File:Using-magnetics-in-petroleum-exploration fig14-6.png

Figure 14-7 shows a fault separating basement blocks of different lithologies and magnetic susceptibilities. If the average magnetic susceptibilities (k₁ and k₂) 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 (Heiland, 1946; Jakosky 1950; Dobrin, 1960) 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.