Open Collections

On recovering distributed induced polarization information from time-domain electromagnetic data

University of British Columbia

The electrical conductivity of earth materials is frequency-dependent. This is due to a phenomenon known as induced polarization (IP), wherein electrical charges build-up under the application of an electric field. Macroscopically, rocks may be considered chargeable, as they act like electric capacitors. The goal of this thesis is to show how IP data can be extracted from time-domain electromagnetic (TEM) data, then inverted to recover information about chargeable targets. Although both frequency and time-domain electromagnetic (EM) surveys measure IP signals, this dissertation will focus solely on TEM. To recover chargeability information, the following TEM-IP inversion workflow is developed. (1) Extract a background conductivity model that is assumed free of IP signals. (2) Decouple the TEM and IP signals by subtracting the fundamental responses estimated using the background conductivity. (3) Invert the resultant IP data to recover pseudo-chargeabilities at multiple times for a set of 3D volumes. This is used to infer the location and dimensions of chargeable targets. (4) Carry out further analyses of pseudo-chargeabilities at multiple times to estimate intrinsic parameters such as Cole-Cole chargeability and its associated time constant. For grounded sources, the workflow is implemented for a synthetic gradient array example. Results show that the early time signals, which are often discarded, can be used to estimate the background conductivity. Applying the workflow to inductive sources such as airborne EM (AEM) is more challenging, as steady-state electric fields are not produced. This was overcome by developing an IP function which (1) accurately characterizes how electric fields from inductive sources behave in the earth and (2) allows the recovery of a 3D chargeability by solving a linear inverse problem. The efficacy of the aforementioned approach is validated using field AEM surveys over the Mt. Milligan porphyry deposit in British Columbia and Tli Kwi Cho kimberlite deposit in the Northwestern Territories. For the kimberlite deposit, the recovered chargeability information is able to distinguish two distinct kimberlite units. To validate the approach, a 3D rock model for Tli Kiw Cho is constructed using the recovered chargeability and background conductivity. This model is compared against geological models obtained through drilling and shows good agreement.

Text

2018-03-05

Attribution 4.0 International

http://hdl.handle.net/2429/64757

Geophysics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by/4.0/