What lies beneath: geophysical mapping of a concealed Precambrian intrusive complex along the Iowa-Minnesota border
This paper describes the geologic interpretation of airborne magnetic and gravity gradient data, constrained by two boreholes and regional geologic concepts, in the vicinity of Decorah, Iowa. The focus is on poorly understood crystalline Precambrian rocks that are buried under several hundreds of meters of younger sedimentary rocks. Several different geologic elements are interpreted, including a Yavapai-age gabbro intrusion, dikes of the Midcontinent Rift system, and a series of mafic-ultramafic and silicic-intermediate intrusions. In particular, a circular intrusion in the vicinity of Decorah is noted for its geophysical similarities to intrusions in Canada that host nickel, copper, and platinum group element deposits.
The U.S. Geological Survey, in collaboration with the University of Iowa IIHR - Hydroscience and Engineering, evaluated the use of the Ott Hydromet Qliner using laboratory flume tests along with field validation tests. Analysis of the flume testing indicates the velocities measured by the Qliner at a 40-second exposure time results in higher dispersion of velocities from the mean velocity of data collected with a 5-minute exposure time. The percent data spread from the mean of a 100-minute mean of Qliner velocities for a 40-second exposure time averaged 16.6 percent for the entire vertical, and a 5-minute mean produced a 6.2 percent data spread from the 100-minute mean. This 16.6 percent variation in measured velocity would result in a 3.32 percent variation in computed discharge assuming 25 verticals while averaging 4 bins in each vertical. The flume testing also provided results that indicate the blanking distance of 0.20 meters is acceptable when using beams 1 and 2, however be am 3 is negatively biased near the transducer and the 0.20-meter blanking distance is not sufficient. Field testing included comparing the measured discharge by the Qliner to the discharge measured by a Price AA mechanical current meter and a Teledyne RDI Rio Grande 1200 kilohertz acoustic Doppler current profiler. The field tests indicated a difference between the discharges measured with the Qliner and the field reference discharge between -14.0 and 8.0 percent; however the average percent difference for all 22 field comparisons was 0.22, which was not statistically significant.