USGS CMG Menlo Park Particle Size Analysis Labs

Room	Particle Size Analysis Laboratories
Location	M1065, M3009, M3009
Size	2439 sq. ft.
Contact	Michael E. Torresan
Description	The CMG Team has three laboratories that house analytical equipment used to conduct particle-size analysis based on both sieve and sedimentation analysis. The systems used in these laboratories range from classical techniques employing sieves and pipettes, to modern techniques using automated systems that employ sedimentation analysis as it applies to particles settling in columns of water or to light or x-ray attenuation through the sedimentation of a homogeneous, dispersed sediment suspension. One CMG laboratory ( M3009A) is dedicated to the determination of the size distribution of mud, a second laboratory ( M1065) houses settling tubes employed for the sedimentation analysis of sand-size particles, and the core splitting laboratory (previously described) houses sieves and a rotap machine utilized to analyze gravel-and sand-size distributions. Sedimentation analysis requires that all the samples must be pre-treated and disaggregated to remove flocculating agents, cements, organic matter, solubles, etc., in order to obtain an accurate reading of the grain size distribution. Preparation is always conducted in the M3009 sediment preparation laboratory.
Specialized Equipment

Grain-size analysis, the determination of the particle-size distribution of grains that compose a sediment, can be performed in a variety of ways (see Carver, 1971, and Syvitski, 1991), including sieve analysis (the determination of the particle-size distribution in a sediment by measuring the weight percentage of particles that pass through and are retained on a standard set of sieves of various sizes), and sedimentation analysis (the determination of particle settling velocity by allowing particles to settle in a water-filled column called a settling tube). Sedimentation analysis employs Stokesian particle settling theory, and is guided by the realization that most sediment is deposited in the process of hydraulic transport. Thus, it is desirable that a characteristic measure related to this depositional process be universally used. Sedimentation analysis employing pipettes, x-ray beam attenuation, and settling tubes are proven methods, as is sieve analysis. All of these methods are performed in the CMG sediment laboratories. In sedimentation theory particle settling velocity is converted to sedimentation diameter by comparing the measured velocity to that of a reference sediment of known size and density, usually quartz spheres. Thus, the sedimentation diameter is expressed in terms of hydraulic equivalence, or the equivalent spherical settling velocity (ESSD) of the reference sediment. The sedimentation diameter is used to model sediment transport processes and to understand depositional systems in both modern and ancient environments. The settling velocity of a particle is a more fundamental dynamic property than the geometric measurement of size obtained from sieve analysis, thus, settling properties are a better reference to particle behavior in a hydrodynamic environment. Sedimentation analysis, coupled with modern computer technology allows the development of a user-friendly and intuitive program structure, which results in high-speed data collection and provides researchers with high resolution data. Computers also provide the concomitant abilities to reduce human errors during data transfer, to display results in real-time, and to statistically manipulate large data sets both during and after an analysis. As a result efficiency and productivity are increased. The major objectives in determining the particle size distribution of a sediment or sedimentary rock are description, comparison, and interpretation. Particle size is a fundamental physical property of sediment or sedimentary rock, and it can tell researchers much about sediment or sedimentary rock origin and history. The conditions of transport and deposition of sedimentary particles can be also be inferred from the particle size distribution, and the size distribution is also an essential property for assessing how soils or sediment behave under loading conditions like storm waves, oceanographic currents, and earthquakes. Sieve analysis, sedimentation analysis, and measurement by thin section or caliper are ways to size, describe, compare, and interpret sediment or sedimentary rocks, but unlike sieve analysis or microscopic measurement that determine the physical size of a sediment, sedimentation analysis is a sizing technique that employs settling velocity as defined by Stokes¹ law. All deposits are laid down in a specific time and place because of the settling velocities of their constituent particles, which are a function of physical size, particle shape, particle concentration, particle density, gravity, and the density of the fluid that carries the particle. Since sedimentation analysis utilizes settling velocity, interpretations concerning sedimentological history tend to be more valid than interpretations based solely on physical size. There are four principal size categories of materials, gravel, sand, silt and clay. Gravel is composed of particles 2 mm - 64 mm in size, sand particles range in size from 2 mm to 0.063 mm, silt ranges in size from 0.063 mm to 0.002 mm, and clay is composed of particles finer than 0.002 mm. Mud is the component of sediment that consists of silt and clay. Particle size analysis techniques are dependent on the size classes being analyzed, and range from simple particle counting or the segregating of particles through a stacked set of wire mesh sieves, to modern techniques based on the settling of particles or particle suspensions. In sedimentation analysis one must distinguish between the settling of a homogeneous suspension or particles introduced at the top of a column. Fine-grained particles can also be counted with modern techniques such a laser based particle counters to image analyzers employing traditional light or scanning electron microscopy.