Free-air gravity anomalies along the inside passage of British Columbia and Alaska

Abstract

Continuous measurements were obtained by a LaCoste and Romberg surface-ship gravity meter along canals, bays, channels, and fjords between Skagway, Alaska, and Seattle, Washington. Free-air gravity anomalies were calculated along the traverses, usually at 1.5-km intervals. Seas were calm and navigation excellent during nearly all of the measurements; the uncertainty in the measurements was ±2 mgal. The free-air anomalies are between +25 and −165 mgal; in general, they are small near the coast line and become progressively more negative inland along the fjords. Most of the large negative anomalies north of Vancouver Island are caused by the topographic effect of the rugged terrain; the negative anomalies southeast of Vancouver Island in Puget Sound (northern part and near Seattle) are essentially of geologic origin. A map of the track lines and approximate free-air anomaly contours is presented.