In addition to the above-mentioned field identification signs of faults, in recent years, with the development of geophysical technology and its application in geoscience, fault identification methods have made new developments, mainly including the following methods.
(1) Geophysical methods
The structural geology methods mentioned above are mainly used for fault structures with complete outcrops on the surface, but they are powerless against hidden faults, let alone hidden faults. Understand the propagation of deep faults. Geophysical methods can reveal the location and geometry of large deep faults, and in some cases, can also provide crustal and other information related to the occurrence of tectonic faults. With the increasing development of structural geology research and the gradual expansion of energy and mineral demand, the detection and research of deep geology has become more and more important, and the role of geophysical methods has become more obvious. Geophysical methods mainly include seismic, gravity, magnetic and electrical methods.
1. Seismic method
Seismic method is the most effective method to reveal the crustal structure and deep structures (including faults), and it still shows great potential so far.
As a first-order approximation, the earth as a whole is elastic, and seismic waves propagating from natural earthquake sources or artificial blasting sources are also elastic. When seismic waves pass from one layer to another, they are refracted if the wave speeds of the two layers are different; or when they arrive from one layer to another, they are reflected at the interface between the two layers. Groups of wave speeds with different wave speeds that cause refraction can be used to identify different layers; interfaces that cause reflection can be used to identify interfaces of different layers. When such a wave velocity group or reflective interface is intercepted by a fault, the resulting discontinuity will be reflected in the seismic travel time diagram or seismic profile. Therefore, the principles and methods of seismic refraction and reflection can be used to identify structural faults, especially large faults. Figure 11-27 is the seismic section and interpretation section of a shovel-type normal fault and its hanging wall normal fault.
Figure 11-27 A shovel normal fault seismic profile and interpretation profile (according to Wernicket and Burchfiel, 1982)
Figure 11-28 Gravity profiles showing different anomaly types (Part 2) and the corresponding fault cross section (top) (according to Hatcher, 1990)
2. Gravity method
The gravity method belongs to the potential field method, and the gravity method mainly uses gravity anomalies to explain geology structure. This is because gravity anomalies are caused by differences in the density of rocks distributed within the Earth, and this difference is often related to geological structures. If there were no differences in the density of rocks within the Earth, the premise for using gravity anomalies to explain geological structures would be lost. The difference between the measured value of gravity and the reference value (estimated value) is defined as a gravity anomaly. The measurement of this difference in gravity field is measured in milligal (mGal). Figure 11-28 shows the gravity anomaly map of the fault.
The main instrument for measuring gravity anomalies is the gravimeter. Measurements can be made on land and sea, or on aircraft and satellites, but the latter are more difficult and less reliable.
Gravity anomalies caused by faults usually have the following characteristics:
1) Structural faults appear as asymmetric gravity anomalies on the gravity anomaly profile.
2) The faults appear as gravity anomalies with obvious linear distribution on the gravity anomaly plane map.
3) The fault appears as a steep gradient zone on the gravity anomaly map.
4) Faults appear on gravity anomaly maps as distortions of gravity anomaly zones, gradient zones or contours or sudden interruptions along their direction.
5) Faults appear as sharp dividing lines between gravity fields with different characteristics. These fields have obvious contrasts in the magnitude and type of gravity anomalies.
3. Magnetic method
The geomagnetic method also belongs to the potential field method. From the perspective of geological applications, geomagnetic field research focuses on magnetic anomalies and paleomagnetic aspects, while applications in fracture research focus more on magnetic anomalies.
The main basis for applying geomagnetic methods to fracture research is that geological structures control the magnetic changes of rocks distributed inside the earth, which are reflected in the distribution of magnetic anomalies. Magnetic anomalies refer to the difference between the measured value of the Earth's magnetic field and the reference value (or assumed average value), that is, the deviation of the measured value from the reference value. Since the geomagnetic field is not constant but changes significantly, there is no internationally recognized standard reference field, and an assumed average value is used as the reference value.
It is generally believed that rocks containing a high percentage of magnetically sensitive minerals (such as magnetite) produce positive anomalies, and rocks containing a low percentage of magnetically sensitive minerals produce negative anomalies. Magnetic field strength can be measured with magnetometers on land and sea, or from aircraft.
Similar to gravity anomalies, magnetic anomalies can also be used to infer deep structures including faults, but magnetic anomalies reflect differences in the magnetic properties of rocks rather than differences in rock density. The structural signs shown by magnetic anomalies are morphologically quite similar to the fracture signs shown by gravity anomalies. Their main characteristics are as follows:
1) Faults appear as asymmetric magnetic anomalies on the magnetic anomaly profile.
2) The magnetic anomaly plane map shows obvious linear distribution of magnetic anomaly bands.
3) The magnetic anomaly map shows a steep gradient zone.
4) Magnetic anomaly maps appear as distortions of magnetic anomaly bands, gradient bands or contours or sudden interruptions in their directions.
5) A sharp dividing line between magnetic fields with different characteristics. These fields show obvious differences in magnetic field strength, magnetization direction, etc.
4. Electrical method
Materials in the earth's crust have varying degrees of resistivity. Measuring the electrical conductivity and resistivity of rocks in the earth's crust can determine the distribution of materials in the earth's crust. Electrical measurement usually involves inserting electrodes into the ground to measure conductivity and resistivity, and recording the data at varying intervals. Electrical methods can be used to detect adjacent rock formations or ore bodies with different conductivities and resistivities, but they can only roughly determine the depth of the conductive layer. At the fault development site, the resistivity abnormal layer will generally be interrupted suddenly along its direction, so the electrical method can indirectly identify the fault.
(2) Remote sensing method
All surface objects on the earth have the ability to reflect electromagnetic radiation from the sun or emit electromagnetic radiation themselves, and this reflection or emission ability changes with the surface objects The types vary, and even the same type of features vary according to their position in the electromagnetic spectrum. In other words, each ground feature has its own reflection or emission spectrum characteristics. The remote sensing method uses airborne sensors to obtain the difference in electromagnetic radiation information emitted or reflected from the sun by the surface and nearby objects to identify ground objects. The electromagnetic spectrum band currently used by remote sensing methods ranges from about 0. 3 μm to 3 m, spans up to 106 orders of magnitude, and belongs to different spectrum bands such as ultraviolet (part), visible light, infrared and microwave. Radiated infrared has certain propagation capabilities in solid media, while other spectral bands generally have no propagation capabilities in solid media. Therefore, only radiative infrared remote sensing can have a certain so-called "see-through" ability to the overburden, while the other spectral bands are basically used to obtain information about features and landscapes near the surface. Remote sensing methods can identify various ground objects, such as vegetation, water, soil, rocks and artificial objects. The electromagnetic radiation information characteristics reflected and emitted by these objects vary widely, and almost all of them can be reflected in different remote sensing data to varying degrees. Therefore, remote sensing methods have become a unique tool for identifying various types of land objects, especially in agriculture, forestry, water resources, soil, geology, energy and mineral exploration. As far as geological applications are concerned, remote sensing methods can be used to identify lithology and geological structures, and are particularly effective in identifying fracture structures. The remote sensing method can also obtain information from a long distance and cover a wide area. It can get a glimpse of different land features, including the correlation between different geological bodies or structural elements, so that it can have a bird's-eye view of the overall appearance of these land features and discover the inner connection between them. Therefore, it has become an important tool for studying geological structures, especially fault structures. As shown in Figure 11-29, the fault structure clearly dislocates the marked strata.
Figure 11-29 Remote sensing image of a strike-slip fault
1. Commonly used remote sensing methods
Commonly used remote sensing methods use different sensors to obtain reflections of ground objects or Emitted electromagnetic radiation information. The sensor is carried by aircraft or satellites. There are two ways for the sensor to obtain information: photography and scanning. The former directly images (limited to 0. 3 ~ 1. 1 μm), while the latter converts the information obtained by scanning into analog signals and digital signals. Indirect imaging. Some commonly used remote sensing methods are introduced below.
(1) Panchromatic remote sensing
Generally, it is direct optical imaging remote sensing. The sensor is a camera (instrument), and the band range is mainly limited to visible light (0. 4 ~ 0. 7 μm) . The ability to resolve ground objects varies from sensor to sensor.
Black-and-white aerial photos and satellite photos obtained by full-color remote sensing are the basic data for topographic mapping, geological survey, and structural interpretation.
(2) Color and color infrared remote sensing
This is color camera remote sensing including visible light and reflected infrared (i.e. near infrared, 0. 7 ~ 3 μm) band, but The imaging segment is limited to within 0. 4 ~ 1. 1 μm. Conventional color photography is suitable for geological body development areas with significant natural color contrast. For example, light-toned granite rock bodies and dark-toned mafic rock bodies appear completely different on such remote sensing images and are easily distinguishable. The main difference between infrared color photography and conventional photography is that because the spectral reflectivity of vegetation suddenly increases in the infrared region of the camera, the infrared region of the infrared color film is located within the wavelength of the red image layer exposure, which makes the vegetation type in the infrared color film. Medium is easier to identify than in conventional color film and helps infer the nature of the underlying soil and rock.
(3) Thermal infrared remote sensing
The thermal infrared band is the mid- and far-infrared band (3 ~ 14 μm). Thermal infrared remote sensing uses the thermal sensor of an infrared scanner to obtain radiation information emitted by ground objects. On a conventional thermal infrared black and white image, the darker and lighter tones respectively represent the colder and warmer radiation temperatures, which is different from the conventional black and white image. On a regular black and white image, darker and lighter tones represent low and high reflectivity, respectively. As mentioned earlier, since thermal infrared radiation can propagate in solid media, the thermal radiation of materials at a certain depth below the layer can propagate from bottom to top to the surface. Therefore, thermal infrared sensors can detect this type of information and can identify objects with thermal properties. Different materials radiate contrast to obtain information about tectonic fractures. Because the thermal radiation contrast between water, rocks, and soil is relatively obvious, thermal infrared remote sensing is very suitable for water resources surveys. For geological applications, predawn thermal infrared images are far superior to images after sunrise, which are subject to terrain effects due to different solar illumination and obscuration.
(4) Multispectral remote sensing
It is generally indirect scanning imaging remote sensing, and the sensor is a multispectral scanner (MSS). The band range and resolution capabilities vary greatly with different sensors. Narrower bands include the near-infrared bands of visible light, such as Landsat and Multiband Scanner (MSS) images. Because multispectral remote sensing can simultaneously obtain ground object information through finely separated channels, it has a strong ability to identify ground objects, and it is convenient for multi-band optical or digital image processing. In addition, the selection of band range and channel separation is more flexible and the resolution is better. There is also potential for further improvement, and it is therefore the most commonly used remote sensing method.
(5) Microwave remote sensing
The range of microwave remote sensing is 0. 3 ~ 300 cm. Different ground objects have different abilities to radiate microwaves, and their radiation intensity, spectral distribution, and polarization direction depend on the physical temperature (in K) of the ground object and the properties of the radiating surface. Microwave remote sensing is divided into two categories. One is passive microwave remote sensing, which uses microwave radiometers to detect microwave information radiated by ground objects, including part of the radiation power from external sources reflected or scattered by the ground. The other type is active microwave remote sensing, which uses side-looking radar to emit electromagnetic radiation generated by it onto the surface of the object to be measured, and receives and records the electromagnetic radiation reflected back from the surface of the object and backscattered in the direction of the radar (called Radar echo), the intensity and nature of the echo provide a lot of useful information about the size, shape, electrical properties, etc. of the object.
2. Image signs of fractured linear bodies
The most significant and intuitive image signs of fractures are that they have line features and “grayscale edge” features. This is why breaks are generally always called image linear bodies or linear bodies. However, the types of ground objects or landscapes reflected by the image linear body are relatively wide and do not exclusively reflect fractures, although a considerable part of them are or may be fractures. Therefore, in order to distinguish linear bodies caused by fractures from non-fractured linear bodies, certain restrictive signs must be attached, and these additional signs can usually correspond to geological signs, landforms, and water system signs. In order to effectively identify structural faults on remote sensing images, the best way is to first establish the image stratigraphic units in the image area based on preliminary image geological interpretation, combined with necessary field geological verification, determine their image interpretation signs, and preliminary Understanding the image frame within the zone before interpreting the fractured image may help reduce false positives.
According to the above, the image signs for identifying fractures can be attributed to the following two aspects.
(1) General image marks
This type of image mark, or image linear mark, can be used as a preliminary interpretation mark and the basis for further fracture interpretation. This type of mark Yes:
1) Line or strip-like image features, which have obvious contrast in grayscale or structure between themselves and the areas on both sides. Such image linear bodies can be regarded as linear radiation. Anomalies or grayscale anomalies.
2) Line or strip-like image features, with obvious contrast in grayscale or structure between the areas on both sides. Such linear bodies appear as "grayscale edges".
3) The grid water system that extends in a straight line or has right-angled turns, or a linear ravine shown in the image can be regarded as a reflection of the linear body in the landform and water system.
4) The triangular cliffs are arranged in a linear shape, most of which are images of fractured landforms.
5) Lines or image features show a preferred orientation, which is a manifestation of the spatial characteristics of a linear body that may have tectonic origins.
6) The high-frequency component on the image is the attribute of the linear body in the spectrum domain.
In addition to possible faults, linear bodies interpreted based on these signs also include boundaries between lithological layers or stratigraphic units, stratigraphic unconformity lines, sign layers, large joints, and non-tectonic origins. Linear landforms and water system elements, etc., therefore require further interpretation based on the following additional restrictive signs.
(2) Constraint signs
This type of sign can be used to roughly determine the cause of fracture of a linear body. Such signs are:
1) Linear body In the strike direction, it is parallel to the boundary of the image stratigraphic unit. However, in the direction across its strike direction, the image stratigraphic unit appears asymmetrically and repeatedly, or is partially missing.
2) The linear body is parallel to the regional structural line in strike, but there are truncated fold turning ends.
3) The linear body is oblique or orthogonal to the regional structural line in strike, and the former is offset from the latter; or the fold core widths on both sides of the linear body are completely different, and the stratigraphic boundaries are offset.
4) Linear body offset annular body (the annular body may be an intrusive rock mass, volcanic structure or annular fault).
5) The ring-shaped bodies are arranged in a band, which is probably an image display of faults controlling intrusive bodies or volcanic structures.
6) Sudden interruption or dislocation of the linear arrangement of the annular body.
7) On the thermal infrared image, the grayscale abnormal points are arranged in a linear manner (the grayscale abnormal points change between bright and dark during the day and night, which may be the image display of spring water), which may be the fracture that controls the spring point. Image characteristics.
8) Different rivers turn simultaneously along a linear image, which is usually a display of active fractures.
9) Sudden interruption or dislocation of ancient river channels as shown by images.
It is easy to see from the above that these additional restrictive signs can generally correspond to the geological and geomorphological signs that determine the fault. Therefore, better interpretation results can be obtained by combining the fault interpretation of remote sensing images with comprehensive geological interpretation.