Systematic error analysis of demagnetization and implications for magneticinterpretation

Demagnetization can affect the interpretation of magnetic data significantl y. However, little attempt has been made to understand its effects by analy zing systematically the differences between demagnetization corrected and u ncorrected magnetic properties. A systematic error analysis is made in this paper using a 2-D elliptic cylinder model. Generally, demagnetization chan ges the effective susceptibility and remanence or the effective magnetizati on in bath magnitude and direction. Error analyses show that demagnetizatio n causes the magnitude of effective magnetization of a magnetic body to be less than its intrinsic magnetization. This implies that a theoretical anom aly computed without accounting for demagnetization will overestimate the a mplitude of the anomaly associated with the body. The decrease in magnetiza tion magnitude depends on the intrinsic magnetic susceptibility of a body a s well as on the body's geometry (flattening ratio) and its relative orient ation (magnetic dip) in the geomagnetic field. The magnitude of the effecti ve magnetization, relative to the intrinsic magnetization, decreases with i ncreasing intrinsic magnetic susceptibility. This factor dominates the body 's effective magnetization. When intrinsic magnetic susceptibility is less than 0.1 SI, the demagnetization effects are generally insignificant and ma y be ignored in magnetic anomaly modeling. The magnetic dip and flattening ratio only cause minor fluctuations in the effective magnetization. Demagne tization also changes the direction of the effective magnetization vector b y making it approach the plane of flattening of any flattened body. The dif ference between the inclinations of the effective and intrinsic magnetizati on changes the horizontal positions of extreme values of an anomaly, which may affect the precision of magnetic interpretations. Generally, the inclin ation difference is significant far magnetic dips of 30 degrees to 70 degre es and increases with increasing susceptibility and decreasing flattening r atio. In particular, for large flat-lying magnetic geological units located at middle magnetic latitudes (30 degrees to 70 degrees), significant magne tic inclination deflections are expected because of demagnetization effects . Theoretical, experimental, and practical examples of magnetic interpretat ion are presented to illustrate these demagnetization effects.