Correcting airborne gravity data for overburden thickness using airborne transient electromagnetic data

A new methodology is presented that corrects gravity data for lateral changes in overburden thickness through the creation of a bedrock topography (BedTopo) map. The methodology results in a Bouguer anomaly map where bathymetry, overburden thickness, and bedrock are reduced to a reference datum. This methodology applies to airborne, ground, and gravity gradiometry surveying. Also presented is a working methodology for inverting helicopter transient electromagnetic (HTEM) survey data to resolve the overburden-bedrock contact using discrete layered-earth modelling in a highland terrain for the purpose of creating a BedTopo map. A study site with a glacial overburden within the Nechako Plateau of British Colombia, Canada is used as a case study. AeroTEM III HTEM survey data is inverted using BC Wells water well data, which provides ground-truth to guide inversion. Water well inversions show that the HTEM methodology can estimate the depth of bedrock with an sRMS % error of 61% of the total thickness indicated by the water wells. The HTEM methodology was found to be inaccurate in areas where 3D topography effects are prominent and over areas where a conductive anomaly is hosted within the bedrock. AeroTEM III HTEM survey data is inverted in order to create a BedTopo map. AIRGrav gravity data is reduced using digital elevation model, BedTopo, and bathymetry data in order to create a BedTopo corrected Bouguer anomaly map of the study site. Inversion results of the study site used 4- and 5- layer models with results showing that overburden varies in thickness from 0 to 265 m, with an average of 35 m, and an inverted resistivity that ranges from 3 to ~680 Ω•m. Bedrock resistivity ranged from 0.5 to over 10,000 Ω•m. An analysis and comparison of Bouguer anomaly maps with and without a BedTopo correction was conducted. Results show that variations in overburden thickness that are < 30 m may not be detectable by an airborne gravimeter system due to GPS noise, and that overburden thickness variations >100 m, which create anomalies larger than 3 mGal, if not considered may contribute to interpretation errors on a Bouguer anomaly map.