Figure 4-21.-Simplified comparison of natural field density and submarine anomaly

Figure 4-20.-Dip angles. are known as angles of dip (fig. 4-20). At any given point between the equator and the magnetic poles, the relationship of the angle between the earth’s surface and the magnetic lines of force is between 0° and 90°. This angle is determined by drawing an imaginary line tangent to the earth’s surface and to the line of force where it enters the earth’s surface. The angle thus formed is called the DIP ANGLE. If the same lines are traced only a short distance, 300 feet for instance, their natural changes in variation and dip over such a short distance (short-trace) are almost impossible to measure. However, short-trace variation and dip in the area of a large mass of ferrous material, though still extremely minute, are measurable with a sensitive anomaly detector. This is shown in figure 4-21. The dashed lines represent lines of force in the earth’s magnetic field. View A shows the angular direction at which natural lines of magnetic force enter and leave the surface of the earth. Note that the angles of dip are considerably steeper in extreme northern and southern latitudes than they are near the equator. View B represents an area of undisturbed natural Figure 4-21.-Simplified comparison of natural field density and submarine anomaly. 4-16