LAUSR

Abstract Magnetotelluric (MT) soundings were conducted in a 4 × 4 km area at the Aquistore CO2 sequestration site at Estevan, Saskatchewan, Canada in 2013, 2014 and 2015, prior to CO2 injection. The MT response is locally one-dimensional but spatially variable at short periods ( 30 s). Responses corresponding to Williston Basin rocks are uniform across the area. A representative MT response was inverted using constraints derived from a resistivity well-log. Beneath the Jurassic Watrous to Vanguard formations (at depths greater than 1240–1600 m), the resistivity of the inversion model is consistently 20–30% lower than in the reference well-log model, consistent with the MT method sensing the longitudinal resistivity of the more strongly stratified units at these depths. Electromagnetic noise in the MT data set includes high-frequency odd harmonics of the 60 Hz source and a broad-band source, spatially associated with the CO2 pipeline, observed only in 2014. Within the period range 10−4 to 102 s, but outside the bands influenced by the broad-band noise, the off-diagonal MT impedance response at Aquistore can be measured with an RMS repeatability of 1% or better.