Changes

The presence of a fault is a common interpretation of a magnetic increase or decrease. This interpretation assumes the fault throw, which changes the elevation to the top of basement, is the cause of the anomaly. It also assumes uniform lithology and uniform magnetic susceptibility of basement across a fault. Given this (usually incorrect) assumption, we can calculate the depth of the fault and its throw from the shape and amplitude of an observed magnetic curve. If we do not know the exact susceptibility, we can calculate a series of curves to establish a range of probable values of the throw. In all cases, the magnetic high necessarily appears on the upthrown side of the fault.

The presence of a fault is a common interpretation of a magnetic increase or decrease. This interpretation assumes the fault throw, which changes the elevation to the top of basement, is the cause of the anomaly. It also assumes uniform lithology and uniform magnetic susceptibility of basement across a fault. Given this (usually incorrect) assumption, we can calculate the depth of the fault and its throw from the shape and amplitude of an observed magnetic curve. If we do not know the exact susceptibility, we can calculate a series of curves to establish a range of probable values of the throw. In all cases, the magnetic high necessarily appears on the upthrown side of the fault.

In the hypothetical cross section below, basement rock has the same susceptibility across the fault.

In the hypothetical cross section here, basement rock has the same susceptibility across the fault.

[[file:using-magnetics-in-petroleum-exploration_fig14-6.png|thumb|{{figure number|14-6}}Figure 14-7 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|thumb|{{figure number|14-7}}]]

Figure 14-7 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.

 

==Detecting basement hills==

==Detecting basement hills==