Assessing the presence of hard rock along a gas pipeline alignment with airborne EM

Abstract

Over the past decades, airborne electromagnetic (AEM) surveys have mostly been used in connection with mineral exploration and a variety of issues in hydrogeophysical mapping. However, increasingly, AEM is used for a wide range of geotechnical purposes: pollution mapping, geotechnical assessment on road and freeway alignments, bathymetry, and depth to bedrock. We present an investigation using a helicopterborne transient electromagnetic system along the planned trace of a gas pipeline. Oil and gas pipelines are often buried at a depth of a few meters and the cost of construction depends critically on whether the subsurface is composed of soft sediments that can be easily excavated or hard rock formations that require much heavier equipment and possibly have to be blasted. The aim of the AEM survey was to distinguish between the soft, relatively conductive sediments and the hard, relatively resistive bedrock in the upper few meters of the subsurface. Data were collected with a rather small transmitter moment, but a high repetition frequency that simultaneously allowed high acquisition speed, and reliable data quality. Measurements were inverted with 1D models with both vertical and lateral constraints to produce model sections along flight lines. A novel method of statistical analysis of the set of equivalent models for each inverted model, calibrated against boreholes, improved the estimates of the presence of hard rock along the flight lines.

Strictly horizontal lateral parameter correlation for 1D inverse modelling of large datasets

“…Since the publication of Christensen and Tølbøll ()’s work, the LPC method has been used in the inversion of airborne data from several surveys for both hydrogeophysical and geo‐technical purposes (Christensen et al . ; Christensen and Halkjær ; Christensen and Reid ; Reid J., Christensen, and Godber; Lawrie et al a, b).…”

“…Christensen and Tølbøll (2009) showed through both theoretical and field examples that the LPC method has the desired effect that well-determined parameters have more influence on the correlated models than poorly determined parameters. Since the publication of Christensen and Tølbøll (2009)'s work, the LPC method has been used in the inversion of airborne data from several surveys for both hydrogeophysical and geotechnical purposes Christensen and Halkjaer 2014;Christensen and Reid 2012;Reid J., Christensen, and Godber;Lawrie et al 2012a, b).…”

Conditional and marginal probabilities in AEM inversions using multivariate Gaussian statistics

“…We extend upon the results of Christensen and Reid (Christensen and Reid 2012) whereby, instead of using a stochastic approach to answer questions of the inversion results, we instead examine multivariate Gaussian integrals to evaluate our statistics. We show the most probable depth and thickness of a portion of one of the more productive aquifers located along the northern side of the extended borefield reticulation development at Carnarvon.…”