A transient AMT (TAMT) survey was conducted by EMpulse Geophysics at Pasfield Lake, 545 stations were collected at a nominal spacing of 250 m on seven parallel lines, spaced approximately 1500 m apart with each line approximately 20 km long.
Initial interpretation based on two-dimensional (2D) inversions were helpful but the 2D analysis could not replicate the measured data in the structurally complex geologic conditions near the line extremities. Three-dimensional (3D) inversion of the TAMT data was a significant step forward, improving the interpretational power of the TAMT data set. The 3D inversion, done one line at a time, was able to reproduce the measured data for the entire line and produced models in excellent agreement with drill defined lithology and integrated very well with completely independent 3D inversion of airborne potential field data. Grid based 3D inversion to be shown soon.
Please click here to download the results of 3D inversion of our Pasfield TAMT data-set, presented at the 2010 SEG in Denver (4.5 MBytes).
Pasfield Lake is an intriguing project as all the geophysics and drilling to date are consistent with the hypothesis that Pasfield Lake is itself an impact crater.
Regional depth to basement in the Pasfield Lake area is 900 m while metapelitic basement material was found at just over 200 m depth under Pasfield Lake indicating a basement uplift of at least 700 m. This may correspond to a central uplift or "rebound" structure that is common with larger, so called "complex" craters.
The possible central uplift is seen very well in the 3D inverted TAMT data and "ties in" with completely independent 3D inversion of airborne magnetic data, which also responds very well to the proposed uplift. Airborne gravity data produces a 15 mgal low over the Pasfield structure but appears to essentially outline the crater as a whole.
Discrete contact type tipper anomalies are seen near the west shore of Pasfield, this may correspond to the Cable Bay Shear Zone but also to discrete "steps" down into the crater, such terraced walls again can occur with larger "complex" craters.
Large amounts of brecciation and fracturing were seen in drilling, sometimes with extremely poor core recovery. This made the typical exploration model difficult to apply at Pasfield Lake since conductive anomalies in the sandstone were several times not due to clay alteration but rather due to intense fracturing (porosity).
Anomalously deep water at Pasfield Lake, over 220 m in select spots and in general more than 100 m. This is much deeper than the typical 20 m or so water depths encountered in most northern Saskatchewan lakes.
Please click here for a paper, also presented at SEG 2010 in Denver, which attempts to answer the question, is Pasfield Lake a meteorite impact crater ?
