We have developed a joint ultrasonic laboratory facility at the BGS, Keyworth for the physical characterisation of rock samples and for the investigation of acoustic signals in the characterisation of subsurface formations. The rig consists of a large water tank, which forms the central facility in which experimentation is undertaken to characterise and resolve geological targets. The facility allows the investigation of the effect of rock properties such as pore shape and size, porosity, and permeability on the acoustic signals. This novel facility is based on custom-built wideband transducers and is bio-inspired. Bats, whales, manatees and dolphins use sound to communicate, to locate and to characterise their environment, avoiding danger and finding prey. Observations by researchers appear to demonstrate that these creatures have greatly superior capabilities compared to existing technology; bats have the ability to resolve targets many times smaller than should be possible, while dolphins are capable of discriminating different materials, again significantly outperforming current detection systems. Rock properties of particular interest include pore morphology (size and shape), the size, nature and extent of fractures and fracture networks, surface texture effects and sediment characteristics. Our work focuses on relatively low frequency ultrasound in water for physical characterisation and geological applications. The development of a range of bio-acoustic techniques, measuring acoustic properties such as attenuation and velocity for example, will allow investigation of the relationships between acoustic parameters, sediment characteristics and petrophysical parameters. Ultimately these laboratory results may be applicable down hole in characterising subsurface formations. The ultimate goal of improved resolution and better physical characterisation will have application across many sectors.
Acoustic or sonic measurements have been used in petrophysical studies for many years and Ellis and Singer (2007) provide a good background to both the history and the physical principles. The original patent for measuring the velocity of the compressional wave, or strictly the transit time, in the borehole (Schlumberger, 1935) was initially driven by the need for better constraint of time-depth relationships for seismic reflection work but also opened up the opportunity for assessing the rock characteristics remotely, supplementing existing resistivity measurements. Wyllie et al. (1956, 1958) conducted investigations into the factors affecting the velocity in rocks, and this resulted in the time-average equation which enabled porosity to be estimated. Wyllie and others' work also demonstrated clearly that there were other factors attributable to the rock that were affecting the velocity, as well as a realisation that rocks were often far too complex to closely obey this idealized equation. At the same time as this laboratory work considerable efforts were directed towards theoretical studies of the behaviour of fluid saturated porous media. Thus Gassman (1951) and Biot (1956) initiated considerable investigations that continue to this day, with the model originally proposed by Gassman often forming the basis for fluid substitution modelling using downhole sonic data. Many other researchers have investigated the effects of mineralogy, saturation, pore morphology and confining pressure on the transit time (e.g. Timur, 1987; McCann & Sothcott, 1992; Dvorkin et al, 1999).
