- journal_title：Geophysics
- Contributor：Bernhard Siemon
- Publisher：Society of Exploration Geophysicists
- Date：2012-
- Format：text/html
- Language：en
- Identifier：10.1190/geo2011-0371.1
- journal_abbrev：Geophysics
- issn：0016-8033
- volume：77
- issue：4
- firstpage：WB71
- section：Near-surface EM
Frequency-domain helicopter-borne electromagnetic (HEM) data are commonly interpreted using quasistatic approximations in forward and inverse solutions. At high-frequencies (<mml:math><mml:mrow><mml:mi>f</mml:mi><mml:mo>></mml:mo><mml:mtext>some</mml:mtext><mml:mrow><mml:mo> </mml:mo></mml:mrow><mml:mn>10</mml:mn><mml:mrow><mml:mo> </mml:mo></mml:mrow><mml:mrow><mml:mo> </mml:mo></mml:mrow><mml:mi>kHz</mml:mi></mml:mrow></mml:math>f>some 10 kHz), the accuracy of this approach is often insufficient. Implementation of a full solution of the forward problem that includes displacement currents in both subsurface and air, however, may cause singularity problems during numerical evaluation of the secondary field integral using Hankel or Laplace transforms, particularly at high frequencies. These effects can be reduced by a wavenumber shift during numerical evaluation, a transformation of the integral by partial integration, or a combination of both. Based on these corrections, the resulting forward responses obtained with various fast Hankel transforms or numerical Laplace transforms are nearly identical. The inversion of HEM data also requires some modification. For a half-space inversion, the parameter sets, such as look-up tables, linear or polynomial approximations have to be modified for frequencies above some 10 kHz. The quasistatic formulas used in multi-layer inversion procedures have to be replaced by the accurate formulas. Fortunately, singularity problems are restricted to the forward calculation as the derivatives of the Jacobian matrix are not affected. Accurate high-frequency forward and inverse modeling enables a successful usage of helicopter-borne electromagnetics for near-surface investigations and expands the range of applications in environmental and geotechnical issues.