive mineral search model to guide the exploration work and as the basis for data interpretation. According to the idea of "modeling", there are many successful examples of mineral searching at home and abroad (He Jishan, 1997; Zhao Wenjin, 1991). However, the model can only represent the sum of people's understanding of the characteristics and genesis of mineral deposits at that time. Changes in the geological situation are very complex, and it is very difficult to encounter exactly the same situation. Therefore, it is necessary to pay attention to the model for finding minerals, and at the same time, it is also necessary to consider whether there will be new types of deposits or new mineral resources that are not included in the generalized model for finding minerals. Especially in a new area do not stick to a certain model.
3)Improvement of forward and reverse methods: Geological phenomena are very complex, and the mathematical representation of their physical field characteristics is not accurate enough, which is often the cause of inaccurate forward and reverse. For example, a nonlinear problem, often due to inappropriate treatment with linear approximation, do not get good results. Therefore, geophysicists should constantly absorb the latest results of mathematics and other related disciplines to improve the geophysical forward and inverse methods in order to achieve reliable geological results.
4)Multi-parameter joint inversion: joint inversion of the observation results of more than two physical fields of the same research object, or the results obtained by more than two different observation modes of the same physical parameter is one of the effective ways to minimize the effect of non-uniqueness of the solution (Wang Jiaying, 1997).
5) Integrated data management: In order to effectively realize the integrated application of multiple information, the integrated management of data is one of the key factors. The diversity of geophysical and geological data types and the increasing volume of data make the task of data management more complex. In order to be able to effectively store and manage a large amount of exploration data, the concept of data warehousing is proposed in order to create conditions for multiple data integration.
Summary
Through a brief introduction to the classification, essence, characteristics of physical exploration methods and the role of geophysical exploration in resource exploration, geophysical exploration is faced with the tasks, problems and development trends, to motivate students to learn enthusiasm, confidence, and strive to master the physical exploration technology.
Review reflection questions
1. What is geophysical exploration?
2. geophysical exploration facing the task?
3. geophysical exploration in resource exploration?