Locate the maximum slope on the magnetic intensity profile or magnetic anomaly chart created by using the magnetometer. The maximum slope is the most steep part of the magnetic anomaly line on the graph.
Draw a straight line through the maximum slope, extending beyond the slope's top and bottom end and intersecting with the chart's x-axis beyond the bottom end; this is the "slope line."
Draw a vertical line, connecting the maximum slope's top end with the chart's x-axis. Measure the vertical line's height with the ruler and mark its middle point.
Draw a straight line connecting the slope line's intersection with the x-axis and the middle point of the vertical line; this is the "half-slope line."
Add two straight lines parallel to the half-slope line and tangent to the magnetic intensity line, touching the curve, but not intersecting it.
Draw two new vertical lines, starting from the two points were the tangent lines touch the curve and ending at the x-axis. Measure the distance between these two vertical lines and note the value as distance. Values on the x-axis are in feet, meters or kilometers, so ensure the distance agrees with the units of the x-axis.
Assign the object an index value or "proportionality factor," ranging from 1.2 to 2, depending on the object's size. For example, large objects, like a buried car, would have a value approaching 2, while a smaller item, such as a handgun, would be around 1.2. There is no formula to assign specific index values to objects of certain sizes, so the value depends on your perception of the object's size, with the typical index value at 1.6.
Divide distance by index value to find the depth of the object. The depth will be in feet, meters or kilometers, depending on the unit of distance. If you are not aware of the object's size, divide the distance by both 1.2 and 2 to determine the depths between which the object is buried.