Application of TEM Based on HTS SQUID Magnetometer in Deep Geological Structure Exploration in the Baiyun Gold Deposit,NE China

Exploration of deep mineralization,particularly where the mineralization of interest is covered by a conductive overburden,is still a challenge for the conventional transient electromagnetic(TEM)method,which measures TEM response using induction coils as the sensor.However,sensors such as fluxgate and superconductive quantum interfere device(SQUID)magnetometers can measure the B-field directly,which can provide more reliable deep information for mineralization exploration.In this paper,we report on the research and development of our newly developed high-temperature su-perconductor(HTS)SQUID magnetometer,which is cooled by liquid nitrogen at 77 K,and its applica-tion in TEM measurement for deep exploration in a gold deposit in China.This improved SQUID magnetometer version has a good performance with noise(60 fT/√Hz),slew rate(0.8 mT/S),dynamic range(100 dB),sensitivity(6.25 mV/nT),and bandwidth(DC-20 kHz).To find deep and peripheral ore in the Baiyun gold deposit located in Liaoning Province,NE China,both the SQUID magnetometer and induction coil were used for TEM data acquisition.Results show that TEM can detect the distribution of local strata and the faults contained within them.Results also indicate that the SQUID magnetome-ter has superior response performance for response over geological targets with slower decay time when compared to the induction coil signals.The SQUID magnetometer is more sensitive at observing the induced-polarization effect which is closely related to the ore-controlling faults.

Inversion of IP-Affected TEM Responses and Its Application in High Polarization Area

The transient electromagnetic(TEM)responses measured in areas with highly pola-rizable media commonly show induced polarization(IP)effects,which poses difficulties to the TEM data interpretation and inversion with conventional methods.When present,the IP effects break the monotony in TEM decay curves and can even cause sign reversals,and lead to the singularity and non-monotony of inversion method.It is therefore important to take the IP effects into account when processing those TEM data.In this paper,a new inversion method is developed to invert TEM data with IP effects.The Cole-Cole model was introduced to the forward modeling solver so that the IP effects can be accurately calculated.The regularization terms were adapted to the objective function and a global optimization method,the particle swarm optimization algorithm,was used to solve the optimization problem,which weakened the singularity and non-monotony of the inversion greatly.Field data were collected in Wulong gold mine where IP effects were detected.The data were in-verted using the method developed in this paper,and the inversion results were able to identify the depth extension of NO.163 ore-bearing quartz veins and their associated fine-grained diorite veins.The global inversion method in this paper primarily solved the inversion problem of IP-affected TEM responses,and recovered the polarization parameters,which can be used as an aid to the geo-logical interpretation.