Using The Hodograph For IdentificationWe have previously mentioned about using the hodograph for identifying targets. But what is the hodograph and what it is for? The received signal is characterized not only by its amplitude and phase, but it is also a vector quantity. Amplitude and phase depend on the electro physical parameters of objects such as conductivity, magnetic permeability, depth, geometry, etc. To accurately describe the nature of the received signal from an object is very difficult, given the variety of influencing factors. However, it is possible to point out some general patterns. Earlier, we mentioned that the signal is a vector quantity characterized by amplitude and phase. If you hold any metal object to the coil it is evident that the magnitude of this vector will vary. The end of the vector will describe, on a coordinate plane, some shapes (lines, loops, etc.). These shapes are called Hodographs. The latter most closely describes the complex interactions of the coil with metal objects.
When analyzing the hodographs should be remember a few general rules:
- Hodographs of small and medium-sized ferromagnetic objects are in the left quadrant (i.e. have a negative vector angle);
- Hodographs of objects of non-ferrous metals and large ferromagnetic objects lie in the right quadrant (positive vector angle);
- The larger the area of the surface of an object the higher the electrical conductivity, hence the greater the slope of the hodograph to the right;
- Travel times of medium and large ferrous objects, as a rule, have the shape of a loop;
- Hodographs of non-ferrous metal objects are mainly straight;
- A properly ground balanced unit will have a vector along the horizontal axis. Examples of the hodographs of some objects are shown in Fig. 14. Thus, analyzing the shape and position of the hodograph, along the coordinate plane, there can be a certain degree of probability as to the identity of the object type. It should be noted that the hodograph examples given are idealized and do not take into account the effect ground mineralization.