Laboratory Ultrasonic Measurements Research Articles

Exploring for the Future: new geomechanical data in frontier Australian basins

Led by Geoscience Australia, Exploring for the Future (EFTF) is a A$225 million Australian Government program dedicated to exploring Australia’s resource potential and boosting investment. The EFTF program energy component aimed to attract industry investment by delivering a suite of new precompetitive geoscience data in prospective Australian sedimentary basins. Through EFTF, Geoscience Australia has acquired significant amounts of new geomechanical data from underexplored onshore sedimentary basins with identified hydrocarbon prospectivity, from both legacy and newly acquired samples. These data were acquired to build a better understanding of basin sediment rock properties, particularly looking at the reservoir and seal potential of postulated unconventional and conventional targets. Four major datasets are presented herein, representing prospective intervals from the Paleozoic Canning Basin of Western Australia, the Neoproterozoic-Paleozoic Officer Basin of South Australia and Western Australia, the Paleo-Mesoproterozoic South Nicholson region of the Northern Territory and northwest Queensland, and the Paleo-Mesoproterozoic Birrindudu Basin of the Northern Territory and Western Australia. Additionally, the Paleo-Mesoproterozoic McArthur Basin of the Northern Territory is represented by a small number of analyses. Tests include unconfined compressive strength tests, laboratory ultrasonic measurements, single and multi-stage triaxial tests and Brazilian tensile strength tests. These datasets are a precompetitive resource that can facilitate investment decisions in frontier regions, helping to identify elements of conventional and unconventional hydrocarbon systems as well as providing essential data to assess geological storage opportunities.

Anisotropic 4D seismic response inferred from ultrasonic laboratory measurements: A direct comparison with the isotropic response

AbstractPore‐pressure depletion causes changes in the triaxial stress state. Pore‐pressure depletion in a flat reservoir, for example, can be reasonably approximated as uniaxial compaction, in which the horizontal effective stress change is smaller than the vertical effective stress. Furthermore, the stress sensitivity of velocities can be angle‐dependent. Therefore, time‐lapse changes in reservoir elastic anisotropy are expected as a consequence of production, which can complicate the interpretation of the 4D seismic response. The anisotropic 4D seismic response caused by pore‐pressure depletion was investigated using existing core velocity measurements. To make a direct comparison between the anisotropic 4D seismic response and the isotropic response based only on vertical velocities, pseudoisotropic elastic properties were utilized, and the two responses were compared in terms of a dynamic rock physics template. A comparison of the dynamic rock physics templates indicates that time‐lapse changes in reservoir elastic anisotropy have a noticeable impact on the interpretation of 4D seismic data. Changes in anisotropy as a result of pore‐pressure depletion cause a time‐lapse amplitude variation with offset response as if there is a reduction in VP/VS (i.e., pseudoisotropic VP/VS decreases), although the vertical VP/VS increases. The impact of time‐lapse changes in anisotropy on the amplitude variation with offset gradient was also investigated, and the time‐lapse anisotropy was found to enhance changes in the amplitude variation with offset gradient for a given case.

Can laboratory ultrasonic measurements on core plugs reproduce results obtained from sonic well logging in carbonates?

In the absence of wireline data in the open literature, the use of core plugs extracted from outcrops or subsurface reservoirs is significant for establishing the link between geology, petrophysical and elastic properties. Establishing such a link, known as rock physics transform, is an essential task in rock physics studies. The use of core plugs raises however questions about representation and upscaling, especially in heterogeneous rocks like carbonates. This study aims to investigate whether or not ultrasonic and petrophysical measurements on core plugs can reproduce results obtained from wireline logs in carbonates. The comparison between core plugs and downhole data is conducted in terms of values and trends. Specifically, we compare the data obtained from core plugs with wireline data in terms of 1) the magnitude of velocity and porosity at the same corresponding depth, and 2) the velocity-porosity relationship establishing a rock physics transform. Laboratory ultrasonic and porosity measurements were conducted on 128 carbonate core plugs extracted from subsurface oil field in the Middle East and results were compared with wireline data from the same well. Laboratory measurements were done under reservoir pressure and at dry conditions, while the fluid impact was incorporated using Gassmann's fluid substitution theory. The results show that laboratory-derived porosity and velocity failed significantly (error percentage of 81% and 10% respectively) to reproduce the wireline values at the same corresponding depth. Nevertheless, the velocity-porosity relationship and the resultant rock physics transform established based on core plugs were consistent with those obtained from wireline data. Such a result confirms, using experimental and field data, the hypothesis about rock physics transform scale independence which was only theoretically proposed in earlier studies. One main point here is that such scale independence is valid even in rocks with heterogeneous microstructure like the studied carbonates. The results suggest that one approach for upscaling cores-derived results is to establish and use relationships (trends) between different measurable properties rather than directly using (or attempting to upscale) their magnitude at the corresponding downhole depths. Moreover, the results show the validity of using Gassmann's theory to incorporate the impact of fluids based on dry ultrasonic measurements conducted at reservoir pressure, and thus providing a valuable tool for establishing rock physics transforms in carbonates. The conclusions presented here should be valid in carbonate reservoirs as long as no large-scale features such as fractures or karsts are dominant.

Geoscience Poster G8: Geomechanical rock properties of the Officer Basin

Poster G8 The Officer Basin spanning South Australia and Western Australia is the focus of a regional stratigraphic study being undertaken as part of the Exploring for the Future (EFTF) program, an Australian Government initiative dedicated to increasing investment in resource exploration in Australia. Despite numerous demonstrated oil and gas shows, the Officer Basin remains a frontier basin for energy exploration with significant uncertainties due to data availability. Under the EFTF Officer–Musgrave Project, Geoscience Australia acquired new geomechanical rock property data from forty core samples in five legacy stratigraphic and petroleum exploration wells that intersected Paleozoic and Neoproterozoic aged intervals. These samples were subjected to unconfined compressive rock strength tests, Brazilian tensile strength tests and laboratory ultrasonic measurements. Petrophysical properties were also characterised via X-ray computerised tomography scanning, grain density and porosity-permeability analysis. Accurate characterisation of static geomechanical rock properties through laboratory testing is essential. In the modern exploration environment, these datasets are a precompetitive resource that can simplify investment decisions in prospective frontier regions such as the Officer Basin. To access the poster click the link on the right. To read the full paper click here

Mantle Wedge Water Contents Estimated From Ultrasonic Laboratory Measurements of Olivine‐Antigorite Aggregates

AbstractWhile it is known that serpentinized peridotite acts as a water reservoir in the mantle wedge of a subduction zone, the spatial distribution and quantity of the water stored in these reservoirs, as well as the acoustic velocity of the olivine‐antigorite aggregates, are not well constrained. Here, we report the propagation of seismic waves through synthetic olivine–antigorite aggregates, which are used as proxies for various mantle wedge lithologies, by varying the amount of antigorite at pressures up to 8 GPa. Our results indicate that the acoustic velocity is strongly dependent on the proportion of antigorite in the sample and somewhat dependent on pressure. We empirically explore the relationship between the acoustic velocity and the degree of serpentinization as well as pressure to map the water content in the mantle wedge. Our estimations show that the water content among the subducting slabs around the Pacific Ocean is between 0.5 and 5.0 wt%.

Geomechanical rock properties of the Officer Basin

The Officer Basin spanning South Australia and Western Australia is the focus of a regional stratigraphic study being undertaken as part of the Exploring for the Future (EFTF) program, an Australian Government initiative dedicated to increasing investment in resource exploration in Australia. Despite numerous demonstrated oil and gas shows, the Officer Basin remains a frontier basin for energy exploration with significant uncertainties due to data availability. Under the EFTF Officer–Musgrave Project, Geoscience Australia acquired new geomechanical rock property data from forty core samples in five legacy stratigraphic and petroleum exploration wells that intersected Paleozoic and Neoproterozoic aged intervals. These samples were subjected to unconfined compressive rock strength tests, Brazilian tensile strength tests and laboratory ultrasonic measurements. Petrophysical properties were also characterised via X-ray computerised tomography scanning, grain density and porosity-permeability analysis. Accurate characterisation of static geomechanical rock properties through laboratory testing is essential. In the modern exploration environment, these datasets are a precompetitive resource that can simplify investment decisions in prospective frontier regions such as the Officer Basin.

Acoustothermoelasticity for Joint Effects of Stress and Thermal Fields on Wave Dispersion and Attenuation

AbstractThe stress field and thermal field are usually coupled in the Earth's subsurface. The mechanism of the joint effects of stress and thermal fields on wave propagation in rocks is not well understood. To fill this gap, this paper combines the acoustoelasticity theory with heat conduction theory to propose an acoustothermoelasticity mechanism to describe the joint effects of stress and thermal fields. The stress‐ and thermal‐dependent wave velocities and attenuation factors are formulated with 2‐D Fourier transform analysis for dynamic equations, which predict four wave modes of propagation including two longitudinal waves, namely, P wave and T (thermal) wave, and two shear waves, namely, fast S wave and second S wave. The stress‐induced anisotropy accounts for the S‐wave splitting phenomenon and T wave presents the thermal diffusive behavior, which depend on the magnitude and direction of stress and thermoelastic parameters. Stress and thermal effects on the wave velocity dispersion and attenuation are fully analyzed. Modeling results show that T wave is coupled with the P wave and fast S wave rather than the second S wave inducing the dissipation energy in the pre‐stressed thermoelastic rocks. The predicted wave velocities show reasonable agreement with ultrasonic laboratory measurements under uniaxial and triaxial stresses. Our model and results help to better understand mechanical properties and wave propagation in pre‐stressed thermoelastic rocks.

Laboratory seismic measurements for layer-specific description of fluid mud and for linking seismic velocities to rheological properties.

The velocities of the seismic waves propagating in the fluid-mud layer are governed by the rheological properties and density of the fluid mud. Performing seismic transmission measurements inside the fluid mud can give good estimates of the seismic velocities and, thus, of the rheological properties and density. Laboratory ultrasonic transmission measurements of the wave velocities in the fluid-mud layer and their temporal evolution are shown. It is found that the shear-wave velocity and yield stress are positively correlated. Performing a seismic reflection survey for characterization of the fluid-mud layers could be more practical because it allows towing the sources and receivers above the top of fluid-mud layer. Interpretation of the results from a reflection survey, though, is influenced by the water layer above the fluid mud. Applying seismic interferometry to reflection measurements can eliminate the influence of the water layer and retrieve a reflection response from inside the fluid-mud layer. This eliminates the influence of the temperature and salinity of the water layer to obtain information about the seismic properties of the fluid-mud layer. To introduce the approach of retrieving and extracting the reflection response from inside the fluid-mud layer, data from laboratory measurements are used. The obtained compressional- and shear-wave velocities are validated by comparing them with values from current transmission measurements.

Brittleness index of coal from the Upper Silesian Coal Basin

A growing interest in the field of coal bed methane (CBM) extraction in Poland shows the demand for rock mechanics data, used to design hydraulic fracturing operations. The elastic response of the rock is typically determined by sonic logging calibrated with laboratory tests. This paper presents the laboratory ultrasonic measurements of the core samples, performed to determine the elastic moduli and brittleness index (BI) of the coal. Tests were performed on 20 core plugs from four coal mines located in the central and southern part of the Upper Silesian Coal Basin (USCB), Poland, characterized by varied maceral composition and mineral additives. The samples were cored out in three directions: perpendicular, parallel, and at a 45° angle to the bedding planes, and tested with the given effective pressure. The majority of the samples were saturated by water with a potassium chloride additive (swelling inhibitor). A P- and S-wave velocity upward trend was observed when the mineral content in the samples increased. Elevated velocities in samples of high mineral content resulted in exceeding the Ed to vd limits for coal as proposed in literature. With increased BI, upward trends in the liptinite and inertinite content as well as a downward trend in the vitrinite content were observed. The dynamic elastic moduli of the measured samples were compared to the available literature data.

Ultrasonic characterization of anisotropic rocks: Impact of transducers’ size and geometry on data interpretation

Typical ultrasonic laboratory measurements of rock physical properties are conducted with ultrasonic transducers that are relatively large compared to the rock sample. We have determined the impact of the transducer size on the resulting velocity estimations. To improve data interpretation, we explore two complementary avenues: (1) explicitly account for the finite size of the transducers as part of the data interpretation/correction workflow, rather than assuming point sources, and (2) reducing the effective size of the transducers at the hardware design level. Both approaches as well as their combination have been tested successfully on multiple data sets including artificial homogeneous and isotropic samples as well as natural anisotropic rocks such as shales.

Simultaneous static and dynamic bulk modulus measurements under hydrostatic stress conditions and without applying strain gauge

The static properties of subsurface rocks are needed for geomechanical applications, but the dynamic properties are usually more extensively available and can be acquired with much less effort. Therefore, it is important to know the relationships between the static and dynamic properties. Studying these relationships based on traditional triaxial testing yields confusing results, partly due to intrinsic deficiencies of the experimental setup and testing procedures. We have developed a new approach to study the relationship under hydrostatic stress conditions. The traditional ultrasonic velocity measurement system is sufficient for all of the testing, and it requires no application of the traditional strain gauge. During the pressure-dependent ultrasonic velocity measurements, the rock sample experiences static deformation when the pressure changes are in the order of megapascals. If the pore pressure is kept constant, the pore volume will vary with the confining pressure. The pore volume change can be monitored accurately by the pore pressure controlling pump, and it can be related to the volume strain of the rock for estimating the static bulk modulus of the rock sample. Based on the laboratory ultrasonic measurements available in the literature, we analyzed the effects of the differential pressure, clay content, crack density, and pore fluid on the relationship between the static and dynamic bulk moduli. From the analyses, we inferred that most of the laboratory-observed differences between the static and dynamic property may not exist for the reservoir rock under in situ conditions.

Laboratory observations of frequency-dependent ultrasonic P-wave velocity and attenuation during methane hydrate formation in Berea sandstone

SUMMARYKnowledge of the effect of methane hydrate saturation and morphology on elastic wave attenuation could help reduce ambiguity in seafloor hydrate content estimates. These are needed for seafloor resource and geohazard assessment, as well as to improve predictions of greenhouse gas fluxes into the water column. At low hydrate saturations, measuring attenuation can be particularly useful as the seismic velocity of hydrate-bearing sediments is relatively insensitive to hydrate content. Here, we present laboratory ultrasonic (448–782 kHz) measurements of P-wave velocity and attenuation for successive cycles of methane hydrate formation (maximum hydrate saturation of 26 per cent) in Berea sandstone. We observed systematic and repeatable changes in the velocity and attenuation frequency spectra with hydrate saturation. Attenuation generally increases with hydrate saturation, and with measurement frequency at hydrate saturations below 6 per cent. For hydrate saturations greater than 6 per cent, attenuation decreases with frequency. The results support earlier experimental observations of frequency-dependent attenuation peaks at specific hydrate saturations. We used an effective medium rock-physics model which considers attenuation from gas bubble resonance, inertial fluid flow and squirt flow from both fluid inclusions in hydrate and different aspect ratio pores created during hydrate formation. Using this model, we linked the measured attenuation spectral changes to a decrease in coexisting methane gas bubble radius, and creation of different aspect ratio pores during hydrate formation.

Laboratory ultrasonic measurements: Shear transducers for compressional waves

The ultrasonic measurements technique is well established to measure the elastic properties of rocks in the laboratory for seismic and well-log data interpretation. The key components of every laboratory ultrasonic setup are piezoelectric transducers, which generate and register elastic waves in rock samples. The elastic properties of rocks are determined through the velocities of elastic waves, which are measured by the times of the waves' travel from the source to the receiver transducer. Transducers can be specifically designed to generate P-waves (P-transducers) or S-waves (S-transducers). In limited studies, the measurement of P-wave velocities with S-transducers is mentioned. Such measurement is possible due to specific aspects of the operation of S-transducers. Namely, S-transducers are known to emit parasitic low-energy P-waves, which travel faster than high-energy S-waves and hence can be registered. However, no justification or elaboration of this method of measuring P-wave velocities was reported. To fill this gap, we first compare P-wave velocities measured with S-transducers against P-wave velocities measured with P-transducers in different rocks and materials. We show that the discrepancy between velocities measured with the two methods in homogeneous materials is less than 1% and can be up to 4% for natural rocks. Second, we numerically simulate the operation of S-transducers, show that parasitic P-waves have a dipole structure, and explain how the receiver transducer can register this compressional dipole. Finally, we use laser doppler interferometry to measure the displacement of the free surface of a sample caused by elastic waves emitted by the source S-transducer. We observed the dipole structure of the sample's surface displacement upon P-wave arrival on the surface.

On the interpretation of ultrasonic laboratory measurements in anisotropic media

There is an ongoing debate on whether laboratory ultrasonic measurements in anisotropic media yield phase or group velocities. The main problem here is that the size of the transducers used in the laboratory for ultrasonic measurements is comparable with the size of the rock sample or propagation distance. As a result, it is not clear how to interpret the measured traveltimes of ultrasonic waves. The main question is how to define an appropriate effective travel path, its length, and orientation. To answer this question, we look in detail at the full wavefront generated by a finite-size transducer using a specifically designed experimental setup, and a synthetic material (phenolic grade CE). We devise an algorithm for the interpretation of traveltimes measured in the laboratory with finite-size transducers arbitrarily oriented with respect to the symmetry planes of the material. To illustrate/validate the proposed algorithm, we designed a special testing program. The results of the experiments are presented and turn out to be in very good agreement with the analytical predictions on which we base our interpretation algorithm.

Shear Modulus Dispersion in Cracked and Fluid‐Saturated Quartzites: Experimental Observations and Modeling

AbstractThe effect of pore fluids on acoustic wave dispersion in rocks with low aspect ratio crack porosity is important for the interpretation of laboratory and field observations in hard rock mineral exploration environments. Here we make laboratory measurements of shear modulus dispersion at frequencies 0.01–1 Hz and at 1 MHz with different saturating fluids (dry, argon, and water saturated) in two thermally cracked quartzite samples with ~2% total porosity. Measurements are made across a range of effective pressures (10–150 MPa), with the resulting very low permeabilities of the samples varying from 1–300 × 10−21 m2. Moduli across the 0.01–1 Hz band were typically independent of frequency. The shear moduli measured at sub‐Hz frequencies are unaffected by fluid saturation, as expected for the saturated isobaric (Gassmann) regime. In marked contrast, water saturation of the cracked rocks results in very large increases in the shear moduli measured at 1 MHz and low effective pressures, indicative of saturated isolated conditions. Thus, at an effective pressure of 20 MPa, the shear moduli for the two water‐saturated quartzites increase by 74% and 98% from 1 Hz to 1 MHz. The contrast in elastic moduli between dry and water‐saturated conditions is well represented by the theoretical model developed by Walsh and others. The observed dispersion highlights the need for care in seismological application of results obtained at MHz frequencies from laboratory ultrasonic measurements.

Rock anisotropy and brittleness from laboratory ultrasonic measurements in the service of hydraulic fracturing

Rock anisotropy and brittleness from laboratory ultrasonic measurements in the service of hydraulic fracturing

Petrophysical Study of Szolnok Formation, Endrod Gas Field, Hungary

Investigation of rock porosity and permeability is highly beneficial for geologists, petro-physicist and petroleum engineers in order to evaluate reservoir pore space geometry through the time and space. Clastic reservoir quality and classification could perform based on the petrophysical data correlations. Study of the Szolnok formation was our target. It is composed mainly of sandstones with clay-marlstone and siltstones. Two hundred thirteen-core samples of Upper and Lower Pliocene and Miocene age were subjected for petrophysical investigations. Pore size distribution using MICP, Mercury and Helium porosity, horizontal and vertical permeability were measured for studied core samples. The Szolnok Formation has two main lithologic facies: a. 141 clean sandstone samples and b. 72 siltstone and clay-marlstone samples. Ultrasonic laboratory measurements were carried out for only 30 selected sandstone core samples. Sonic viewer-120 is used to measure sonic velocities and other mechanical properties such as rigidity, bulk modulus and Young’s modulus. Gas permeability and Helium porosity were plotted versus sonic wave velocity indicates that both permeability and porosity could be outlined from either compressional or shear wave velocity. Effective pore radius is outlined from both of them. The highest sample porosity was recorded for the Miocene in age followed by the Lower Pliocene and then for the Upper Pliocene samples respectively. Miocene samples are relatively clayey free followed by Lower Pliocene samples because they have higher sonic velocity (Vp and Vs) than the Upper Pliocene samples. The Miocene and Lower Pliocene samples have relatively lower dynamic mechanical parameters than Upper Pliocene samples which represent good gas reservoirs in the Endrod field. Several regression line equations with high coefficient of correlation have been calculated to predict Szolnok reservoir parameters.

Stress-Associated Intrinsic and Scattering Attenuation from Laboratory Ultrasonic Measurements on Shales

Seismic attenuation is sensitive to stress-induced subtle changes in the physical state of rocks. In this study, the stress- and frequency-associated attenuation is quantified through ultrasonic measurements on three differently oriented cylindrical shale samples under various axial stresses. As an improvement to the single-scattering model, the elastic Monte Carlo method is employed to investigate multiple-scattering attenuations by incorporating the boundary reflections and wave conversions. Our results show that, as the axial stress increases, the intrinsic attenuation decreases in all directions, while the scattering attenuation decreases slightly in the direction perpendicular to the bedding but increases largely and nonlinearly in other directions. These discrepancies result from different attenuation mechanisms. Both the intrinsic and scattering attenuation are found to be largest in the direction 45° to the bedding, but least in the perpendicular direction. The S-wave attenuation is larger and more sensitive to stress changes than P-wave attenuation due to its shorter wavelength. As expected from sandstone examples, the scattering attenuation in shales is significantly larger and more sensitive to stress changes than the intrinsic attenuation. The frequency dependence of scattering attenuation suggests that the peak frequency with the maximum scattering attenuation is independent of axial stresses, but varies in different directions of an individual rock with different heterogeneity and anisotropy scales. The peak frequency of S-coda is smaller and its peak scattering attenuation is larger than P-coda. In conclusion, the stress and frequency dependence of ultrasonic attenuations in shales differ largely in various directions, indicating significant anisotropy and heterogeneity.

Elastic properties of rock salt: Laboratory measurements and Gulf of Mexico well-log analysis

Rock salt (essentially halite) is a special type of sedimentary rock that has played a large role throughout tectonic and economic history. The unique physical properties of halite (ductility, low density, flowability, and impermeability) can be critical factors in hydrocarbon traps and underground storage. However, seismic imaging and interpretation can be challenging when salt structures are present due to their complex geometry and large impedance contrasts relative to surrounding rocks. To investigate the properties of rock salt in terms of elastic parameters, we use ultrasonic laboratory measurements and well logs. In the laboratory, we have analyzed the effects of composition, crystalline structure, pressure, and temperature on the elastic behavior of a variety of rock salt samples. The samples include pure halite (>95 wt%) from the Gulf of Mexico (GOM) area, argillaceous rock salt from the Zipaquirá Mine, Colombia, and crystalline salt from the Goderich Mine, Canada. Current measurements suggest that the GOM salt cores behave isotropically in general. The Zipaquirá salt samples show velocity and density variations on account of their heterogeneous composition. The Goderich halite crystals display distinct cubic anisotropy. Measurements on the GOM samples at varying confining pressures and temperatures indicate that increasing pressure elevates velocity whereas increasing temperature decreases velocity. From the analysis of 145 log suites from boreholes drilled through rock salt in the northern GOM, we found that, within the salt formations, P-wave velocities increased slightly with depth (approximately [Formula: see text] per km). The S-wave velocities from three wells range from 2280 to [Formula: see text]. Bulk densities from all the wells cluster at [Formula: see text]. These laboratory and log measurements provide new values for the elastic properties of rock salt, which can assist in velocity model building, synthetic seismogram generation, and the understanding of the rock physics of halite.

Modelling ultrasonic laboratory measurements of the saturation dependence of elastic modulus: New insights and implications for wave propagation mechanisms

Seismic time-lapse techniques are a valuable tool used to estimate the mobilization and distribution of stored CO2 in depleted reservoirs. The success of these techniques depends on knowing the seismic properties of partially saturated rocks with accuracy. It is commonplace to use controlled laboratory-scale experiments to determine how the fluid content impacts on their properties. In this work, we measure the ultrasonic P- and S-wave velocities of a set of synthetic sandstones of about 30% porosity. Using an accurate method, we span the entire saturation range of an air-water system. We show that the rocks’ elastic behaviour is consistent with patchy saturation and squirt flow models but observe a discontinuity at around 90% gas saturation which can be interpreted in two very different ways. In one interpretation, the responsible mechanism is frequency-dependent squirt-flow that occurs in narrow pores that are preferentially saturated. An equally plausible mechanism is the change of the mobile fluid in the pores once they are wetted. Extrapolated to seismic frequencies, our results imply that the seismic properties of rocks may be affected by the wetting effect with an impact on the interpretation of field data but would potentially be unaffected by the squirt flow effect. This provides strong motivation to conduct laboratory-scale experiments with partially saturated samples at lower frequency or, ideally, a range of frequencies in the seismo-acoustic range.