A new vertical continuation procedure for airborne electromagnetic field data from the modified image method

Abstract

Airborne electromagnetic (AEM) data measured by equipment in a bird tethered to a helicopter have large variations caused by the unavoidable vertical excursions of the helicopter as it traverses its flight path. Such large changes tend to mask the smaller changes in field strength caused by lateral variations in the earth’s electrical conductivity along the flight path, which is the information that is the goal of AEM surveys. Signals produced by conductivity anomalies such as sea‐ice keels and pipelines in marshes or in the shallow ocean are enhanced and may be apparent directly in the continued fields. Furthermore, electronic or environmental noise is more easily detected in the continued fields and reduced by various methods of filtering and signal processing. In the modified image method (MIM) formalism for AEM fields, the algebraic expression for the secondary to primary field ratio [Formula: see text] is given in terms of R, where R is the total complex vertical separation of the primary and image dipoles [Formula: see text] scaled to the coil spacing ρ, [Formula: see text] is the complex effective skin depth, and h is the altitude of the bird. An inverse algebraic relation gives R as a function of [Formula: see text]. In this paper we present a simple and accurate method of continuing the field by way of continuing R. Because R is linear in h, the vertical continuation of R from h to h0 is accomplished by a simple linear translation. This method is applied to a flight line of an AEM survey of Barataria Bay, Louisiana, which includes both the marsh and a near‐shore region of the Gulf of Mexico. The smoothnesss of the continued data over the Gulf implies that the variability of the continued data over the marsh is attributable to horizontal variation in salinity, soil porosity, and water depth rather than noise. To produce more accurate values for R, we have also included details of an extended half‐space renormalization function which, in effect, removes residual differences between the fields calculated from the MIM algebraic and the numerical evaluation of the exact Sommerfeld integral representations of the [Formula: see text] field.

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

“…The penetration of the field depends on the conductivity of the ground or water and the frequency of the field (Kovacs et al, 1995). Various schemes have been implemented for inverting frequency-domain AEM fields to obtain layer thickness and conductivities (see, for example, Anderson, 1979;Sengpiel, 1983;Bergeron, 1986;Won and Smits, 1987;Pelletier and Holladay, 1994;Huang and Fraser, 1996;Ellis, 1998 Good agreement has been found between modified image method (MIM) inversion results in the Gulf of Mexico with conductivity/temperature/depth (CTD) casts along flight lines and ground-based measurements in this area (Bergeron et al, 1998(Bergeron et al, , 1999(Bergeron et al, , 2001). This demonstrates the validity of the MIM inversion technique and the stability of the AEM measurement system.…”

“…Figure 1 shows the survey area with these flight lines overlaid. The AEM system primary waveform was digitally constructed from the cosine functions of six frequencies: 29 970, 11 670, 4530, 1770, 690, and270 Hz (Pelletier andWu, 1989;Mozley et al, 1991;Pelletier and Holladay, 1994;Bergeron et al, 1998Bergeron et al, , 1999Bergeron et al, , 2001. The amplitude and phase of the secondary fields at these frequencies were determined by digitally convolving the measured secondary field with the original cosine functions.…”

“…Those samples were used to calibrate the AEM fields in the lateral footprint of these points (Bergeron et al, 1998(Bergeron et al, , 1999(Bergeron et al, , 2001). …”

“…Therefore, variations in the secondary field caused by vertical excursions of the AEM system can hide changes in the field associated with changes in the conductivity along the flight path. Hence, a continuation procedure is applied to the field data to remove the altitude-induced variation in the secondary field (Bergeron et al, 1989b(Bergeron et al, , 1998(Bergeron et al, , 1999.…”

“…The survey was conducted by U.S. Navy personnel based at Stennis Space Center, Mississippi, using a Naval Research Laboratory AEM system (Pelletier and Wu, 1989;Mozley et al, 1991;Pelletier and Holladay, 1994;Bergeron et al, 1998Bergeron et al, , 1999Bergeron et al, , 2001). This survey was performed over the marsh and estuarine waters of the Barataria basin as a part of an integrated study of the hydrology and carbon cycle of the basin.…”