نوع مقاله : مقاله تحقیقی (پژوهشی)
1 دانشکده مهندسی معدن، پردیس دانشکدههای فنی، دانشگاه تهران، ایران
2 مؤسسة ژئوفیزیک دانشگاه تهران، ایران
عنوان مقاله [English]
The normalized full gradient (NFG) method defined by Berezkin (1967, 1973 and 1998) is used for downward continuation maps. Analytical downward continuation is a method of estimating the field closer to the source, and, consequently, it results in a better resolution of underground rock distribution. However, the usefulness of this process is limited 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 source level. In the presence of noise, the amplification of high frequencies is so strong that it quickly masks the information in the original profile. Low-pass Fourier filtering, while suppressing such noise, also blurs the signal, overcoming the purpose of sharpening by downward continuation.
Despite the above-mentioned problems, most geophysical experts have long been interested 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 subsurface features, especially where there is no other geophysical or well-logging data. A good analytical downward continuation process could provide subsurface general images, allowing an enhanced interpretation. Also, analytical downward continuation has the ability to determine accurately both the horizontal and vertical extents of geological sources.
The NFG method nullifies perturbations due to the passage of mass depth during downward continuation. The method depends on the downward analytical continuation of normalized full gradient values of magnetic data. Analytical continuation discriminates certain 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 is applied also for detecting oil reserviors and tectonic studies. One of the important parameters to estimate the accurate shape of the deposit is true selection of the harmonic number. In this paper, the correct harmonic number is determined and then this method will be tested for noise-free and noise-corruption synthetic data. Finally, 2D and 3D of this 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 Aliabad village. Hematite and magnetite are the major minerals in the mine in which magnetic exploration has been performed to find anomalies around the mine, near outcrops of the region. From the IGRF model, the geomagnetic field of this region is calculated at 47680 nT (inclination = 54.7°, declination= 4.5°).
The results of the method were satisfactory for a dike. After applying the method, a general 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 the region are under study, as well. The final result of this method shows that the deposit begins from the low depth to approximately less than 100 meters. This modeling yeilded satisfactory results for drilling in the region. The results of the drillings show that the lowest depth of the deposit is about 6 meters.
Finally, 2.5D modeling of th edeposit was done using Modelvision software that had the same results. This method can easily be applied for gravity and microgravity data.