Interpretation of airborne electromagnetic data using the modified image method

Abstract

The modified image method is used to invert active electromagnetic (AEM) data from a 1984 U. S. Navy survey of Cape Cod Bay. The high‐frequency data (7200 Hz) give a robust value for the altitude of the helicopter‐towed AEM bird and for the first‐layer skin depth and, hence, for the first‐layer conductivity. The inversion of low‐frequency (385 Hz) bottom‐probing signals produces more noise‐sensitive estimates for the water depth and for the conductivity contrast K, the ratio of the bottom to water conductivities. The results show good agreement with “ground‐truth” radar altimeter, sea conductivity, and sonar depth data. To demonstrate the accuracy of the modified image method of inversion, we incorporate ground‐truth measurements along a flight line and the experimental frequencies in a forward Sommerfeld calculation to generate synthetic data which then are inverted using this technique.

MIM and nonlinear least‐squares inversions of AEM data in Barataria basin, Louisiana

“…The MIM and both of the nonlinear least-squares two-layer models are in very good agreement for the layer depth over water. If the first-layer parameters are approximately correct in a two-layer model, the second-layer conductivity can be attributed to the conductivity of the lower half-space (Pelletier and Holladay, 1994;Bergeron et al, 1989aBergeron et al, , 1998Bergeron et al, , 1999. In areas where the water depth is at least 1.5 m, both nonlinear least-squares twolayer models agree well with the MIM results for the first-layer conductivity, as shown in Figure 2b.…”

“…The computed value of R is given in terms of the measured field. For a horizontal coplanar transmitter and receiver coil configuration, R is related to the ratio Z of the secondary and primary fields (Bergeron et al, 1989a(Bergeron et al, , 2001 as…”

“…The modified image method is an algebraic representation of the secondary field computed from the measured fields (Bergeron, 1986;Bergeron et al, 1989aBergeron et al, , b, 2001; therefore, it is a computationally fast, efficient method of calculating layer thicknesses and conductivities. In the MIM representation the source of the secondary field is an approximate image of the primary dipole located at a complex depth below the earth's surface.…”

Multilayer MIM inversion of AEM data: Theory and field example

“…Relative locations of the transmitting dipole (coil), receiver coil, and image dipole; h is the altitude of the bird, ρ is the coil spacing, δ eff is the effective complex skin depth. from numerical evaluations of the Sommerfeld integral, are essentially removed by correction factors which bring the MIM field into excellent agreement with the Sommerfeld field for 2h/δ > ∼ 1 (Michel, 1986;Bergeron et al, 1989). The first of these is a half-space correction, which we call a renormalization function F, defined as the ratio of the Sommerfeld to the MIM field for half-space models: …”

“…This result is approximately correct only for A = 2h/δ > 1 (Bergeron, 1986;Wait, 1991). The inverse relationship which gives R in terms of the measured data field Z (data) is given as follows (Bergeron et al, 1989).…”