ADDITIONAL CORRECTIONS APPLIED TO ORIGINAL DATA SET:

In the course of interpretation of the data set provided by Dighem, we became aware of errors in the data which caused difficulty in obtaining reliable layered-earth inversion models (Deszcz-Pan et al., 1998). These errors were traced to problems with calibration and problems with the bird tow-cable length.

Calibration errors were corrected using a procedure described in Deszcz-Pan et al. (1998), outlined in Figure 1. Calibrations are performed on the ground at a site that is assumed to have negligible conductivity. Calibration consists of three parts: 1) adjustment of the system gain, 2) phasing of the receiver coil, and 3) adjustment of the zero level. The first two steps are typically completed on the ground, while step 3 is done with the bird at altitude so that the influence of the ground is removed.

The gain is calibrated by placing a Q coil of known dimensions and electrical properties at a specified distance from the bird receiver coil (R). Currents induced in the calibration coil produce an offset on the chart recorder (Figure 1a). The gain of the electronics is adjusted to make the recorded deflection agree with the theoretically calculated value. The calculated value is usually computed assuming that the conductivity of the half-space below the measurement site is negligible. When the resistivity at the measurement site falls below 100 ohm-m errors of greater than 5 percent are introduced into the calibration especially at frequencies above 50 kHz (Fitterman, 1997, 1998). Errors are also introduced by imprecise positioning of the calibration coil. Positioning errors can be eliminated through the use of jigs which rigidly hold the calibration coil at the proper location with respect to the bird. The gain was adjusted once at the beginning of the survey.

The phasing adjusts the receiver time-base so that the inphase signal is synchronous with the transmitter wave form. This is done by placing a ferrite bar next to the receiver coil (R) and rotating so that it is maximally coupled to the primary field of the transmitter. This configuration should only produce an inphase signal. The phase f is adjusted so that the quadrature signal is zero. For this survey, phasing was adjusted daily.

Sixty-four time-domain electromagnetic (TEM) sounding were collected along or near selected helicopter flight lines and inverted (Fitterman et al., 1999) to obtain the resistivity-depth structure at the sounding location (Figure 1c). Using the radar altimeter data from the HEM survey to estimate the bird height and the resistivity-depth function from 44 TEM inversion and 11 induction logs from nearby wells, the computed HEM response ( ) is determined.

The tow cable length is adjusted for lift effects which are a function of the airspeed of the bird. This was based upon an average survey flying speed and an airspeed-lift relationship provided by Dighem. The correction for lift effectively shortened the tow cable by 2.9 m. This correction, as well as a second one to compensate for the use of a shorter tow cable, resulted in an overall decrease in tow cable length of 5.3 m. The altitude of the bird is then determined by subtracting the tow cable length from the radar altimeter reading. The radar altimeter is mounted on the helicopter so that there is a small error due to bird swing. To avoid errors due to radar reflections from trees, the TEM soundings were made in clear areas—usually freshwater marshes.

A least squares estimation technique is used to determine a correction factor consisting of gain ( ), phase (), and bias terms ( ) (see Figures 1d and 1e). The corrected data are then obtained through the relationship:

where is the observed data and is the corrected data. Finally, the corrected HEM data are inverted to determine the resistivity-depth function throughout the survey area (Figure 1f). (See Fitterman and Deszcz-Pan (1998) for an example of the results.) Usually the bird height is solved for in the inversion.

The correction procedure is slightly more complicated than described here as various parameters of the correction factors need to be computed using subsets of the entire HEM and TEM data sets. This is required because the setting of gain, phasing, and zero levels is determined by survey logistics.

Separate gain correction factors ( ) were computed for each coil-pair. Because the gain of the system was determined only at the beginning of the survey, the same correction factors were used throughout the survey. These factors are summarized in Table 1.

Phase was adjusted in the field daily, so phase corrections were computed for each day of the survey. The phase corrections are summarized in Table 2. On the fourth day of the survey (12 Dec 1994), the phase corrections have been set to zero because of insufficient time-domain soundings along the flight lines for that day. The largest phase adjustment was 3.1^o for the 874-Hz horizontal coplanar channel.

Bias corrections were always set to zero because insufficient TEM data. Furthermore, HEM processing usually requires shifting of line levels to remove effects of offsets in zero levels due to instrument drift. These shifts are usually adjusted to produce smooth apparent resistivity maps, thus the measurements are subject to a somewhat arbitrary bias making estimation of it through our correction procedure superfluous (Huang and Fraser, 1999).