Helicopter electromagnetic & magnetic surveying applied to a pipeline construction project in Canada

Abstract

For construction Companies, the cost of a pipe laying project in a remote region depends on the risks taken when selecting a final pipe path according to the expected ground conditions within the first three to five metres of soil. Proper evaluation of the three engineering soil type distribution: unconsolidated, swampy deposits, loose ground (easily excavated), and hard rock (require drilling and blasting), is crucial to define the construction and environmental risks. It requires a costly and time consuming on-site investigation based on outcrop mapping and hand auger drilling. At best if access and logistics permit it, discontinuous soil sampling may be carried out along the proposed pipe path. For major pipeline projects, the selection of an appropriate path with respect to the lithological conditions is ensured through a helicopter-borne geophysical investigation combining electromagnetic and magnetic techniques. Electromagnetic data depict the surficial and near-surface apparent resistivity and discriminate between conductive layers (clay, marl, graphitic schist, salty formation..) and resistive ones ( dry sand, gravel, limestone, siltstone, sandstone, quartzite, gneiss, granite and volcanic rocks) . Magnetic data allow to distinguish between sedimentary and igneous rocks based on their magnetic susceptibility, which is several decades higher for igneous rocks. Methodology: a primary electromagnetic field is generated through transmitting coils using different frequencies in the 400 Hz – 100 000 Hz range. In a conductive body, eddy currents induce a secondary field, the component values of which (phase and amplitude) depend on the soil conductivity and are measured by a receiving coil. Transmitted frequency and depth of penetration share an inverse relationship whereby the higher the frequency, the shallower the depth, as expressed by the following formula: P = ½ Π √ 10 ρ/f where P is the depth of penetration (km), ρ is the ground resistivity (Ωm) and f is the frequency (Hz). This also means that the higher the soil conductivity (1/ρ), the shallower the penetration. The total intensity of the geomagnetic field is measured with a cesium vapor magnetometer simultaneously. Dighem method: the geophysical instruments are housed in a 8 metre long « bird » which is towed 30 m below a helicopter . Five pairs of coils (two coaxial and three coplanar) constitute the Dighem V electromagnetic system. A Geometrics 822A magnetometer is located between the transmitting and receiving coils. The helicopter flies lines at an altitude of 60 m and a speed of 100 km/h, maintening a ground clearance of 30 m for the bird. Along each line, an electromagnetic measurement is taken every tenth of a second, which represents a distance on the ground of about 3 m. The lines are positioned using an on board Ahstec GG24 GPS receiver, giving a lateral accuracy of about one meter.