Full magnetic tensor gradient survey with MEMS sensor, a case study on the Jafarkhan iron ore deposit

Magnetometry is one of the geophysical methods that has wide applications. In traditional magnetometry, one sensor so-called magnetometer is used for measurements. In order to obtain the gradient in a specific direction, mathematical derivation can be used. Alternatively, A more accurate method of obtaining the gradient is using two sensors simultaneously measuring the magnetic field. In this case, the gradient is obtained in the direction where the two sensors are placed relative to each other. To obtain the Full Magnetic Tensor Gradient (FMTG) of the Earth’s magnetic field, although direction derivatives can be used, the more accurate method is to use four sensors that measure the Earth’s magnetic field simultaneously. In this way, the FMTG matrix can be obtained. So far, Superconducting Quantum Interference Device (SQID) sensors have been used the most in obtaining the FMTG. But these sensors are very expensive and have accurate performance in a small range of temperatures. By developing the Micro Electro Mechanical System (MEMS) sensor with its variety of applications, geophysicists are also becoming interested in using such sensors. In this study, one of the precise MEMS magnetometers has been selected. Although the MEMS sensors have low sensitivity compared to SQUID magnetometers, due to their availability, being cheap, and lightness, they are a suitable option for FMTG measurements.

Although these sensors do not have the resolution of SQUID magnetometers and they cannot be used on small anomalies that have caused changes of less than 160 nT on the Earth’s magnetic field, these kinds of sensors are able to measure the magnitude of the magnetic field in three perpendicular directions and have an acceptable sensitivity. In this study, four sensors of this type have been set up in a cross arrangement. In order to solve the problem of the low sensitivity of these sensors compared to SQUID sensors, the distance between the sensors was chosen to be one meter so that the gradient value would be larger enough. Then, a survey has been performed on an iron ore deposit near Jafar Khan village, Saqqz City, Kurdistan province. In order to validate elements of the FMGT matrix, they have been compared with the directional derivatives obtained from a sensor. This comparison shows that the elements Signal-to-noise ratio of elements Gxx, Gyx, Gzx, Gzy, and Gzz have increased around zero, 4.79, 10.80, 83.83, and 8.46, respectively, compared to their corresponding directional derivatives. This issue shows the ability of MEMS sensors to capture the full tensor of the magnetic gradient.