3.3.5.1 Method introduction
Nuclear magnetic resonance technology is the cutting-edge technology in the world, and its application in groundwater exploration research has only a history of more than 20 years. In 1978, the former Soviet Union developed the first nuclear magnetic resonance tomography water finder. In 1994, France purchased the Russian water finder patent and began to develop a ground-based nuclear magnetic induction system (NUMIS). In 1996, the French IRIS company produced 6 units. Improved NUMIS system. In 1992, the China University of Geosciences NMR scientific research group conducted domestic and foreign research on this method. From 1995 to 1996, China University of Geosciences and the Aerospace Remoteness Center completed the "Preliminary Research on the Effects of NMR Water Finding Methods" project. In 1997 In 2008, China University of Geosciences introduced my country's first NUMIS system, which filled the gap in using geophysical methods to directly find water, and made our country one of the world's leading companies in using high-tech methods to directly find water. It has been deployed in Hubei, Henan, Guangxi, and Hunan. Experiments in other provinces and regions have achieved good results. However, the current exploration depth of nuclear magnetic resonance technology is shallow, and the reliable depth is less than 100m. Moreover, NMR sounding is a volume exploration, that is, a comprehensive reflection of the aquifer within the coil range, coupled with the influence of factors such as electromagnetic noise interference and local magnetic bodies. , the application effect is not obvious in some areas. At present, domestic data processing and inversion interpretation programs are mainly based on one-dimensional single-point interpretation. However, Germany has developed two-dimensional data processing software in terms of nuclear magnetic resonance technology to find water. It is in a leading position in the world and has far higher processing accuracy. in one-dimensional data processing software. And new technologies for field observation are being developed and researched. Multiple coils are arranged at one time, one coil emits alternating current, and multiple coils receive signals to achieve the purpose of improving efficiency and effect [5].
3.3.5.1.1 Basic Principle
It is the application of nuclear magnetic induction system to realize the detection of groundwater resources. It is a geophysical method to directly find water. Hydrogen nuclei in water have nucleon paramagnetism and a non-zero magnetic moment. They are the nuclei with the highest abundance and the largest magnetic spin ratio among the nucleon paramagnetic substances in the formation. Under the influence of a stable magnetic field, hydrogen nuclei precess around the direction of the geomagnetic field like a gyroscope, and their precession frequency (Larmor frequency) is related to the strength of the geomagnetic field and the magnetic spin ratio of the atomic nucleus. When an alternating current with a frequency of Larmor frequency is supplied to the coil (transmitting coil) laid on the ground, the alternating current in the ground forms an alternating magnetic field. Under the excitation of the magnetic field, the hydrogen nuclei in the groundwater form a macroscopic magnetic field. Moment, this macroscopic magnetic moment produces precession motion in the geomagnetic field, and its precession frequency is unique to hydrogen nuclei. When the current pulse is cut off, the same coil (receiving coil) is used to pick up the NMR signals generated by different excitation pulse moments. The signal strength or attenuation speed is directly related to the number of protons in the water, that is, the amplitude of the NMR signal is related to the amount of protons in the detected space. Directly proportional to free water content. The NMR water finder uses the difference in relaxation characteristics of hydrogen nuclei (protons) in water to observe and study the changing patterns of NMR signals generated by protons in groundwater, and then detect whether there is water. That is to say, within the scope of nuclear magnetic resonance sounding detection, when the signal-to-noise ratio is appropriate, if there is free water in the formation, there will be an NMR signal response. The more water (hydrogen nuclei) there is in the formation, the NMR signal will The stronger the signal, otherwise the signal will be weak or unresponsive. From the amplitude and attenuation time constant of the signal, a special inversion program can be used to obtain the changes in hydrogeological parameters with depth after quantitative interpretation [6].
3.3.5.1.2 Scope of application and applicable conditions
It can solve a large number of problems related to hydrogeology and water environment. It is mainly used to determine the rock structure and distribution of each aquifer within the exploration depth range of this method; to quantitatively evaluate the thickness, burial depth, and water content of aquifers; to evaluate the horizontal and vertical distribution between different aquifers; to determine well locations and judge the properties of filling materials. wait.
Since the NMR signal amplitude is very weak, it is susceptible to interference from electromagnetic noise and humanistic noise. At the same time, local magnetic bodies existing in and near the measurement area will also interfere with the NMR signal. Therefore, the work area should try to avoid power lines, Motors, electric locomotives and igneous rock distribution areas; the detection target burial depth should be less than 100m.
3.3.5.1.3 Work arrangement principles and observation methods
Correctly select the excitation frequency (Larmor frequency): the geomagnetic field measurement error is required to be less than 10 nT, and pay attention to the vertical direction of the geomagnetic field. According to the depth and water content of the aquifer to be explored in the work area and the strength and direction of the electromagnetic interference in the work area, the coil shape should be optimized and the coil should be laid scientifically. Usually, a square with a side length of 75m and a circular antenna with a diameter of 100m are used. If the environmental noise is greater than 1500 nV, choose an ∞-shaped coil that can reduce the noise level; selection of acquisition parameters: measurement range, recording length, pulse duration, pulse The number of moments, the number of superpositions, and the measurement range settings in the entire area should be unified. Generally, 4 times the average environmental noise value is taken; the observation parameters include the initial amplitude E0, the initial phase φ0, and the attenuation (relaxation) time.
The current single-coil observation method is used, that is, the same coil is used for transmitting and receiving, and the NMR signal is received through a switch. It is a purely abnormal observation and is less affected by terrain and geological factors.
3.3.5.1.4 Data compilation and interpretation of results
Since the NMR signal is weak and easily affected by various factors, in order to improve the reliability of interpretation, zero-time analysis of the measured data is required. Extension, transformation into standard observation values, and noise filtering preprocessing. The processed and qualified data is subjected to various inversion processes, and various result maps are compiled: aquifer parameter (water content, decay time) changes with depth charts and tables; NMR sounding section diagrams; comprehensive interpretation results diagrams, etc. Convert the observed geophysical data into hydrogeological parameters, obtain the depth, thickness, water content and average porosity of each underground aquifer, delineate the water prospecting area or provide water well locations, or use it to distinguish other geophysical water prospecting methods abnormal properties.
3.3.5.2 Test situation
In order to understand the vertical and horizontal zoning characteristics of karst development and its water richness, an area with favorable electrical sounding results was selected in the experimental area to In single-point form, 15 NMR points are arranged in four areas. Wan Mu Orchard has 2 points, Dayi Village has 3 points, Sanjia Village has 5 points, and Daxingbao has 5 points. As a result of the work, 10 water-rich and favorable target areas were circled. Verified by four boreholes, the aquifer deduced by magnetic resonance (NMR) within 100m is basically consistent with the actual situation.
This work uses the nuclear magnetic resonance system (NUMIS) of the French IRIS company, and the reference frequency is 1985 Hz; the Sanjiacun measuring point and Daxingbao No. 1 point use an ∞ shape with a diameter of 50m due to large interference. Coil, detection depth 60m; the other 9 measuring points use a square frame with a side length of 75m, detection depth 100m; the measurement range is 4 times the average environmental noise; the recording length is 240ms; the pulse duration is 40ms; the number of pulses is 10; The number of stacks is 80 to 140 times.
3.3.5.3 Main results
3.3.5.3.1 million acres of orchards
Second- or third-layer aquifers were found at both sites, No. 1 There are three main aquifers underneath, 15~25m, 25~40m, and 64~100m. The water content and decay time are 2.2%, 219.7ms; 1.1%, 639.6ms; 5.9%, 157.4ms respectively. The deep water content is the largest and the average porosity is the smallest. There are two main aquifers under point 2, 25-40m and 40-64m, with decay times of 77.6-148ms, indicating that the average porosity is small and the water content is 3.9% and 0.7% respectively. The first layer has the largest moisture content. When the two points are 60m apart, there is a big difference in the NMR sounding results, which also illustrates the complexity of the lateral distribution of karst in the area.
The construction drilling at Point 1 has verified that it enters the dolomite section below 5m, and the main aquifer is 83 to 200m. It is dominated by ant food-like dissolved pores, with medium water richness, and is consistent with nuclear magnetic *** It corresponds to the third aquifer, but the inferred depth is 20m shallower than the actual depth. The drill core is broken into a sandy shape, and NMR reflects the characteristics of small porosity and high water content.
3.3.5.3.2 Dayi Village
2 to 3 aquifers were found at all three sites (Figure 3-14). There are three main aquifers at site 1, 10 to 3 layers. 16m, 25~40m, 64~100m, water content is 1.4%, 1.6%, 2.3%, decay time is 54.2ms, 69.1ms, 77.5ms.
The decay times of the three layers are all short, indicating that the average porosity is small, but the third layer has the highest water content. There are two main aquifers at Point 2, 40-64m and 64-100m, both with water contents of 1.6% and decay times of 455.4ms and 730.0ms. The water content of both layers is not large, but the porosity is large. There are two aquifers under point 3, 13-22m and 60-100m, with water contents of 2.5% and 6.1%, and decay times of 76.7ms and 147.3ms. The water volume and porosity of the second layer are larger than those of the first layer. Points 1 and 2 are 30m apart, and the NMR sounding results are also different, which also illustrates the complexity of the karst lateral distribution.
Figure 3-14 Comparison of NMR sounding interpretation results and borehole data in Dayi Village, Xiaojiang River Basin, Luxi
Verified by drilling at point 1, the aquifers are 18.3 to 31m and 31 ~57m, with medium water richness. These two layers correspond to the second aquifer layer of NMR, but the actual thickness of the aquifer is greater than the thickness inferred by NMR. The 57-120m aquifer has weak to medium water richness, corresponding to the third aquifer of NMR. The 120-160m aquifer has exceeded the NMR detection depth. Overall, the water content of each aquifer is not large, which is consistent with the actual drilling results.
3.3.5.3.3 Sanjia Village
The five points are composed of multiple aquifers, among which there are two main aquifers, 20~40m and 40~60m. The first The water content of the layer is less than 2.3%, and the decay time varies greatly, ranging from 148 to 864ms; the water content of the second layer is 1.4% to 4.8%, generally greater than 2%, and the decay time is 400 to 750ms, indicating that the deeper the water content, the greater the water content. The more broken the rock, the greater its porosity. Due to the large interference in this area, the ∞ coil method is used, and the exploration depth is shallow, up to 60m. The NMR sounding results show that points 4, 5, 3, and 2 are favorable areas for finding water. Based on a comprehensive analysis of hydrogeological conditions, point 2 was selected to arrange drilling. According to the drilling results, the dolomite section below 8.9m has developed cracks, weak to medium water richness, and average water content, which is basically consistent with the conclusion that the water content is not large (2.4%) from the sounding results of Point 2.
3.3.5.3.4 Daxingbao
Point 1 is located next to Yanye Station. Two 180m deep exploration wells have been constructed at this site. The rock mass is relatively complete and the water volume is extremely large. Small, not yet a well. Using the ∞ coil method, the exploration depth is 60m, and an aquifer is displayed within 40 to 60m (Figure 3-15). The water content is 4.6%, the decay time is 767ms, and the porosity of the aquifer is large. The NMR sounding results are consistent with the exploration and production well results. Inconsistent. Due to the large lateral changes in karst, it is not completely certain that there is no aquifer within the line frame of Point 1.
Figure 3-15 Comparison of NMR sounding interpretation results and borehole data at Daxingbao in the Xiaojiang River Basin in Luxi
All other points have more than two aquifers (Figure 3- 15), taking points 5, 3, and 4 as favorable water-finding areas, the verification hole is selected at point 5. Point 5 has three main aquifers, 16-25m, 40-64m, and 64-100m, with water contents of 0.7%, 1.3%, and 5.5%, and decay times of 116ms, 106ms, and 178ms. The water content at 64 to 100m is relatively large. The drilling results show that below 20.55m, the dolomite section is entered. The rock mass is broken, joints and fissures are developed, and the water content is strong, which is consistent with the NMR sounding results.
3.3.5.4 Conclusion
In summary, the NMR detection results of the Luxi Karst Basin better reflect the layered structural characteristics of the karst aquifer, and the main aquifers correspond to The attenuation (relaxation) time of the NMR signal is generally in the range of 100 to 200 ms, and the water content is 1.4% to 6%. It has been verified by 4 boreholes that the aquifer inferred by NMR within 100m is basically consistent, but the aquifer inferred by NMR is basically consistent. The depth is slightly shallow, with a difference of about 5 to 20m. The results of a small number of NMR sounding points are different from the actual drilling results.
This is the first time that the NMR method has been applied in the search for water in karst areas of Yunnan, and only a few test points have been conducted. Due to the generally uneven development of karst, karst water in many areas is mainly concentrated in underground caves and pipes. The burial and distribution locations of these caves and pipes are random and change complexly. It is difficult to accurately identify karst by various methods currently used. The location of groundwater is the biggest problem facing the development of karst water resources.
The current exploration depth of nuclear magnetic resonance technology (NMR) is shallow, and the reliable depth is less than 100m. Moreover, NMR sounding is a volume exploration. The single-point interpretation result is a comprehensive reflection within the frame of the transmitting coil (70m×70m), which is very important for determining The specific well location has a great influence, especially the large lateral changes in the karst aquifer, resulting in differences between the interpretation results and the actual situation. Therefore, for the anomalies delineated by NMR, other methods must be used to narrow the target area, such as using electrical sounding method to carry out encrypted measurements within the NMR abnormal range so that they can be positioned more accurately.