Question 9 of 10: Reflection Seismic Source/Sensor Combinations — Engineering, Petroleum and Marine, and Reaching the Moho

Question 9: Reflection Seismic Source/Sensor Combinations — Engineering, Petroleum and Marine, and Reaching the Moho (16.66% of paper)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Shallow engineering/geotechnical survey. Source: a small, low-cost, repeatable impulsive source such as a sledgehammer struck on a steel plate, or an accelerated weight-drop unit, generating a broadband, high-frequency pulse; sometimes a small Betsy-gun (shot-cartridge-fired) source for slightly greater energy. Sensors: closely spaced (1–2 m), high-frequency (100 Hz or higher natural frequency) vertical geophones on a short spread (tens to a couple hundred metres), often deployed as a land streamer for rapid, repeated coverage. The whole system favours high vertical resolution over penetration, appropriate for imaging the shallow tens-of-metres depth range relevant to foundations, pipelines or shallow bedrock.

b) Deeper petroleum exploration survey. Source: multiple heavy Vibroseis trucks sweeping simultaneously (or, in remote/environmentally sensitive settings, dynamite in shot holes) to deliver much greater seismic energy at lower dominant frequency, needed to penetrate kilometres of section with acceptable signal-to-noise. Sensors: large arrays of many low-frequency (10 Hz or lower) geophones summed per receiver group to boost signal and attenuate random noise, recorded on high channel-count telemetry systems along spreads kilometres long, giving the large offsets and long two-way travel times (several seconds) needed to image reservoir-depth targets.

c) Marine reflection survey. Source: an array of airguns (releasing compressed air to generate a repeatable, broadband acoustic pulse in the water column), fired in a tuned array to reinforce the primary pulse and suppress the bubble-pulse signature. Sensors: hydrophone streamers (pressure-sensitive sensors in a fluid-filled cable), one or several towed behind the vessel at fixed depth, requiring no ground coupling at all — a key practical advantage of marine acquisition, since streamer positions can be maintained precisely by GPS and the whole spread simply repositioned by sailing a new line, rather than physically re-planting geophones as on land.

Reaching the Moho at 35 km. Ordinary petroleum-scale reflection specifications (b) are far too shallow-penetrating for crustal-scale imaging. Modifications needed: much larger source energy with more low-frequency content (several Vibroseis trucks in unison sweeping roughly 10–50 Hz rather than the higher bands used for reservoir imaging, or large explosive charges in deep shot holes, since high frequencies are strongly attenuated over tens of kilometres); longer receiver spreads (of order 10–20 km for near-vertical-incidence profiling) to record the offsets needed for a target this deep, often supplemented with true wide-angle/refraction recording at offsets of 100 km or more rather than near-vertical reflection alone; much longer recording times (two-way travel time to 35 km at typical crustal velocities is on the order of 10–12 seconds, versus a few seconds for petroleum targets); wider receiver-group and shot-point spacing (trading some lateral resolution for the much greater required penetration and practical survey cost/logistics over such a large area); and specialized deep-crustal processing (velocity analysis and stacking tuned to near-vertical-incidence Moho arrivals, and careful multiple suppression given the very long record length) — the same general approach used in Canada's LITHOPROBE crustal reflection and refraction transects.