A Rapid Graphical Solution for Determining the Presence of Remnant Magnetization in the Basement Rocks of Western Iraq

Abstract

A graphical method of determining the possible presence of a remanent component of magnetization in two dimensional igneous masses within the basement is described. It is required to assume that the same source produces well defined gravity and magnetic anomalies. The method involves taking the ratios of distance between peaks of maximum values of gravity and magnetic for various angles of field inclinations. Ratio of maximum positive and maximum negative values for various angles of field inclinations are also used. These ratios are plotted against field inclinations. Examples of anomalies are taken from the western desert area of the stable shelf in Iraq. ــــــ ــــ ــ ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ ــــــــــــــــــــــــــــــــــــــــــ ل ةعیرس لولح تاططخم يف ةدعاقلا روخص يف ةیقبتملا ةیسیطانغملا دوجو ىلع فرعتل زجلا قا رعلا نم يبرغلا ء ينادهشملا دومحم میظعلا دبع ينقتلا دهعملا / ىونین صخلملا ماـسجلأا يف ةیقبتملا ةیسیطانغملا دوجو ةیناكمإ ىدم ةفرعمل ةیسایق تاططخم داجیإ ىلإ ثحبلا فدهی بسملا يرانلا مسجلل يبذجلاو يسیطانغملا ذوذشلا سایق ىلع دامتعلااب ةدعاقلا روخصل ةیرانلا ذاوشلا كلتل ب . دـنع ةیـسیطانغملاو ةـیبذجلا ذاوـشلل اـیلعلا میـقلا نیـب تافاـسملا بـسن ذـخأب ةیــسایقلا تاـططخملا دادـعإ مـت ثـیح ةیسیطانغملا ذاوشلل ایلعلا ةبلاسلا میقلاو ایلعلا ةبجوملا میقلا نیب ةبسنلا كلذكو يسیطانغملا لیملا ایاوز فلتخم يسیطانغملا لیملا ایاوز فلتخم دنع . أ يـسیطانغملا لـیملا اـیاوز عـم تمـسر هلاـعأ ةروكذـملا بـسنل . هذـه تـقبط ةسا ردلا ةیبرغلا ءا رحصلا يف قا رعلا يف رقتسملا فرجلا نمض ةعقاولا . ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ ـــــــ ــــــــــــــــــ INTRODUCTION In analyzing the aeromagnetic maps of Iraq, it is usually assumed that the observed anomalies are caused by homogeneously magnetized bodies forming the basement configuration that is concealed beneath a thick cover of sediments. Many unknowns are faced with in interpreting magnetic anomalies related to deep sources. Among these, direction of magnetization in these sources is most important (2) Iraqi Journal of Earth Sciences, Vol. 8No. 2November2008 unknown, with gravity, on the other hand, the maximum values lies directly over the center of gravity of the source. As such, a vertical line passing through the maximum (or minimum) gravity anomaly can be reference line for the location of maximum (or minimum) magnetic anomalies. Previous studies related to interpretation of aeromagnetic anomalies in Iraq , have assumed that the basement magnetic sources have no remanence and its magnetization is made only by induction from the present earths magnetic field(Aziz, 1986; Baban, 1983; Mahmood, 1981). Many authors such as(Ates and Keary, 1995) have discussed the magnetization direction but have restricted the discussion to pseudo-gravity anomaly. In this paper, it will be considered that the induced magnetization of the source from the present field will be affected if there is remanence in whatever direction within the source. In this case the remanence will have some component parallel to the present field magnetization and hence altering it. The following method is designed to estimate the amount of change in the inclination of the resultant magnetization field. The Method Magnetic anomalies are calculated over a line running perpendicular to the strike of an hypothetical two dimensional source using different inclinations of the magnetizing field (Heirtzler, et al., 1962). the gravity anomaly along the same line over the same source is also calculated (Talwani, et al., 1959). These anomalies are shown in fig.( 1). It can be seen that the peak of the gravity anomaly coincides with the positive peak of the magnetic anomaly at magnetization field inclination of 90, and the two shift apart at different horizontal distances at different field inclinations . At each field inclination, the distance between the maximum positive magnetic anomaly and the maximum positive gravity anomaly (D ) is measured. Also measured for each field inclination is the distance between the maximum negative magnetic anomaly and maximum positive gravity anomaly (D ). Then for each field inclination the ratio R1=D /D is calculated. A theoretical curve of R1 against field inclination is constructed (Fig.2) and shows exponential increase of R1 as field inclination decrease . With actual gravity and magnetic surveys, R1 can be measured from the observed anomalies and the field inclination corresponding to it can be read from the curve of Fig. 2. Any variation in the inclination thus measured from the actual field from the area of survey will indicate the presence of a remanent component. Another ratio (R2) which is the ratio between the maximum positive amplitude of the magnetic anomaly to its maximum negative amplitude from the curves of Fig.1 can be measured at each inclination angle. If R2 is plotted against magnetic field inclinations the results will show the exponential relations of Fig. 3. The ratios of R2 (+ve max/-ve max) as shown in Fig. (3) are more accurate and precise by using the equation of Heirtzler for two dimensional igneous body, while the ratios R2 or (Ar)(+ve max/-ve max) as mentiond by Sharma (2004) for calculating the inclination angles which represent an approximate values as in Table (1). (3) Iraqi Journal of Earth Sciences, Vol. 8No. 2November2008 0 1 2 3 4 5 6 7 8 9 10 90 80 70 60 50 40 30 20 10 0 Inclination angles R1 =D +/ D Fig. 2: Standard curve showing the relationship between the ratio (R1) and inclination angles. Fig. 1: Theoritical calculation for gravity and magnetic anomalies at different inclination angles for two dimensional igneous body (4) Iraqi Journal of Earth Sciences, Vol. 8No. 2November2008 Table 1: Variation of R2 values with magnetic inclination angles Using Heirtzler equation Sharma Inclination R2(+ve max/-ve max) Inclination *(R2 or) Ar (+ve max/-ve max) 20 0.4 0-20 1 30 0.6 30-50 2-----5 40 0.77 60-75 6-----20 50 1.1 -----------60 1.55 -----------70 2.1 ----------80 3.3 ----------90 5.8 ----------* R2=Ar , As presented from the research.