Introduction
Summary

In 2007, three time-domain electromagnetic (TEM) airborne surveys were flown for Uranium One in northern Arizona over thick, generally flat-lying sedimentary sequences. Each survey was flown with a different system (MegaTEM, GeoTEM, and VTEM), and data was also collected with each system over a test area for calibration. Ground TEM data was later collected at this site in 2008. The availability of data from three airborne TEM surveys at the test area allows for a unique opportunity for a comparison between these systems and with TEM ground methods. The purpose of our study was to determine whether a single 1D model could be found that is consistent with all of the EM data available at the test area and geological data, as well as to understand the differences in resolution between the different systems.

Airborne TEM is a popular geophysical method in mineral exploration, allowing large areas to be surveyed. The data is typically mapped to identify anomalies of interest, and modeling and inversions may be utilized to understand the response. Ground geophysics is used to develop a better understanding of the structure. A model that fits the airborne data can be compared with ground results to see if they agree, and attempt to determine the reason for any differences. While the ground and airborne surveys differ in their resolution, the general structure that they find should be consistent. The collection of airborne data is complicated by the movement of the plane, and it is important to have confidence that what is seen on the ground is actually what is being measured in the air.

Several different airborne TEM systems exist, including fixed-wing systems such as MegaTEM and GeoTEM (Fugro Airborne Surveys), and helicopter systems such as VTEM (Geotech Ltd.) and AeroTEM (Aeroquest) with in-loop receivers but there are few studies to determine the quantitative differences between these when mapping sedimentary environments.

Electromagnetic Data
Geologic Setting

The test site is located on the so-called North Rim some distance from the Grand Canyon, an area that is actively being explored for breccia pipe uranium deposits. The host environment for the breccia pipes is a sequence of sedimentary rocks including limestones, sandstones, and shales.

At the surface is the Moenkopi Formation, comprised of sandstone and siltstone. Below the Moenkopi are the Kaibab Limestone and Toroweap Formation, which include limestone, sandstone, and gypsum. The Coconino Sandstone, which is quite thin at the test site, and the Hermit Shale underlie these. Below the Hermit Shale is a series of formations known as the Supai Group, the uppermost of these formations being the Esplanade Sandstone.

Information on the geology of the area is available from site work by Uranium One just south of the test area. Drill logs extend into the Hermit Shale.

The following data were collected at the test site: Ground Data: 1. Fixed Loop TEM collected with a Protem system using a TEM67 transmitter (Geonics) in May 2008. 400 m x 400 m loop, centered at (750E, 5200N). Data was collected on two north-south lines (650E and 750E) between 2900N and 6000N at 100 m station spacing. Base frequency was 30 Hz, and all three components were collected.

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