What is a hyperbola?
A hyperbola is the reflection created when the radar wave bounces off a single point object in the ground. It can also appear when you, for example, move across a buried utility with your GPR equipment. It is a distinctive, up-side-down U -shaped reflection that will be indicative of the location of interesting features in the ground, but also provide the surveyor with important information about the soils in the survey area and the point targets themselves.
The hyperbola shapes in radargrams are created as the radar signal is transmitted into the ground in a cone shape (and not straight down). This cone shaped pattern is often referred to as the radar footprint. As a result of this transmission pattern the radar will in effect “see” objects both before it is on top of the actual object and after you have passed it with your GPR equipment. The correct depth of the buried object will be measured when the antenna is directly on top of it.
The shape of the hyperbola from a single point object will differ due to the size of the object, where larger objects at the same depth as a smaller object will result in a wider hyperbola shape. But if objects of the same size are located at different depths the hyperbola from the deeper lying object will appear larger than the shallower object. The velocity of the ground will also have an impact on the shape of the hyperbolas where a “faster” soil will result in wider hyperbolas and vice versa.
Also remember that the shape of the hyperbola, when being reflected off a utility, will depend on the angle between the surveyed line and the direction of the utility itself. If the surveyed line is not measured exactly perpendicular to (and over) the utility, the hyperbola will appear wider and subsequently provide you with the wrong values for your velocity/depth estimations.
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