Application of transient electromagnetic method in geological survey of water area

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Application of transient electromagnetic method in geological survey of water area

 Abstract: In the engineering geological survey under water conditions, if it is necessary to find out the undulating shape of bedrock, the direction and development scale of hidden faults, etc., the general geophysical prospecting method is difficult to achieve ideal results. Taking the Yangtze River crossing project of the Jiangjin section of the natural gas pipeline network as an example, the transient electromagnetic method was used to conduct surveys under water conditions, and the depth of the river water, the thickness of the overburden layer and the undulating shape of the bedrock were detected. After drilling verification, the data consistency is relatively good. Good; it shows that the transient electromagnetic method has a good application prospect in water area survey.

Key words: transient electromagnetic method; water area; crossing project; apparent resistivity of the whole area; engineering geological survey



In recent years, natural gas has become an indispensable resource in people's lives. Engineering geological surveys of natural gas pipeline networks can be seen everywhere, but the work is more difficult in rivers and other waters. Generally, the geophysical prospecting methods used on water include high-density resistivity method and water seismic reflection wave, refraction wave method, etc. The former is feasible on small-scale rivers and canals, but it greatly increases the difficulty of survey layout on rivers, and the data is also very difficult. It is difficult to achieve the ideal effect; the latter also has the problem of actual layout, and because the wave impedance of water is much smaller than that of bedrock, it has strong multi-wave reflection between water areas, which increases the difficulty of data processing. Reduced resolution. In the Yangtze River crossing project of the Jiangjin section of the natural gas pipeline network, the transient electromagnetic method (TEM) was used. This method adopts the non-grounded coil detection method, which solves the problem of measuring line layout well. Although water has certain obstacles to the transmission of electromagnetic signals, the important data information is still obtained by using the data analysis of the apparent resistivity of the whole area. Retained and achieved better results.

1 Working principle of transient electromagnetic method

The transient electromagnetic method is also called the time-domain electromagnetic method. It uses an ungrounded return line or a grounded line source to emit a pulsed magnetic field underground, and uses a coil or a grounding electrode to observe the secondary eddy current field during the intermittent period of the first pulsed magnetic field.

When the exploration target layer is covered with a low-resistance layer, the low-resistance layer will have a shielding effect on the transient electromagnetic field, especially when the surface is a low-resistance layer, it will have a great impact on the detection depth of the transient electromagnetic field, while water The resistivity is low, and the water surface is equivalent to a low-resistance layer.

Date received: 2015-04-15

 

When , the detection time required for transient electromagnetic exploration is prolonged, resulting in a decrease in the signal-to-noise ratio of late data. Therefore, during water exploration, attention should be paid to the influence of low-resistivity layers on transient electromagnetic exploration.

In order to eliminate this effect, before the survey, the parameters must be tested to obtain better data.

2 Overview of the work area

The purpose of a natural gas outer ring pipeline network crossing the Yangtze River geological survey project is to find out the water depth elevation, overburden thickness, bedrock undulation shape and hidden fault development within the site area.

The site is located on the east wing of the Beibei syncline in Chongqing, and the rock formations are monoclinically produced, and the bedrock is exposed on both sides of the river. The strata in the field are mainly composed of Quaternary Holocene residual slope silty clay, alluvial and diluvial fine sandy soil, pebble gravel soil, and Shaximiao Formation (J2s) mudstone and sandstone of Jurassic Middle System.

Dielectric and resistivity of each layer in the work area: river water, <60 Ω•m; pebble layer, 60-200 Ω•m; bedrock, 200-400 Ω•m.

A total of 1 object detection line was arranged for this survey, with a total length of 610 m and a point spacing of 5 m. W1' and W3-W3' are on both banks, and W2-W2' are on the river.

3 Field data collection and data processing

The MSD-1 transient electromagnetic instrument produced by Changsha Baiyun Instrument Factory was used. Experiments were carried out on the river bank and the near river first to determine the parameters of the instrument, the transmitting and receiving wire frame was 5m×5m, the transmitting coil had 2 turns, and the receiving coil had 4 turns;

The electric current is 1-20 A; the transmission frequency is 25 Hz, and the number of superimpositions is 1024 times. Due to the wide river surface, the receiving coil and the transmitting coil are overlapped to work, which not only facilitates the work, but also makes it have the best coupling with the geological body to be detected, improves the detection ability, and is affected by the side geological body. The impact is also minimal. When surveying in the water, the wire frame is tied to the stern of the boat on the bamboo raft, and the hull moves slowly. At the collection point of the data record, the point number is recorded by GPS at the same time to ensure the validity of the collected data.

After the measurement is completed, it is necessary to check and accept the original data, eliminate the abnormal points of the data, draw the section diagram of the apparent resistivity contour line, analyze the electrical changes and trends of the site, and conduct comparison, comprehensive analysis and judgment based on the site engineering geological survey data , leading to the interpretation of the results. Multiple sets of different inversion coefficients were used in the inversion calculation. Firstly, the electrical anomaly area was qualitatively analyzed based on the high-smoothness inversion section, and then qualitatively explained based on the low-smoothness inversion section in combination with relevant geological results.

4 Data Interpretation

Figure 2 shows the transient electromagnetic measurement profile of the three sections of the survey line. The total length of the W1-W1' measuring line is 120 m. The upper and lower layers in Figure 2a

 

The resistivity has an obvious boundary, the depth is 16.8 m, it is inferred to be the bedrock surface, the depth within the range of the survey line is 6.5-16.8 m, the resistivity is low, it is inferred to be the overburden and pebble layer; the depth of 16 .8 m, the resistivity is high, and it is speculated that the measuring line section is interbedded with sand and mudstone.

The total length of the W3-W3' measuring line is 30 m. Figure 2b shows that the resistivity of the surface layer is low, and the depth of the survey line ranges from 1 to 3 m, which is presumed to be a pebble overburden layer. From the resistivity contour profile, it is inferred that the thickness below 3 m is sand-shale interbed. On the three survey lines, although they are not layered, the continuity of the data here is good. Combined with the analysis of geological data, it is speculated that there should be no fault development in this place. Fig. 3 shows the stratigraphic section according to the receipt of geophysical interpretation results.

The total length of the W2-W2' measuring line is 460 m. The measuring line of this section is on the river surface, and the resistivity interface is also obvious in Fig. 2c, but there is a transition layer in between. According to the analysis of the survey line data, the water depth within the survey line range is 4.5-10.5 m; the resistivity is higher than before, and it is speculated that the depth of the covering layer is about 6-7.5 m. Pebble layer and silt; from the section in the figure, it is speculated that the underlying bedrock is because it is interbedded with sand and mudstone.

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