Seismic Borehole Geophysics
Seismic borehole geophysics applies principles discovered in earthquake-based deep earth investigations to environmental exploration, which requires high resolution information of the shallow subsurface. Seismic methods consider how density contrasts effect seismic energy and, because material density depends upon material type, can often distinguish subsurface materials, formations, and structures. Seismic borehole methods are those that emplace a transmitter (i.e., active seismic source) and/or receiver (i.e., geophone or hydrophone) within a borehole during measurement.
A seismic wave is acoustic (i.e., sound) energy that propagates through and interacts with the subsurface as it travels from its place of origin (e.g., earthquake focus, shot point). While influenced by a seismic wave, constituent material particles oscillate about their resting positions, which, depending on wave characteristics, stretches or compresses the bulk material. After this influence, materials resume original positions, size, and shape. Thus, seismic energy induces elastic deformation, so seismic methods are based upon wave theory in elastic media (Paillet and Cheng, 1991).
Seismic energy can travel in many types of waveforms, each having distinguishing characteristics and geophysical applications. The broadest classification of seismic wave type is the distinction between surface waves and body waves. Surface waves propagate only along surfaces (i.e., they cannot penetrate the ground surface) or boundaries. Alternatively, compressional (i.e., P or primary) and shear (i.e., S or secondary) waves can travel through geologic bodies and are, thus, predominantly utilized within seismic exploration methods.
Driven by compression, P-wave propagation resembles a slinky that is released after being stretched along its longitudinal axis. P-waves temporarily displace material in a direction parallel to that of travel and are thereby considered longitudinal waves. P-waves travel through materials at the highest velocity (i.e., first to arrive at receiver) and can pass through all states of matter. Because they are least affected by fractures/faults, unconsolidated sediments, and borehole/pore fluids, p-waves are the most measured wave type for seismic investigations.
S-waves are classified as transverse, as their propagation resembles the movement of a sidewinder snake and results in particle displacement perpendicular to travel direction. S-waves are transmitted by the lateral displacement of particles in elastic media with sufficient shear strength and, therefore, cannot travel through liquids or gasses. Relative to P-waves, S-waves typically have higher amplitudes, which increase the attenuation rate, and generally are the second arrivals at a receiver due to lower travel velocities (Paillet and Cheng, 1991).
The integration of a borehole into the subsurface generates an additional wave (i.e., Stoneley or tube wave) that can be useful in seismic borehole geophysics. Low frequency, high amplitude Stoneley waves are produced as a result of the seismic source traveling through the borehole fluid and into the subsurface materials. Because they travel along the borehole wall (i.e., fluid-rock interface), Stoneley waves are related to interface conditions and can indicate properties of borehole-adjacent formations.
An acoustic wave travels through a specific isotropic, homogeneous material at a constant seismic velocity, which has a magnitude dependent upon material properties and wave type. Seismic velocities are functions of factors such as density, elasticity, lithology/mineralogy, porosity/permeability, saturation, and pore-fluid composition and have been mathematically related to such in laboratory analyses. Thus, the measured travel times of various seismic wave arrivals can be used to interpret and differentiate the identity, characteristics, and/or structures of subsurface materials.
The amplitude of a seismic wave decreases over time as it travels through the subsurface. This attenuation occurs as seismic energy disperses via certain mechanisms (e.g., absorption, scattering, reflection, refraction, etc.) that typically depend upon material contrasts. Though also dependent on wave frequency, which is inversely proportional to depth of penetration, attenuation rate can indicate the presence of certain materials. Generally, seismic energy attenuates quickly in poorly consolidated formations and slowly/negligibly in dense formations (Paillet and Cheng, 1991).
Seismic borehole tools measure one or more of the four main seismic wave properties (i.e., velocity, amplitude, attenuation, and frequency) to interpret subsurface conditions. Tools are commonly designed to measure P-, S-, and Stoneley wave travel times, which are used to determine seismic velocity, and amplitudes. P- and S-wave information allows for lithology and formation fluid interpretations, while Stoneley wave analysis has been successful for fracture detection and relative permeability estimation (Paillet and Cheng, 1991).
Seismic borehole tools provide information unobtainable by other seismic methods, and site characterization increases by integrating various surface- and borehole methods. Seismic borehole methods correlate travel time with known sensor depths, which thereby eliminate the depth and/or time uncertainties/estimations of surface-seismic methods. Borehole methods typically employ higher frequency source waves, which, along with the smaller transmitter to receiver distance(s), produce higher resolution data compared to surface-seismic methods. The seismic borehole-geophysical methods most applicable to environmental investigations include:
References
Paillet, F.L., and Cheng, C.H., 1991, Acoustic Waves in Boreholes: Boca Raton, Florida, CRC Press, 264 p.