Changes

Variations in the local magnetic field are due mainly to the following:

Variations in the local magnetic field are due mainly to the following:

* Lithologic changes of basement rocks with corresponding differences in magnetite content

* Lithologic changes of [[basement]] rocks with corresponding differences in magnetite content

* Elevation changes on the top of basement where basement is of uniform magnetic susceptibility (k)

* Elevation changes on the top of basement where basement is of uniform magnetic susceptibility (k)

[[file:using-magnetics-in-petroleum-exploration_fig14-8.png|300px|thumb|{{figure number|3}}Basement hill and magnetic anomaly.]]

[[file:using-magnetics-in-petroleum-exploration_fig14-8.png|300px|thumb|{{figure number|3}}Basement hill and magnetic anomaly.]]

The basement hill and obvious magnetic anomaly shown on the left side of [[:file:using-magnetics-in-petroleum-exploration_fig14-8.png|Figure 3]] assumes a uniform magnetic susceptibility for basement. However, given that basement is usually block faulted, is this type of feature detectable? If we are looking at a topographic prominence centered on a basement block, the detection problem becomes that shown on the right side of the figure. A series of adjacent basement blocks having different magnetic susceptibilities results in a residual magnetic pattern of alternating highs and lows (solid lines).

The [[basement]] hill and obvious magnetic anomaly shown on the left side of [[:file:using-magnetics-in-petroleum-exploration_fig14-8.png|Figure 3]] assumes a uniform magnetic susceptibility for basement. However, given that basement is usually block faulted, is this type of feature detectable? If we are looking at a topographic prominence centered on a basement block, the detection problem becomes that shown on the right side of the figure. A series of adjacent basement blocks having different magnetic susceptibilities results in a residual magnetic pattern of alternating highs and lows (solid lines).

When the basement block on which the hill is carved is more magnetic than surrounding blocks, the hill contributes slightly to the magnetic high over the block as shown. The slight increase in anomaly amplitude due to the hill (top dashed line) generally is not distinguishable from a similar increase due to a slightly higher magnetic susceptibility for the whole block; hence, the hill is not generally detectable. If the block on which the hill is carved is less magnetic than the adjacent blocks, then the hill results in a lesser amplitude of the magnetic low over that block, but the low is still present (bottom dashed line). The hill generally is not detected.

When the basement block on which the hill is carved is more magnetic than surrounding blocks, the hill contributes slightly to the magnetic high over the block as shown. The slight increase in anomaly amplitude due to the hill (top dashed line) generally is not distinguishable from a similar increase due to a slightly higher magnetic susceptibility for the whole block; hence, the hill is not generally detectable. If the block on which the hill is carved is less magnetic than the adjacent blocks, then the hill results in a lesser amplitude of the magnetic low over that block, but the low is still present (bottom dashed line). The hill generally is not detected.