## 2 Principle of Distance MWD Based on Dual-Electrode Current Excitation in Drilling Wells  
The principle of the active MWD method based on dual-electrode current excitation is illustrated in Figure 1. The detection MWD device comprises a central current control and acquisition unit, an upper/lower low-frequency magnetic field detection unit, and a upper/lower electrode unit, with upper/lower representing the upper and lower parts and the central current control and acquisition unit being symmetrically arranged. The upper and lower electrode units should be equipped with insulating sub-sections, and the shell surface of each sub-section in the middle should be sprayed with insulating wear-resistant materials or glass fiber-reinforced plastic rings to minimize the impact of drilling mud and metal shells on the excitation current circuit.  
The central current control and acquisition unit are equipped with a middle electrode. The internal current source injects current into the formation by two or three poles (three electrodes can constitute four current excitation modes) through three electrodes arranged at the top, middle, and bottom. The current generates radiant flow in the formation near the drilling well. If there is metal material near the formation (such as a drill pipe and casing in the target well), the current will flow on it, with most of the current direction being consistent to avoid the cancellation and decay of the upper and lower currents. The gathered current in the target well produces a specific induced magnetic field in the formation, which is measured by the upper/lower low-frequency magnetic field detection unit. Combined with geomagnetic data, well inclination, excitation current, the potential between loop electrodes, and other relevant measurements, using the appropriate positioning algorithms can obtain the orientation, distance, and angle between the drilling well and the drilled well to achieve the access-independent active measurement of adjacent distances while drilling.  
The four reflux modes, composed of three electrodes, are illustrated in Figure 1. All four modes are based on two electrodes forming a current loop, as described below.  
1. Upper electrode reflux: the positive and negative electrode current loops are formed by the middle electrode and the upper electrode [14].
2. Lower electrode reflux: the positive and negative electrode current loops are formed by the middle electrode and the lower electrode.
3. Full loop reflux: the positive and negative electrode current loops are composed of the upper electrode and the lower electrode.
4. Double loop reflux: the upper and lower electrodes are in parallel for the negative electrode, with the middle electrode serving as the positive electrode to form the current circuit loop.  
Among these four current loop modes, the full loop reflux mode has the longest current loop. As a result, excitation currents can be injected deeper into the formations, leading to its longest effective detection distance. The other three modes are primarily used for non-ideal ranging environments (e.g., well position crossing). The relative distances between the center point of the upper/lower loops (the low-frequency magnetic field detector positions) and the target wells can be obtained through both the upper/lower electrolyte reflux mode and the double loop reflex mode.