Marine Seismic Survey Airgun Operation Explained

Depth conversion is the process of converting seismic data from time domain to depth domain. This requires velocity information about the subsurface rocks. Common methods to obtain velocity data include Common Depth Point (CDP) analysis and Common Midpoint (CMP) analysis. Depth conversion allows geologists to interpret subsurface structures at actual geological depths rather than two-way travel times.

Seismic data acquisition involves sources, receivers, cables, and stations. Raw data contains multiple wave types: direct waves (straight lines), reflected waves (hyperbolas), refracted waves (offset straight lines), and surface waves. Environmental vibrations from trees and machinery add noise. Processing separates useful signals from noise using filters and characteristic wave patterns. Modern software enables rapid processing with mouse clicks, replacing manual hand-drawn analysis. Time records are converted to depth records using known velocities, transforming temporal sections into geological cross-sections for interpretation.

Seismic data can be filtered by depth using the 'awk' command with conditional statements. The command checks specific columns in the data file (e.g., column 5 for depth) and extracts data meeting certain criteria (e.g., depth ≤ 60 km for shallow earthquakes). This creates separate data files for different earthquake depth categories.

Seismic data processing involves: (1) Data acquisition - recording raw seismic signals; (2) Pre-processing - filtering, noise reduction, and signal enhancement; (3) Migration - correcting for wave propagation effects; (4) Interpretation - identifying geological structures. Noise sources include equipment noise, environmental noise (wind, traffic), and natural seismic noise. Noise reduction techniques include stacking multiple traces, filtering, and using directional geophones. High-quality seismic data requires careful noise management throughout the survey process.

The main objectives of seismic data processing include: (1) Removing the effects of the input signal to isolate the reflectivity series, (2) Determining velocity functions to convert time axes to depth axes, (3) Determining acoustic impedances of different layers to understand subsurface properties. These processing steps are essential for obtaining interpretable seismic data that reveals subsurface geological structures.

Marine seismic surveys implement comprehensive environmental protection measures including: (1) 500-meter mitigation zones monitored by marine mammal observers, (2) Passive acoustic monitoring during nighttime and poor visibility, (3) Seabird observers from academic institutions, (4) Fishery liaison officers for local industry coordination, (5) Chase boats maintaining safety perimeters. Marine life actively avoids survey areas, with dolphins, seals, and penguins relocating outside the mitigation zone. Research spanning over 50 years by the US Bureau of Energy Management found no documented evidence of adverse effects on marine mammal populations or coastal communities. Once surveys conclude, marine life returns to normal patterns within days to weeks.

Since the late 1980s, marine scientists observed sharp increases in sperm whale strandings coinciding with intensified oil and gas exploration using seismic surveys. These operations use huge pressure cannons that produce air bubbles collapsing to send pressure waves shaking the ocean floor thousands of meters deep. Sound travels through sound channels between water layers of the same temperature, radiating sideways for thousands of kilometers. Marine biologists observed that strandings always occurred during winter migration when bulls returned to their Azorean clans. Laboratory analyses showed whales appeared healthy, yet they abandoned traditional routes toward shallow North Sea waters at depths of only 200 meters, suggesting acoustic disturbance as a primary driver of this behavioral shift.

Seismic surveys impact marine mammals through three documented categories: (1) Physical/physiological effects including temporary and permanent threshold shifts in hearing, and potential decompression sickness from rapid surfacing; (2) Behavioral disruption including avoidance of noise areas, changes in vocalization patterns, and altered movement; (3) Indirect effects such as prey displacement that may break up krill or plankton balls, making feeding more challenging. The impacts can occur over tens or hundreds of kilometers. However, responses are highly variable by species, time of year, water depth, and latitude, making it difficult to establish hard-and-fast rules for mitigation.

Seismic surveys produce sound only approximately 1% of the time (about 1/10th second pulses every 10 seconds), with operations stopped 20-30% of the time for maintenance, weather, and mitigation measures; the actual surveyed area is extremely small compared to permitted zones, and marine mammals are sparsely distributed and often avoid the sound source, resulting in minimal environmental impact on marine life.

Marine seismic surveys, which use airgun arrays to map geological structures beneath the ocean floor, generate significant low-frequency acoustic noise that can potentially impact marine life, including physical effects (hearing damage), behavioral changes, and indirect effects on food sources; however, comprehensive research by Geoscience Australia demonstrates that impacts are highly context-dependent and vary significantly across species, with no universal negative effects observed in all cases, emphasizing the need for site-specific monitoring and integrated scientific approaches to balance resource exploration with environmental protection.

Vibroseis is a seismic source technology developed in the 1950s that provides a safer and environmentally friendly alternative to explosive sources by using vibrating sources that impart moderate energy over longer time periods, achieving comparable signal-to-noise ratios through multiple sources and signal stacking, while avoiding the costs, regulations, and safety concerns associated with explosive sources.

This comprehensive section traces the evolution of marine seismic source technologies from early water guns through modern innovations. It covers conventional air gun arrays with their bubble dynamics, the Gigan source achieving 20% primary-to-bubble ratios, and the Blueprint source mitigating high-frequency footprints. The presentation then transitions to the e-seismic continuous wave field method, which distributes small air guns continuously rather than firing simultaneously, enabling better source-side sampling and reduced environmental impact. Marine vibrators are introduced as the controlled-source alternative, transmitting energy over 10-30 second periods instead of releasing 99% of energy in the first 30 milliseconds as impulsive air guns do. This fundamental difference enables significantly lower environmental impact while maintaining comparable subsurface illumination. The section concludes with the JIP vibrator system (2,500 kg units, 36 per survey) and Aqua Vib (shallow water deployment), demonstrating practical implementations achieving 20+ dB lower peak amplitudes than conventional air guns.

Vibroseis (Vibroseis®) is a patented vibratory source that is highly repeatable, linear, and inexpensive for large-scale surveys. It provides frequency control from 10-200 Hz and can image reflectors through the entire crust. However, trucks cost about $500,000 each, and mobilization costs $80,000 per truck. Mini Vibroseis (Mini Sosie) uses pavement whackers as low-cost vibratory sources, though only one crew remains in the US. Marine vibrators and borehole vibrators can operate at kilohertz frequencies.

Marine seismic surveys use scientific vessels towing cables carrying geophones and air guns. The air guns expel high-pressure compressed air, generating waves that travel through the seabed and subsurface formations. These waves bounce back to be identified by geophones and analyzed by geologists to map underwater geological structures.

Research conducted in late 2021 tested alternative seismic source technologies designed to reduce impacts on marine life. Animals exposed to these alternative sources showed significantly better responses compared to conventional air guns, with animals appearing 'a lot more happy.' This suggests that technological modifications to seismic sources may offer a pathway to reducing environmental impacts while still enabling necessary geological exploration for hydrocarbon resources.