Changes

[[: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.

The hypothetical cross section shows a fault juxtaposing basement blocks of different lithologies and susceptibilities. The curves above the cross section are the magnetic profiles where the magnetic field is vertical for ''k''₁ > ''k''₂ and ''k''₁ < ''k''₂. It assumes no throw on the fault (''d'' = 0). The dashed curves show the magnetic response if the fault has a finite throw (''d''). Note how little impact the fault throw has on either profile.

The hypothetical [[cross section]] shows a fault juxtaposing basement blocks of different lithologies and susceptibilities. The curves above the cross section are the magnetic profiles where the magnetic field is vertical for ''k''₁ > ''k''₂ and ''k''₁ < ''k''₂. It assumes no throw on the fault (''d'' = 0). The dashed curves show the magnetic response if the fault has a finite throw (''d''). Note how little impact the fault throw has on either profile.

==Detecting basement hills==

==Detecting basement hills==