Resolution of airborne VLF data
2006 (English)In: Journal of Applied Geophysics, ISSN 0926-9851, Vol. 58, no 2, 158-175 p.Article in journal (Refereed) Published
The interpretation of airborne VLF data represents an important aspect of geophysical mapping of the upper few hundred meters of the Earth's crust, especially in areas with crystalline rocks. We have examined the ability of the single frequency VLF method to provide quantitative subsurface resistivity information using two generic models and standard airborne parameters with a flight altitude of 70 m and a frequency of 16 kHz. The models are long thin conductor (10 in thick, 10 Omega in resistivity and 1 km long) and a wider buried conductive dike (100 Omega m resistivity and 500 m wide). Using standard regularized inversion it turned out that for both models the conductivity of the conductors are underestimated and the vertical resolution is rather poor. The lateral positions of the minimum of the resistivity distributions coincide well with the true positions of the shallow conductors. For deeper conductors the position of the minimum resistivity moves from the edges of the conductor into the conductor. The depth to the minimum of the resistivity anomalies correlates well with the true depth to the top of the conductors although the latter is always smaller than the former.
Interpretation of field airborne data collected at 70 in flight height resolved both small scale and large scale near surface conductors (conductance similar to 1 S). Deeper conductors show up in the VLF data as very long wavelength anomalies that are particularly powerful in delineating the lateral boundaries of the conductors. Many of the VLF anomalies in the Stockholm area are dominated by these deep conductor responses with some near surface conductors superimposed. The deep conductors often follow topographic lows coinciding with metasediments. We interpret the frequent absence of near surface responses at 70 in flight height as a result of weak coupling between the primary VLF wave and the small scale (in all three dimensions) near-surface conductors.
Radio magnetotelluric (RMT) ground measurements were carried out along a short profile coinciding with part of an airborne profile. Using data at 9 frequencies (14-250 kHz) small scale conductors in the upper few tens of meters, not identified from the airborne data, could be well resolved. Large scale deeper conductors could be identified by both methods at nearly the same positions.
Place, publisher, year, edition, pages
2006. Vol. 58, no 2, 158-175 p.
airborne, RMT, modeling, tipper, 2D, VLF
Earth and Related Environmental Sciences
IdentifiersURN: urn:nbn:se:uu:diva-91557DOI: 10.1016/j.jappgeo.2005.06.002ISI: 000241096600006OAI: oai:DiVA.org:uu-91557DiVA: diva2:164330