Fast 3D forward modeling of the magnetic field and gradient tensor on an undulated surface

Magnetic field gradient tensor technique provides abundant data for delicate inversion of subsurface magnetic susceptibility distribution. Large scale magnetic data inversion imaging requires high speed and accuracy for forward modeling. For arbitrarily distributed susceptibility data on an undulated surface, we propose a fast 3D forward modeling method in the wavenumber domain based on(1) the wavenumber-domain expression of the prism combination model and the Gauss–FFT algorithm and(2) cubic spline interpolation. We apply the proposed 3D forward modeling method to synthetic data and use weighting coefficients in the wavenumber domain to improve the modeling for multiple observation surfaces, and also demonstrate the accuracy and efficiency of the proposed method.

Using electrogeochemical approach to explore buried gold deposits in an alpine meadow-covered area

Exploration for buried gold ores and other deeply buried ores, especially in high altitude localities, is one of the tough challenges facing the geological world today.Fast and efficient ore prospecting methods are badly needed to deal with the situation. This paper documents a test that, for the first time, uses an electrogeochemical approach to prospect ores in the alpine meadow-covered Bangzhuoma area and its periphery in Qinghai–Tibet Plateau.The results were compared with conventional soil measurements from a 2 D prospection, and an ideal model of electrogeochemical anomaly formation in the area was established based on the comparison in order to provide theoretical guidance to buried ore prospecting in areas with similar conditions. The research shows that:(1) For exploration of deeply-buried mineral deposits, an electrogeochemical approach is better than soil measurements in terms of correspondence between element content values and anomaly forms and spatial distribution of known deposits in sections. Anomalies of high to low temperature element associations(Bi–Mo; Au–Ag–As–Bi and Au–Ag)and clear zonation were also observed along vertical vein runs in the sections. Based on integration of the observation with geological characteristics of the sections, we propose to use Au, Ag and As as the electrogeochemical indicators and Bi and Mo as the electrogeochemical tracing elementsto guide further analysis.(2) Judging from element statistics and the scale, intensity, and range of anomalies in plan maps, we found that an electrogeochemical approach is less affected by topography and secondary actions. The plan maps also show that elemental differentiation coefficients of the study area are in an ascending order of Ag(0.67) \ Mo(0.85) \ Bi(0.97) \ Au(1.51) \ As(2.35),better representing the element distribution in the area and yielding more striking and concentrated anomalies for known deposits than that of the soil measurements. Apart from that, electrogeochemical anomalies were observed in the south of lines 002 and 003 and the central of lines 008 and 009. We suggest the existence of deeply-buried mineral deposits based on analyses of element combinations and gold grade variations in samples taken from exploratory trenches in the area.(3) A three-stage(referring to the ore body dissolution, the mineralogenetic particle migration, and the mineralogenetic particle unloading) electrogeochemical ideal model was established for the study area, which takes into account moderate rainfall, high altitude, low air pressure, well-developed vegetation and roots, and an Upper Triassic Nieru Formation carbonaceous sandy slate as overburden.