What is Job Shadowing?

The shadow method, also known as terrain rendering method or landscape rendering method, is to simulate the lightness and darkness generated by sunlight on the ground, and use grayscale tones or color output to obtain a hue that changes approximately continuously with the luminosity to achieve sensitive contrast of the terrain. The distribution, undulations and morphological features of the landforms have a certain three-dimensional sense, which can intuitively express the undulations of the ground ^[1] .

[Radio wave perspective method] radiowave penetration method is also called shadow method. It is based on the physical properties that good conductive ore bodies have stronger absorption capacity of electromagnetic energy than ordinary rocks. When working, the tunnel can be used to transmit electromagnetic waves, to receive electromagnetic waves in the tunnel or on the ground or in the well, and to receive two schemes in the well. For example, when a high-frequency electromagnetic wave is emitted in a tunnel, and when the ground surface is observed point by point, if a ore body exists between the transmitting source and the receiving point, a region where the electromagnetic field strength is significantly weakened will be observed, that is, a shadow area, and in a place where there is no ore body , Still can observe a strong electromagnetic field strength. The frequency of radio wave perspective is mostly several megacycles / second. ^[2]

Radio wave perspective works as shown. In one wellbore, a transmitter is used to transmit electromagnetic waves of a certain frequency (a few tenths of a megahertz to tens of megahertz), and in another wellbore, a receiver is used to receive the electromagnetic waves absorbed by the medium. When there is a well-conducting orebody or a water-filled cave in the wellbore, the electromagnetic wave is strongly absorbed and its energy is greatly reduced. Therefore, a low value anomaly of the field strength curve (or called For shadows). The existence of geological bodies can be inferred from the relationship between the receiver and the transmitter and the location of the anomaly. ^[3]

There are two observation methods of radio wave perspective method: synchronous method and fixed-point method.

Synchronization method is to send the transmitter (or transmitting antenna) and receiver (or receiving antenna) into two wells respectively, and then move up and down synchronously to observe. If the transmitter and receiver remain at the same height, the synchronous measurement is called the horizontal synchronization method. If the transmitter and receiver are at different heights, the synchronous measurement is called the height difference synchronization method. The size of the height difference generally depends on the same distance, well depth, and rock formation.

The fixed-point measurement is generally to fix the transmitter (or receiver) at a preselected position in the wellbore, and then place the receiver (or transmitter) in another wellbore for continuous measurement. In actual work, the approximate location of the geological bodies between wells is generally known by the synchronous method, and then the boundary and contour of the abnormal body are further determined by the fixed-point method. ^[1]

The record obtained by the radio wave perspective receiver is the electric field value expressed in microvolts. After finishing, the change curve of the measured field strength E along the profile can be plotted. When plotting, the ordinate uses arithmetic coordinates to indicate the depth of the well, and the abscissa uses The logarithmic coordinates represent the field strength E or E. (the difference between the measured field strength E and the normal field strength Eo).

In radio wave perspective, the choice of frequency is also very important, it directly affects the distance of the perspective and the ability to resolve anomalies. Generally speaking, the frequency is high, the wavelength is short, and the ability to distinguish anomalous objects is strong, but with it comes absorption enhancement and shorter transmission distance. Conversely, low frequencies, long wavelengths, weak absorption, and larger transmission distances are more suitable for studying changes in geological bodies over a larger area, but the ability to resolve anomalous bodies is also reduced.

The simplest and most commonly used interpretation method of the radio wave perspective in the well is the intersection method. First, based on the wellbore cross-section corrected for the well deviation, a sectional view including the wellbore is drawn at a certain scale. Horizontal synchronization, height difference synchronization, or fixed points In the profile data, straight lines are drawn from the transmitting points to the abnormal boundaries of the receiving wellbore on the cross-sectional view. As a result of the intersection of these straight lines, a shaded area can be obtained. The space, location, shape and scale of this common shadow area basically reflect the range and contour of the low-resistance anomaly. ^[1]