Changes

==Lithologic changes due to a fault==

==Lithologic changes due to a fault==

[[file:using-magnetics-in-petroleum-exploration_fig14-7.png|300px|thumb|{{figure number|2}}Fault separating basement blocks of different lithologies and magnetic susceptibilities. From Gay, 1995; courtesy International Basement Tectonics Assoc.]]

[[file:using-magnetics-in-petroleum-exploration_fig14-7.png|300px|thumb|{{figure number|2}}Fault separating basement blocks of different lithologies and magnetic susceptibilities. From Gay;<ref name=Gay1995>Gay, S. P., Jr., 1995, The basement fault block pattern: its importance in petroleum explo- ration, and its delineation with residual aeromagnetic techniques, in R. W. Ojakangas, ed., Proceedings of the 10th International Basement Tectonics Conference, p. 159–207.</ref> courtesy International Basement Tectonics Assoc.]]

[[:file:using-magnetics-in-petroleum-exploration_fig14-7.png|Figure 2]] shows a fault separating basement blocks of different lithologies and magnetic susceptibilities. If the average magnetic susceptibilities (''k''₁ and ''k''₂) of the basement blocks are unknown, then we cannot determine the amount of throw of the fault—we cannot even determine the direction of throw if the signal resulting from susceptibility overrides that due to throw. Since susceptibilities of basement rocks commonly vary by hundreds, even thousands, of percent<ref name=ch14r4>Heiland, C., A., 1946, Geophysical Exploration: Englewood Cliffs, New Jersey, Prentice-Hall, 1013 p.</ref><ref name=ch14r5>Jakosky, J., J., 1950, Exploration Geophysics: Los Angeles, Trija Publishing Co., 1195 p.</ref><ref name=ch14r1>Dobrin, M., B., 1960, Introduction to Geophysical Prospecting, 2nd ed.: New York, McGraw-Hill, 446 p.</ref> and the ratio of throw to depth of a fault can be, at most, 100%, then it follows that in most cases the magnetic response due to susceptibility overrides that due to throw. The result is that many faults (perhaps as high as 40–50%) show a magnetic low on the upthrown side.

[[:file:using-magnetics-in-petroleum-exploration_fig14-7.png|Figure 2]] shows a fault separating basement blocks of different lithologies and magnetic susceptibilities. If the average magnetic susceptibilities (''k''₁ and ''k''₂) of the basement blocks are unknown, then we cannot determine the amount of throw of the fault—we cannot even determine the direction of throw if the signal resulting from susceptibility overrides that due to throw. Since susceptibilities of basement rocks commonly vary by hundreds, even thousands, of percent<ref name=ch14r4>Heiland, C., A., 1946, Geophysical Exploration: Englewood Cliffs, New Jersey, Prentice-Hall, 1013 p.</ref><ref name=ch14r5>Jakosky, J., J., 1950, Exploration Geophysics: Los Angeles, Trija Publishing Co., 1195 p.</ref><ref name=ch14r1>Dobrin, M., B., 1960, Introduction to Geophysical Prospecting, 2nd ed.: New York, McGraw-Hill, 446 p.</ref> and the ratio of throw to depth of a fault can be, at most, 100%, then it follows that in most cases the magnetic response due to susceptibility overrides that due to throw. The result is that many faults (perhaps as high as 40–50%) show a magnetic low on the upthrown side.