A Method for Extracting Pure Secondary Field Signals Using Semi-Airborne Electromagnetic Method

Abstract

This application applies to the field of geological surveying technology and provides a method for extracting pure secondary field signals using a semi-airborne electromagnetic method. This method involves setting up two observation points at different distances from the source on the flight path (observation line) during the observation process, and comparing the signals observed by the two adjacent observation points to extract the secondary field signal. This method can better filter out the information generated by the primary electromagnetic field from the extracted field signal, improve the signal quality of the semi-airborne method, and lay the foundation for high-quality imaging interpretation.

Description

Technical Field

[0001] This application belongs to the field of geological exploration technology, and in particular relates to a method for extracting pure secondary field signals using a semi-airborne electromagnetic method. Background Technology

[0002] In semi-airborne electromagnetic methods, the transmitter is positioned on the ground, and the observation system is suspended below the aircraft, observing the magnetic field signals as the aircraft flies. The observed signals include a primary electromagnetic field signal from the primary electromagnetic field and a secondary electromagnetic field signal from the secondary electromagnetic field. The primary electromagnetic field is mainly related to the transmitter, while the secondary electromagnetic field is generated by eddy currents in the conductors within the measured area of ​​the strata based on the primary electromagnetic field, and contains information on the conductivity of different underground materials. Since only the secondary electromagnetic field contains information on underground conductivity, the primary field signal is generally removed from the observed signal, leaving only the secondary field signal, in order to interpret and infer the underground conductivity structure.

[0003] like Figure 1 As shown, the observation system mounted under the aircraft observes the secondary electromagnetic field signal. Because the primary electromagnetic field has relatively strong energy, when the receiving coil (the spherical coil in the middle) is working, the compensation coil (the outer ring coil) radiates electromagnetic waves to cancel out the primary electromagnetic field. Therefore, current methods consider the signal received by the receiving coil to be the secondary field signal.

[0004] However, since the primary and secondary electromagnetic fields occur without a clear time interval, they are essentially mixed together when detected. It is difficult to obtain a pure secondary field signal during signal extraction, and inversion interpretation based on the secondary field signal containing the primary field signal cannot accurately reveal the underground structure. Therefore, existing technologies have shortcomings. Summary of the Invention

[0005] The purpose of this application is to extract pure secondary field signals from signals received by semi-airborne electromagnetic methods, thereby improving the accuracy of underground structure interpretation during current inversion.

[0006] This application provides a method for extracting pure secondary field signals using a semi-airborne electromagnetic method, the method comprising the following steps:

[0007] S1. Set up a first observation point and a second observation point along the aircraft's flight path; the distance between the second observation point and the emission source is greater than the distance between the first observation point and the emission source;

[0008] S2. When the aircraft flies to the first observation point, an electromagnetic field observation is performed for one unit of time; when the aircraft flies to the second observation point, an electromagnetic field observation is performed for one unit of time.

[0009] S3. Normalize the observation signal measured at the first observation point to obtain the first total effect; normalize the observation signal measured at the second observation point to obtain the second total effect;

[0010] S4. Calculate the difference between the sum of the second action and the sum of the first action to obtain the pure secondary field signal.

[0011] This application employs a method of setting up two observation points at different distances from the source along the flight path (observation line) during the observation process, and comparing the signals observed at the two adjacent observation points to extract the secondary field signal. This allows for better filtering of the information generated by the primary electromagnetic field from the extracted field signal, improving the signal quality of the semi-airborne system and laying the foundation for high-quality imaging interpretation. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the cancellation of primary field signals using the semi-airborne electromagnetic method in the prior art;

[0013] Figure 2 This is a flowchart illustrating the implementation of the semi-airborne electromagnetic method for extracting pure secondary field signals provided in this application. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0015] The specific implementation of this application will be described in detail below with reference to specific embodiments:

[0016] Example:

[0017] Figure 2 The implementation flow of the semi-airborne electromagnetic method for extracting pure secondary field signals provided in Embodiment 1 of this application is illustrated. For ease of explanation, only the parts related to the embodiments of this application are shown, and are detailed below:

[0018] A method for extracting pure secondary field signals using a semi-airborne electromagnetic method, the method comprising the following steps:

[0019] S1. Set up a first observation point and a second observation point on the aircraft flight path (observation line); the distance between the second observation point and the emission source is greater than the distance between the first observation point and the emission source.

[0020] In practice, observations are continuously conducted as the aircraft flies along the flight path, meaning there are many observation points along a single flight path. This application selects two adjacent observation points as the first and second observation points for the convenience of describing the method described herein.

[0021] S2. When the aircraft flies over the first observation point, an electromagnetic field observation is performed for one unit of time. When the aircraft flies over the second observation point, an electromagnetic field observation is performed for one unit of time.

[0022] S3. Normalize the observation signal measured at the first observation point to obtain the first total effect; normalize the observation signal measured at the second observation point to obtain the second total effect.

[0023] In practice, normalization of the observed signal helps to suppress noise caused by the instrument's structure and circuitry, thereby improving the signal-to-noise ratio.

[0024] S4. Calculate the difference between the sum of the second action and the sum of the first action to obtain the pure secondary field signal.

[0025] In practice, by changing the measurement method, the secondary field signal calculated from the data is more accurate and purer than the method of using a compensation coil to cancel the primary electromagnetic field.

[0026] Furthermore, in step S2, the unit time is greater than the arrival time of the secondary electromagnetic field signal.

[0027] Specifically, the unit time defined in this application is a continuous period of time. Although this unit time is relatively short due to the high speed of electromagnetic wave propagation, it is still possible to ensure the reception of electromagnetic waves reflected back to the receiving antenna by the underground medium, i.e., the secondary electromagnetic field signal.

[0028] Furthermore, in step S3, the normalization method for the observation signal measured at the first observation point is as follows:

[0029]

[0030] The normalization method for the observation signal measured at the second observation point is as follows:

[0031]

[0032] Where M1 is the sum of the first effects; M2 is the sum of the second effects; r1 is the first observation point; r2 is the second observation point; t is the unit time; E1(r1,t) is the time-domain electromagnetic field signal observed by the receiving coil at the first observation point; E2(r2,t) is the time-domain electromagnetic field signal observed by the receiving coil at the second observation point.

[0033] Furthermore, the first sum of effects includes the effect of the primary electromagnetic field excited by the emission source and the effect of the secondary electromagnetic field excited by the strata between the emission source and the first observation point; expressed as: M1 = M s +Ms1 .

[0034] In practical implementation, since this application targets the semi-airborne electromagnetic method, its signal frequency is relatively high. Induced electromagnetic signals generated by the subsurface medium further away from the observation point and source attenuate rapidly and are generally undetectable by instruments. Therefore, such signals are ignored, and this application considers the observation point to only observe the induced electromagnetic signals generated by the formation medium directly below it. The observation of the second observation point follows a similar assumption.

[0035] The second sum of effects includes the effect of the primary electromagnetic field excited by the emission source, the effect of the secondary electromagnetic field excited by the strata between the emission source and the first observation point, and the effect of the secondary electromagnetic field excited by the strata between the first observation point and the second observation point; expressed as: M2 = M s +M s1 +M 12 ;

[0036] Among them, M s The emission source is subjected to the action of generating an electromagnetic field; M s1 The effect of generating a secondary electromagnetic field in the strata between the emission source and the first observation point; M 12 This is the effect of generating a secondary electromagnetic field in the strata between the first observation point and the second observation point.

[0037] Based on the analysis of the components of the sum of the first and second effects, it can be seen that the difference between the two components is exactly the secondary electromagnetic field effect generated by the strata between the two measurement points, i.e., the secondary field signal.

[0038] Furthermore, the difference between the second total effect and the first total effect in step S4 is expressed as: ΔM = M2 - M1 = M 12 Where, ΔM=M 12 .

[0039] The secondary field signal extracted from the sum of the second action by subtraction calculation is, in principle, completely free of the primary field signal, perfectly avoiding interference from the primary electromagnetic field and temporal confusion between the two. Its signal purity is fundamentally different from existing compensation methods.

[0040] Furthermore, it also includes the following steps:

[0041] S5. The difference between the sum of the second and first effects is used to infer the underground conductivity structure between the first and second observation points. This effectively avoids interference from the primary electromagnetic field on the secondary field signal in the observed signal, thus significantly improving the accuracy of the inferred stratigraphic information.

[0042] This application's embodiment is an improvement on the semi-airborne electromagnetic method. During the observation process, two observation points with different distances from the source are set on the flight path (observation line), and the signals observed by the two adjacent observation points are compared to extract the secondary field signal. This allows for better filtering of the information generated by the primary electromagnetic field from the extracted field signal, improving the signal quality of the secondary field signal measured by the semi-airborne electromagnetic method, and laying the foundation for high-quality geological imaging interpretation.

[0043] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for extracting pure secondary field signals using a semi-airborne electromagnetic method, characterized in that, The method includes the following steps: S1. Set up a first observation point and a second observation point along the aircraft's flight path; the distance between the second observation point and the emission source is greater than the distance between the first observation point and the emission source; S2. When the aircraft flies to the first observation point, an electromagnetic field observation is performed for one unit of time; when the aircraft flies to the second observation point, an electromagnetic field observation is performed for one unit of time. S3. Normalize the observation signal measured at the first observation point to obtain the first sum of actions; The observed signal measured at the second observation point is normalized to obtain the second total effect; S4. Calculate the difference between the sum of the second action and the sum of the first action to obtain the pure secondary field signal; In step S3, the normalization method for the observation signal measured at the first observation point is as follows: ； (1) The normalization method for the observation signal measured at the second observation point is as follows: ；(2) in, The sum of the first effects; The sum of the second effects; This is the first observation point; This is the second observation point; t is the unit of time. The time-domain electromagnetic field signal observed by the receiving coil at the first observation point; The time-domain electromagnetic field signal observed by the receiving coil at the second observation point; The first sum of effects includes the effect of the primary electromagnetic field excited by the emission source and the effect of the secondary electromagnetic field excited by the strata between the emission source and the first observation point; expressed as: ; The second sum of effects includes the effect of the primary electromagnetic field excited by the emission source, the effect of the secondary electromagnetic field excited by the strata between the emission source and the first observation point, and the effect of the secondary electromagnetic field excited by the strata between the first observation point and the second observation point; expressed as: ; in, The emission source is subjected to the effect of generating an electromagnetic field. This is to induce a secondary electromagnetic field in the strata between the emission source and the first observation point; This is due to the effect of generating a secondary electromagnetic field in the strata between the first and second observation points; The difference between the second sum of actions and the first sum of actions in step S4 is expressed as: ;in, .

2. The method as described in claim 1, characterized in that, In step S2, the unit time is greater than the arrival time of the secondary electromagnetic field signal.

3. The method as described in claim 1, characterized in that, It also includes the following steps: S5. The difference between the sum of the second action and the sum of the first action is used to interpret and infer the underground conductivity structure between the first observation point and the second observation point.

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