Data collection and reduction procedures for 1900 km of total intensity magnetic field data collected with a truck-mounted system in southeastern Arizona, southwestern Colorado, and northwestern Wyoming

Abstract

This report details the acquisition, reduction, and availability of total intensity magnetic field data collected with a truck-mounted magnetometer from selected regions of southeastern Arizona, southwestern Colorado, and northwestern Wyoming. This dataset is comprised of approximately 381,000 observations along about 1,900 line km of profiles. The dataset and the computer software used in the data reduction are available to users via anonymous FTP on the Internet or World Wide Web servers. Data Acquisition, Reduction, and Accuracy Data Acquisition Total intensity magnetic field data were acquired from locations in southeastern Arizona, southwestern Colorado, and northwestern Wyoming (Fig. 1) during several field excursions, each lasting from a few days to several weeks in the period 1989 to 1993. Much of the Arizona data are located in the Santa Catalina, Galiuro, Santa Rita, and Patagonia Mountains, and the Tucson, San Rafael, and San Pedro basins. Colorado data were taken in the western San Juan Mountains in the Silverton Caldera area and on the Uncompahgre Plateau. The data from Wyoming come from the Wood River area including the Kerwin porphyry in the Absaroka Mountains. The reduced data are archived by year and the size and filename location of data from the various areas are shown in Table 1. Data were collected by the use of a proton precession magnetometer (Geometries airborne magnetometer, model G-811) mounted on a four-wheel drive utility vehicle, with an average sample spacing of 5 meters and a frequency of 1 reading per second. The magnetometer sensor is at a nominal height of 3.8 meters above the surface and 4.6 meters behind the truck. The sensor is compensated against magnetic effects from the truck and magnetic field variation due to heading. However, since the required compensation is a function of magnetic latitude, an additional software heading corrections is also done as part of the data reduction sequence described below. The compensation with software requires that the truck system be used to collect heading versus azimuth data at a magnetically quiet location within the survey area, thus saving the considerable effort required to physically recompensate the system before every survey. The magnetic data are collected onto a computer real-time along with one-dimensional position data, and documentation points used to precisely locate in the dataset features such as navigation points, man-made magnetic sources such as culverts and power lines, and geologic features. The data recorded also include the time of each observation to the nearest second and a fixed 8 character numeric string set by thumbwheel switches which uniquely identifies each dataset. The collected data are written onto floppy diskette media for processing in the office. The data collection configuration is illustrated in Figure 2.