1. Handling the interpretation process
Data processing and interpretation are roughly carried out in the following five steps:
1) conventional processing: according to a series of conventional data processing techniques, various data and maps for interpretation are obtained for shallow sedimentary rocks.
2) Special treatment: depth profile inversion is carried out according to the deep structure and structural characteristics, and the inversion depth reaches 70km.
3) Qualitative interpretation: By analyzing the qualitative map, we can qualitatively understand the ups and downs of the electrical basement, the structural outline and the division of structural units.
4) Quantitative interpretation: On the basis of qualitative interpretation, the quantitative inversion results are analyzed, and the electrical layers are divided and calibrated.
5) Comprehensive geological interpretation: based on the corresponding relationship between the electrical layer and the stratum, with two-dimensional inversion as the main method and one-dimensional inversion as the auxiliary method, combined with geological, seismic, drilling and other data, the geophysical model obtained by quantitative interpretation is finally interpreted and inferred.
2. Handling the workload of interpretation
① The main survey lines in Qaidam Basin 10 MT were reprocessed and interpreted, and five tie lines were reinterpreted.
② Magnetotelluric data in Delingha area and CEMP data in Hongshan and Huaitoutala areas are interpreted according to Carboniferous data.
3. Mainly deal with interpretation technology
(1) static correction technology
The surface lithology of Qaidam basin is complex and changeable, and the shallow electrical properties are obviously uneven. These shallow electrical inhomogeneities will generate an additional electric field near the surface, which has nothing to do with frequency and affects the distribution of the earth current field. The superposition of the additional electric field and the normal electric field causes the abnormal change of the amplitude of the measured electric field, thus changing the apparent resistivity value and making the apparent resistivity curve move up and down along the resistivity axis in the logarithmic coordinate system. The influence of additional electric field on the apparent resistivity curves of TE and TM polarization directions is different due to the different geometric shapes of topographic relief or shallow electrical inhomogeneity. There are different combinations for the translation of resistivity curve: ① the translation of apparent resistivity curve in Te polarization direction; ② Apparent resistivity curve shifts to TM polarization direction; ③ The apparent resistivity curves of TE and TM polarization directions are shifted.
On the isoline profile of apparent resistivity, the static displacement shows that the isoline is steep, dense, high and low, and the change is disorderly, resulting in the phenomenon of "hanging noodles" (Figure 5- 1a). It can also be seen from the profile that the static displacement of the Quaternary coverage area of the basin is small, and the static displacement of the exposed area of the surrounding old strata is large. The results obtained by data inversion before static correction are seriously distorted and cannot be used for interpretation, so the data affected by static displacement must be corrected before data inversion.
Static correction of profile window adaptive spatial filtering: The research shows that with the increase of depth, the influence range of static displacement increases, so for profile spatial filtering, the filtering window should be increased accordingly, which is the idea of CEMP static correction, that is, the adaptive spatial filtering method with the window changing with skin depth. Small high frequency band window, small static correction, large frequency reduction window and large static correction. Therefore, the high frequency band has good fidelity and can reflect the electrical characteristics of exposed strata; The deep electrical layer is not too steep, but relatively flat. This is also the reason why this method was popularized as soon as it was put forward.
Fine-tuning static correction: after pre-processing, it is necessary to further use the visual human-computer interaction data processing system for fine-tuning. The apparent resistivity at the same frequency point is taken as a profile and compared with the known geological structure to determine whether the fluctuation of apparent resistivity along the profile corresponds to the known structure. If not, it means that MT treatment may be excessive or insufficient; Fine-tune according to geological conditions until it is satisfactory. The principle of fine-tuning correction is as follows: ① refer to the shape of phase profile (Figure 5-1b); ② The resistivity of adjacent points should change continuously and be corrected according to its changing trend; (3) When the shallow layer is one-dimensional, make the first branch of the bidirectional curve of the same measuring point coincide.
Fig. 5- 1 Comparison Diagram of Static Displacement Correction
The upward continuation method of magnetotelluric function: Its principle is to cover the upper part of the original observation surface with a layer of uniform electrical property, the top of which is flat and the bottom is consistent with the terrain, and then extend the data on the observation surface to the horizontal plane, so that the magnetotelluric transfer function on the original observation surface has changed, which is related to the transfer function on the original observation surface and the electrical and geometric parameters of the covering layer. After the transfer function is extended, it is equivalent to the observation point being far away from the local inhomogeneity. The response of local inhomogeneity is rapidly weakened due to dispersion and geometric attenuation, while the response of deep structure is almost unchanged due to the limited upwelling height. Although the topographic influence exists, it has been greatly weakened. The parameters of overlying strata are known and can be fixed as known parameters in two-dimensional inversion. This is a new method to eliminate the influence of terrain.
Static correction effect: after correction, the apparent resistivity changes obviously in the lateral direction, but it keeps the trend before correction (Figure 5- 1c), and it has a good correspondence with the phase, indicating that the static displacement has been well suppressed. Using this data inversion, the shape of underground electrical layer can be well reflected.
(2) Depth inversion technology
Two-dimensional continuous medium inversion is a two-dimensional inversion method with terrain correction. It takes the inversion result of one-dimensional continuous medium as the initial model, and iterates repeatedly to obtain the final interpretation profile, which eliminates or suppresses the influence of topography and static displacement on the data to the maximum extent and can be used as the basis for further geological interpretation and inference.
At present, most exploration areas are mountainous areas, with complex terrain, great changes in surface lithology and wide distribution of shallow electrical heterogeneity. This makes the electrical data inevitably affected by topography and static displacement. With these uncorrected data, the shape of the underground electrical layer will be seriously distorted, and it is difficult to get accurate results with it. Generally, the interference caused by terrain response and uneven surface electrical characteristics is confined to space. In this paper, the upward continuation of electromagnetic field transfer function is used to suppress these effects at the same time. Theoretically, after extending upward, the following effects can be achieved:
The upward delayed observation point is far away from the local inhomogeneity of the original surface. Because of the small spatial range of the anomaly, the electromagnetic field propagation decays according to the geometric law, and the corresponding static effect weakens rapidly, while the underground structure has a large spatial range and limited upward height, and its response is almost unchanged. Electromagnetic noise does not have the characteristics of regional anomaly in space, but is more similar to static effect. Locally distributed irregularly in space, the influence of upward extension will be rapidly weakened. After upward continuation, the response of terrain is equivalent to that of shallow structure. Because the parameters of caprock are known, they can be fixed as known parameters in two-dimensional inversion, and their influence can be automatically eliminated in the inversion process. After upward continuation, the sensitivity of observation data of electrical structure from shallow to deep tends to be balanced, which improves the inversion conditions and is beneficial to the simultaneous reconstruction of electrical structure from shallow to deep.
MT inversion can obtain deep electrical profile information, which lays a foundation for deep data interpretation and deep structural layer research.
(3) Optimize the layering technology
In the processing of electrical data, not only the traditional one-dimensional and two-dimensional inversion techniques are adopted, but also new methods and technologies such as electrical data optimization layering technology are adopted on the basis of two-dimensional continuous medium inversion results. Because the inversion process is to restore the possible geoelectric structure according to the measured data, the geoelectric profile is displayed in the form of resistivity changing with depth, which is equivalent to the velocity profile of earthquake, and the geological information such as stratigraphic interface and fault is not intuitive, so the extraction of this information is of great significance to the later interpretation. In this respect, information extraction technology has a good application prospect, so we discuss the extraction of geological structure information from the perspective of electrical data optimization layering technology. In practical work, the application of these new methods and technologies helps us to understand the structural morphology and determine the fault location to some extent.
Optimal layering technology: using the calculation method of interlayer deviation, the electrical inversion data volume is optimized and layered. Its layering goal is to minimize the difference of resistivity properties in each interval and maximize the difference of resistivity properties between intervals, which fully embodies the layering law of underground geoelectric model. By using this method, the position, fluctuation and occurrence characteristics of stratigraphic interface reflected by the processing results are relatively more intuitive, and the information such as fault position, structural style and structural characteristics is more abundant, which can provide useful information for effectively identifying geological structures and structural deformation, and has a good application prospect.
The optimized layering technology of electrical data (Figure 5-2) highlights the geological information without resistivity background, improves the vertical resolution, and the extracted information contains interference factors, so it must be combined with two-dimensional inversion of resistivity profile to make a correct interpretation, which has a good application prospect in theory. In fact, the optimized layering technology of electrical data can reflect the ups and downs and thickness changes of different electrical layers along the survey line, and can directly track the information of electrical interfaces and cracks, which has a good application prospect, but sometimes it may provide some other false information, so it must be used reasonably in combination with two-dimensional inversion resistivity profile in the specific application process.
Figure 5-2 Comparison Diagram of Information Extraction of MT04- 1 Survey Line in Delingha Area