Time-Domain Electromagnetic Resistivity Mapping of a Fjord-Head Delta

Abstract

An electromagnetic (EM) survey was conducted on the Zeballos River delta, approximately [Formula: see text] north of Victoria, British Columbia, to determine whether EM methods can help delineate the three-dimensional architecture of a fjord-head delta. The EM data were collected using a Geonics EM-47 system with a [Formula: see text] transmitter loop. The data were downloaded and subsequently edited and interpreted using the TEMIXGL layered earth inversion package from Interpex. The interpretation provided a picture of the three-dimensional resistivity structure of the delta. Throughout most of the surveyed area, a relatively thick layer of resistive [Formula: see text] sediment (gravel and sand—delta topset and foreset) overlies more conductive [Formula: see text] sediment (silt and clay—delta bottom-set), which in turn overlies resistive [Formula: see text] bedrock. Shallow sediments in the intertidal zone are saturated with sea water and hence are very conductive. However, we were able to determine the depth to more resistive (fresh water) gravel and sand and, in some areas, what we interpret to be resistive bedrock. The results of this study indicate that EM resistivity surveying can provide constraints on sediment type and thickness which, when used in conjunction with geological mapping and borehole logs, help improve our understanding of the three-dimensional architecture of these deltas.