Seismic attenuation: Effects of pore fluids and frictional‐sliding

Abstract

Seismic wave attenuation in rocks was studied experimentally, with particular attention focused on frictional sliding and fluid flow mechanisms. Sandstone bars were resonated at frequencies from 500 to 9000 Hz, and the effects of confining pressure, pore pressure, degree of saturation, strain amplitude, and frequency were studied. Observed changes in attenuation and velocity with strain amplitude are interpreted as evidence for frictional sliding at grain contacts. Since this amplitude dependence disappears at strains and confining pressures typical of seismic wave propagation in the earth, we infer that frictional sliding is not a significant source of seismic attenuation in situ. Partial water saturation significantly increases the attenuation of both compressional (P) and shear (S) waves relative to that in dry rock, resulting in greater P‐wave than S‐wave attenuation. Complete saturation maximizes S‐wave attenuation but causes a reduction in P‐wave attenuation. These effects can be interpreted in terms of wave induced pore fluid flow. The ratio of compressional to shear attenuation is found to be a more sensitive and reliable indicator of partial gas saturation than is the corresponding velocity ratio. Potential applications may exist in exploration for natural gas and geothermal steam reservoirs.

Effect of Partial Water Saturation on Attenuation Characteristics of Low Porosity Rocks

“…However, attenuations are unexceptionally measured for natural rock specimens under a dry condition (e.g., GN1: 0.048, GN2: 0.071, MS1: 0.006, and MS2: 0.010). These experimental measurements represent pure material damping without fluid effect, where the material damping is decided by frictional energy loss between cracks (or mineral grain boundaries) during elastic wave propagation (Winkler and Nur 1982). As shown in Fig.…”

“…Compressional wave velocity changes little until the pore spaces are fully saturated with water because the air in partially saturated pore fluids diminishes the stiffness of the pore fluids and hardly contributes to strengthening the rock frame. On the other hand, when there is an increase in water saturation, the compressional wave attenuation-that is related to energy dissipation-tends to increase more sensitively than velocity (Gardner et al 1964;Toks} oz et al 1979;Mavko and Nur 1979;Murphy 1982;Winkler and Nur 1982;Cadoret et al 1998).…”

“…Several previous researchers have investigated the fluid effect on the wave velocity and attenuation, mainly focusing on high porosity rocks (mostly greater than 20% of porosity), such as limestone (Cadoret et al 1998) and sandstone (Murphy 1982;Winkler and Nur 1982). This study, therefore, examines how water saturation affects the attenuation characteristics of low porosity rocks.…”

Effect of Amplitude on Wave Propagation

“…Limited experimental studies carried out on this effect on velocity dependence on strain provide ambiguous results. Winkler and Nur, (1982) show that the velocity of the waves decreases with the increase of the strain, however Mashinskii (1994Mashinskii ( , 2004Mashinskii ( and 2005 observed increasing of wave velocity with wave amplitude and related that phenomena to microplastic effects in elastic modulus. In both mentioned works the strain amplitude has been estimated and not directly measured.…”

Fracture detection in crystalline rock using ultrasonic reflection techniques

“…This model is commonly used in the geophysical literature (Johnson and Toksoz, 1980a;Winkler and Nur, 1981) in the quantitative analysis of anelastic attenuation in rock.…”