lag behind the solid pipe, creating relative motion, and hence energy dissipation. The same phenomenon occurs in a porous medium, although the tortuous pore space results in additional inertial coupling between the solid matrix and fluid motion. Mesoscopic flow occurs when the wave passes through a porous medium with spatial variation of material (frame or fluid) properties on meso-scale (much larger than pore size but much smaller than wavelength). For instance, when the frame bulk modulus is spatially variable, the more compliant regions will deform to a greater extent than stiffer ones, resulting in fluid pressure gradients and hence WIFF from more compliant into stiffer areas, and, consequently, viscous dissipation. Mesoscopic dissipation factor scales with the variance (squared contrast) in material properties, and hence is only significant when the contrast is large. Two such situations are of particular interest: liquid- saturated rock with mesoscopic fractures and with meso- scale gas-saturated patches. Consider a rock saturated with a mixture of a liquid and gas. At low frequencies, there is enough time for fluid pressure to equalise between liquid and fluid saturated regions, and hence the bulk modulus of the saturated rock is given by Gassmann’s equation with the fluid modulus equal to the modulus of the fluid mixture given by the weighted harmonic average of the bulk moduli of liquid and gas. This mixture modulus is entirely controlled by the modulus of gas, and hence the rock modulus is constant almost in the entire saturation range. In contrast, at ultrasonic frequencies, there isn’t enough time for pressure to equalise, and the overall rock is effectively a mixture of two elastic constituents saturated with liquid and gas. According to the Gassmann theory, these constituents have the same shear moduli and only somewhat different bulk moduli, and hence the overall bulk modulus is a very gradual (near-linear) function of the volume fractions. Local (pore or grain scale) WIFF, also known as squirt, occurs between more compliant voids (cracks, grain- to-grain contacts) and relatively stiff pores. When the rock is compressed, much greater pressure builds up in compliant than stiff pores, resulting in the fluid pressure gradient, pore scale WIFF and dissipation. Similarly to the case in mesoscopic WIFF, when the frequency is low, fluid pressure has enough time to equalise within one half-period of the wave, and hence the compliant pores remain compliant, while at high frequencies, there is not enough time for the pressure to equalise, and hence the pores that are compliant in dry state become much stiffer. Hence materials with binary pore structure exhibit moduli and velocity dispersion. At the same time, presence of compliant pores is responsible for strong pressure dependency of elastic wave velocities. Thus substantial reduction of pore compliance at high (e.g., ultrasonic) frequencies results in much weaker pressure dependency. This effect can be demonstrated by much stronger pressure dependency of the rock bulk modulus at seismic than ultrasonic frequencies, with the difference (dispersion) decreasing with increasing effective pressure. At intermediate frequencies, the dispersion is accompanied by significant attenuation. Attenuation is even stronger in rocks saturated with viscoelastic substances such as heavy oil/ bitumen. At low frequencies (and/or high temperatures), the pore fill is in liquid state, the pressure is equalised between compliant and stiff pores, and the rock is relatively soft. But at high frequencies (or low temperature) the pore fill is near-solid, and won’t flow, making previously compliant pores very stiff, and causing strong dispersion. SEG DISC Lecturer Manika Prasad presents in Australia, August – September 2019 Breaking news from Marina Pervukhina and the ASEG’s Education Committee is that the SEG has just announced an SEG Distinguished Instructor Short Course (DISC) to be offered in Australia over the next 3 months. Physics and mechanics of rocks: a practical approach This one-day short course will be presented by Professor Manika Prasad of the Colorado School of Mines. It will provide the earth scientist and engineer with a foundation in rock physics to describe the physical processes that govern the response of rocks to the external stresses essential for reservoir characterisation. The course will also offer practical guidance to help better analyse existing data. A major goal of this course is to offer practical instruction and provide working knowledge in the areas of rock physics and rock mechanics for rock characterisation (Table 1). More information about the course and the speaker can be found at https://seg.org/Education/Courses/ DISC#sort:path~type~order=​ startdate~datetime~desc|​paging:​ number=​ 10|paging:currentPage=0 Table 1.  DISC lecture dates. Branch Date Time Venue QLD 20 August 09:00 XXXX Brewery (Alehouse), Black Street, Milton VIC 22 August 09:00 Kelvin Club TBC SA 27 August 09:00 128 Rundle Mall, Adelaide, SA, 5000 ACT 29 August 09:00 Geoscience Australia WA 6 September 09:00 AEGC Conference Education matters 37 PREVIEW JUNE 2019