PCL detect magnetic fields Alternate electromagnetic current creates a moving and reversing magnetic field. When the receiver detects the presence of a conductor, signal appears. Afterwards, the strength of the signal is shown on the screen. The operator reads the response of the locator and interprets the result. Figure 4: Alternate electromagnetic current Difference between Passive and Active mode detection For passive detection, the signals are naturally present in conductors.
The only equipment is a receiver. The application is to sweep and search for the existence of conductors buried underground. It emits 50 Hz and radio frequency. Its detection by radio is less definite. It should not be relied upon for depth measurement. For active detection, it requires the use of a signal generator (transmitter) to trace and pinpoint target lines. Signal can be transmitted to the cable by induction or signal clamp. It allows more precise work such as depth measurement and signal strength comparison.
Passive modeActive mode – Direct connection Active mode – Signal clampActive mode – Induction Table 1: Passive mode and Active mode detection Why passive mode is unreliable It is unable to identify the conductors unless tracing to the source to obtain clues. Passive signals can be unobvious. Frequency of signals is not relevant to the voltage. Frequency of signals depends on strength of the current and depth of the line. The result of passive detection can only be taken as reference but not accurate measurement.
The function of a transmitter Transmitter discharges an identifiable signal and applies the signal to the target line. The receiver than traces and locates the lines by detecting the applied signal. It can also flood with signal and energize the lines in that area. Accessories are optional. Direct connection uses a pair lead (red for connection and black for grounding). Signal clamping uses a signal clamp. Direct connection Signal clamping ? Induction Table 2: the three main types of Active mode detection
For direct connection, since contact to cable is necessary, this method is not suitable to detect electric cable but suitable to metallic pipe. Signal clamping is the most effective method. It can be applied to exposed cable (trail hole). However, the size of the cable is limited by the size of the clamp. The jaws of the clamp must be closed completely. For induction method, signal can be applied without access to the line. However, it is not accurate method since signals can induce onto nearby lines as well as the target. It is also inefficient on deep targets.
Ground Penetrating Radar (GPR) For the data collection part of GPR, the elapsed time between radar energy generation, reflection from the ground and the final record of reflected wave at the receiving antenna is measured. The amplitude and wavelength of the reflected radar waves received back to surface are also amplified for processing and viewing on a computer screen (Conyers, 2004). Reflection of radar energy occurs when energy enters into a material with different electrical conduction properties from materials it left.
The amplitude of the reflection depends on the contrast in the dielectric constants of the two materials. High amplitude reflections usually appear when there is a sudden change in water content, lithologic or mineralogic changes (Laskowski W. , 2010). The following shows Schematic illustration of common-offset, single-fold profiling along a line showing major survey specification parameters. Fig. 5 Showing Schematic illustration of common-offset, single-fold profiling along a line showing major survey specification parameters.
According to Conyers (2004), radar energy emits from the GPR antenna downward to the ground surface is of conical shape. Therefore reflected radar energy received by antenna may not come from buried objects that are directly below the antenna but still within the “beam” of propagating waves. Oblique radar wave travel to and from the ground surface is longer in distance and travelling time. These reflected radar wave will still be recorded by the antenna as if directly below the antennas, but deeper in the ground.
When the GPR is propelled forward on the ground in transect, the antenna moves closer gradually to the buried object. The antenna will continually record reflections from the buried object before arriving on top of the object and continue to record after passing it. The following shows a signal paths between a transmitter and a receiver on the surface treated as rays following the paths. Fig. 6 A signal paths between a transmitter and a receiver on the surface treated as rays following the paths. A=direct airwave G=direct ground wave R=reflected wave C=critically refracted wave
These transmitting and receiving radar waves produce a reflection parabola as time for radar wave transmit and reflect back to antenna is longer for point sources of reflection that are locate obliquely from the antenna. Two-way time traveled when antenna is moving close to the object will be shorter until the antenna reaches the top of the object. The situation is the reverse when the antenna moves away from the top of the object. Therefore, the apex of the parabola denotes the actual location of the buried point sources of reflection while the arms of the parabola are generated when the antenna receive the oblique radar wave.