Interpretation of the magnetic anomaly of Zanjan's Morvarid mine using the normalized full gradient method

The normalized full gradient (NFG) method defined by Berezkin (1967, 1973 and 1998)is used for downward continuation maps. Analytical downward continuation is a methodof estimating the field closer to the source, and, consequently, it results in a betterresolution of underground rock distribution. However, the usefulness of this process islimited by the fact that the operation is extremely sensitive to noise. With noise free data,downward continuation is well defined; it is unnecessary to continue below the sourcelevel. In the presence of noise, the amplification of high frequencies is so strong that itquickly masks the information in the original profile. Low-pass Fourier filtering, whilesuppressing such noise, also blurs the signal, overcoming the purpose of sharpening bydownward continuation.Despite the above-mentioned problems, most geophysical experts have long beeninterested in this technique because of its importance to mineral exploration. Furthermore,this method is a fast and cheap way to determine the initial depth of the subsurfacefeatures, especially where there is no other geophysical or well-logging data. A goodanalytical downward continuation process could provide subsurface general images,allowing an enhanced interpretation. Also, analytical downward continuation has theability to determine accurately both the horizontal and vertical extents of geologicalsources.The NFG method nullifies perturbations due to the passage of mass depth duringdownward continuation. The method depends on the downward analytical continuation ofnormalized full gradient values of magnetic data. Analytical continuation discriminatescertain structural anomalies which cannot be distinguished in the observed magnetic field.It can be used to estimate location, depth to the top, and center of the deposit that isapplied also for detecting oil reserviors and tectonic studies. One of the importantparameters to estimate the accurate shape of the deposit is true selection of the harmonicnumber. In this paper, the correct harmonic number is determined and then this methodwill be tested for noise-free and noise-corruption synthetic data. Finally, 2D and 3D ofthis method are applied to real data from Zanjan’s Morvarid mine.The Morvarid mine is located at a distance of 23 km southeast of Zanjan, near Aliabadvillage. Hematite and magnetite are the major minerals in the mine in which magneticexploration has been performed to find anomalies around the mine, near outcrops of theregion. From the IGRF model, the geomagnetic field of this region is calculated at 47680nT (inclination = 54.7°, declination= 4.5°).The results of the method were satisfactory for a dike. After applying the method, ageneral view of deposit structure was obtained that helped us to make a fast interpretation.2D and 3D results of NFG method satisfied the results of trenches and drilling in theregion are under study, as well. The final result of this method shows that the depositbegins from the low depth to approximately less than 100 meters. This modeling yeildedsatisfactory results for drilling in the region. The results of the drillings show that thelowest depth of the deposit is about 6 meters.Finally, 2.5D modeling of th edeposit was done using Modelvision software that hadthe same results. This method can easily be applied for gravity and microgravity data.