Geophysical exploration with audiofrequency natural magnetic fields

Abstract

Experience with the AFMAG method has demonstrated that an electromagnetic exploration system using the Earth’s natural audiofrequency magnetic fields as an energy source is capable of mapping subsurface electrical structure in the upper kilometer of the Earth’s crust. We resolved the limitations of this method by adapting the tensor analysis and remote reference noise bias removal techniques from the geomagnetic induction and magnetotelluric methods to computation of the tippers. After a thorough spectral study of the natural magnetic fields, we designed lightweight magnetic field sensors capable of measuring the magnetic field throughout the year. We also built a digital acquisition and processing system with the ability to provide audiofrequency tipper results in the field. This new instrumentation was used in a study of the Mariposa, California site previously mapped with AFMAG. This study once again demonstrates the usefulness of natural magnetic field data in mapping an electrically conductive body. Reoccupation of one of the sites in several different seasons proves the high level of repeatability of these data. As well as being repeatable, the tensor data provide additional information on the geometry of the conductive body. Different electrical conductivity features can be distinguished using a broad band of frequencies.

Helicopter EM (ZTEM-VTEM) Survey Results over the Nuqrah Cu-Pb- Zn-Au Sedex massive sulphide deposit in the Western Arabian Shield

“…4.1 km length and spaced at 100 m ( Figure 2b). The ZTEM overlapping coverage consisted of in-line (Tzx) and cross-line (Tzy) tipper AFMAG, (Ward, 1959;Labson et al, 1985) and caesium aeromagnetic data. A total 243 km of ZTEM were flown along 56 lines, averaging ~4.3 km in length, and at 200 m line-spacing (i.e., ZTEM flown over every 2 ND alternating line of VTEM).…”

Passive and active helicopter EM survey comparisons over 501 Project Cu- Zn volcanogenic massive sulphide at McFauld’s Lake, northern Ontario

“…After the 501 discovery, a few months later between June-August, 2008, VTEM (versatile time-domain electromagnetic; Witherly et al, 2004) surveys were also flown over multi blocks, including the 501 zone. This was subsequently followed by a ZTEM (z-axis tipper electromagnetic; Lo and Zang, 2008) helicopter AFMAG (Ward, 1959;Labson et al, 1985) survey in August, 2009.…”

3D inversion of ZTEM data for uranium exploration

“…One unique feature of the tipper is that the presence of lateral resistivity variations gives rise to an anomalous response (or vertical field), while 1D layered structures produce no response (Labson et al, 1985). The forward modelling of the tipper is based upon a 3D MT modelling algorithm that employs a staggered-grid finite-difference method (Sasaki, 2001).…”