V Practical aspects and equipment used in seismic surveying for geophysical exploration, detailing key seismic sources, instruments, and the requirements for effective survey setups

Question 1

What are the primary differences among seismic sources, and how do they affect survey depth and resolution?

Answer 1

Different seismic sources are tailored to specific survey needs:

• Sledgehammers and weight dropsare cost-effective, suitable for shallow surveys (up to 50 m), and offer repeatability, but have limited energy for deeper targets.
• Explosivesprovide high-energy signals for deeper targets but come with safety and cost concerns.
• Vibroseisis preferred in oil and gas surveys, generating a controlled signal that can penetrate deep and provide high-resolution data.
• Air gunsare ideal for marine surveys, producing repeatable signals. Selection depends on factors like survey depth, target resolution, budget, and field conditions.

Question 2

What role does a seismograph play in seismic surveying, and what are the key features needed for quality data capture?

Answer 2

A seismograph records seismic waves from multiple channels, digitizing and displaying data from ground sensors (geophones) to analyze subsurface structures. Key features include:

• High Channel Count: At least 48 channels for broader data collection.
• Dynamic Range: Typically 24-bit to capture a range of wave amplitudes.
• Noise Reduction: Low electrical noise ensures clarity.
• Sampling Interval: Small intervals (0.1 to 0.5 ms) prevent aliasing.
• These capabilities allow the seismograph to record clear, detailed data essential for accurate interpretation.

Question 3

How does the design and setup of geophones influence the quality of seismic data collected?

Answer 3

Geophones convert ground movement into electrical signals, and their setup is vital for quality data:

• Ground Coupling: Proper placement and coupling reduce noise (e.g., from wind) and capture signals accurately.
• Frequency Range: Geophones are chosen based on investigation depth (e.g., higher frequencies for shallow depths).
• Spacing: The distance between geophones depends on the survey’s resolution requirements, ensuring that full waveforms are detected without spatial aliasing.
• These elements help capture the seismic wave’s characteristics, enabling precise interpretation.

Question 4

What are the main considerations for ensuring data quality during a seismic field survey?

Answer 4

Data quality depends on:

• Signal-to-Noise Ratio: High-quality coupling of geophones with the ground and positioning to minimize interference from environmental noise.
• Geophone and Sampling Configuration: Geophones should be spaced appropriately to avoid signal aliasing, and sampling intervals must match the frequency of the waves.
• Documentation and Storage: Recording detailed field protocols and redundant data storage protects against loss and ensures repeatable analysis.

Question 5

Why is adherence to Health, Safety, and Environment (HSE) standards essential in seismic surveying?

Answer 5

HSE standards safeguard the crew and environment during field operations:

• Safety: With potentially hazardous sources (like explosives), adherence minimizes risks.
• Environmental Impact: Avoiding ecosystem disruption (e.g., marine surveys) and following local regulations prevent negative impacts.
• Efficiency and Compliance: HSE ensures efficient, legally compliant operations, contributing to sustainable survey practices.