A new automatic method for estimation of magnetization and density contrast by using three-dimensional (3D) magnetic and gravity anomalies

https://doi.org/10.1016/j.pepi.2012.06.003Get rights and content

Abstract

In this paper, a new method estimating the ratio of magnetic intensity to density contrast of a body that creates magnetic and gravity anomalies is presented. Although magnetic intensity and density of an anomalous body can be measured in the laboratory from the surface samples, the proposed new method is developed to determine the magnetic intensity and density contrast from the magnetic and gravity anomalies when the surface samples are not available. In this method, density contrast diagrams of a synthetic model are produced and these diagrams are prepared as graphics where the magnetic intensity (J) is given in the vertical axis and Psg (pseudogravity)/Grv (gravity) values in horizontal axis. The density contrast diagrams can be prepared as three sub-diagrams to show the low, middle and high ranges allowing obtain density contrast of body.

The proposed method is successfully tested on the synthetic models with and without error. In order to verify the results of the method, an alternative method known as root-mean-square (RMS) is also applied onto the same models to determine the density contrast. In this manner, maximum correlation between the observed gravity and calculated gravity anomalies is searched and confirmation of the results is supported with the RMS method. In order to check the reliability of the new method on the field data, the proposed method is applied to the Tetbury (England) and Hanobasi (Central Turkey) magnetic and gravity anomalies. Field models are correlated with available geological, seismic and borehole data. The results are found consistent and reliable for estimating the magnetic intensity and density contrast of the causative bodies.

Highlights

► A new method is proposed to estimate the magnetic intensity/density contrast ratio. ► This new method can be useful when the surface samples are not available. ► The method allows to study each anomaly separately for different rock types. ► All models are correlated with available geological and geophysical data.

Introduction

Magnetic or gravity anomalies, are generally produced by buried bodies having the susceptibility (K=J/H) and density contrasts (Δρ) where J is the magnetic intensity, H is the magnetic field. It is possible to convert the magnetic field to gravity field using the Poisson relation (Baranov, 1957). By using Eq. (1) below, magnetic field is transferred to the gravity field (Blakely, 1995). This converted field is called as the pseudogravity field while the real density of the body is not known.V=-CmγMρmˆ·pUwhere V is the magnetic potential, U is the gravity potential, Cm is the magnetic constant, mˆ is the direction of magnetization, M is the magnetic intensity, ρ is the density, and γ is the international gravity constant.

Interpretation of pseudogravity anomalies is important because it provides comparison of both fields. For instance, gravity anomaly and pseudogravity anomaly obtained from magnetic anomalies over the same area are compared for the same geological causative body (Kearey et al., 2002). Relative amplitudes of these fields can provide a measurement for the ratio of magnetic intensity to density contrast. Comparison of the real gravity anomaly and the pseudogravity sometimes provides additional information about the causative body. There are several studies on this subject by various authors such as Kearey, 1991, Kearey and Rabae, 1993, Ates and Kearey, 1995. Further, Mendonça, 2004, Mendonça and Meguid, 2008 studied automatic determination for the ratio of magnetic intensity to density contrast from the gravity and magnetic anomalies. Recently, the same ratio was also calculated by Doo et al. (2009). In the most recent papers given above, authors calculated only ratio of the magnetic intensity to density contrast. However, there are different rocks which can indicate same ratio. In this paper, we determined magnetic intensity (J) and density contrast (Δρ) separately and then calculated the ratio of magnetic intensity to density contrast. This gives an advantage to the new method to study each anomaly separately for different rock types. However, our error calculations demonstrate that it is possible to calculate the magnetization and density contrast separately.

Utility of this method requires the assumption that the subsurface contains highly correlative magnetic susceptibility and density. Assumptions may also be related to the characteristics of magnetization, such as declination and inclination (Bilim and Ates, 1999, Bilim and Ates, 2004).

Section snippets

Method

Correlation of the magnetic and gravity anomalies to obtain the magnetization and density contrast was illustrated in a flow chart given in Fig. 1. This method is based on two main parts. These are annotated with blue and red boxes numbered with I and II in Fig. 1. In Part I intensity of magnetization and in Part II density contrast of anomalous body is calculated. Main parts I and II have sub steps from i to viii. The method has also Auxiliary steps shown with dashed boxes of (a–c) in Fig. 1.

Synthetic models (without any error)

The method was tested on gravity and magnetic anomalies of synthetic models. In this study, gravity and magnetic anomaly of a single vertical prism model (with 15 × 15 × 4 km dimensions in x, y and z directions) were used. Inclination and declination of the earth magnetic field were taken as 55° and 4°, respectively. These are values in central Turkey and also they are accepted as the values of Case 2 in the following section of this paper. Model parameters of the density diagrams (Table 1) were

Field examples

The method was applied on two different field cases. In case 1, gravity and magnetic anomalies have already been studied and interpreted, while gravity and magnetic anomalies were previously unprocessed and un-interpreted in Case 2.

Discussions and conclusions

In this study, we proposed a new method to estimate the ratio of magnetic intensity to density contrast of a body causing magnetic and gravity anomalies by means of simple correlation. Normally, the magnetic intensity and density of an anomalous body are determined in the laboratory environment after sampling from the surface. However, the magnetic intensity and density contrasts can be calculated from the magnetic and gravity data by using this new developed method in case of having no hand

Acknowledgments

Authors would like to thank the Mineral Exploration and Research Institute of Turkey for the use of gravity and aeromagnetic data that were provided for a TUBITAK Project (Project No: YDABCAG-118). Our appreciations are also extended to the General Directory of Petroleum Affairs (GDPA) for provision of seismic sections. We are the grateful to Dr. Kumar Hemant Singh and other anonymous reviewer for their constructive suggestions. Many thanks are extended to Dr. Kei Hirose for his delicate

References (23)

  • V. Baranov

    A new method for interpretation of aero-magnetic maps: pseudogravimetric anomalies

    Geophysics

    (1957)
  • Cited by (2)

    • Joint interpretation of magnetic and gravity data at the Golgohar mine in Iran

      2021, Journal of Applied Geophysics
      Citation Excerpt :

      In this way, we may deduce the source physical parameters by studying the magnetization to density ratio on a set of moving windows. However, in mineral exploration, there are few studies based on the application of the Poisson's analysis to potential field data (von Frese et al., 1982; Doo et al., 2009; Bektas et al., 2012; Alencar de Matos and Mendonça, 2020). This is probably due to the difficulty on fulfilling the condition on the source homogeneity.

    View full text