Density of Geological Materials

The density of a rock is closely tied to its mineralogy. Sedimentary rocks have a lower density than igneous rocks, with most sandstones having a density close to the density of quartz (ρ = 2.65 g/cm3), suggesting that they are composed primarily of quartz grains and cement. Sandstones with higher densities are composed of quartz mixed with denser minerals such as calcite (ρ = 2.71 g/cm3) and dolomite (ρ = 2.8 to 3.1 g/cm3).

Amongst igneous rocks, mafic rocks contain the high-density minerals pyroxene and olivine, and are thus more dense than felsic rocks which contain more of the lower-density minerals quartz and feldspar. As oceanic crust is primarily mafic rocks like basalt and peridotite, while continental crust is primarily felsic rocks like granite, therefore oceanic rocks are generally more dense than continental rocks.

Similarly, metallic ores are usually more dense than quartz-rich crustal rocks due to a larger component of heavy metallic minerals. The large portion of light ions like hydrogen and sodium in evaporites like salt mean that evaporites have a lower density than crustal rocks. As salt caps are associated with petroleum deposits, this density difference makes gravity surveying effective for petroleum fields. More complex environments such as glacial deposits exhibit a substantial range in density: fluvial-glacial deposits of fine, dry sands are less dense than glacial moraines of cobbles and rocks.

Water content impacts density, both by permeating void space or by absorption into the material structure, potentially leading to substantial changes between dry and wet densities. Porous rocks above the water table are less dense than the same rocks below the water table, as water infills the pores. Clay can absorb substantial amount of water, swelling in volume while also increasing in density.

The grain density of a material is an indicator of the mineralogy. A rock like shale is composed of several minerals of different densities including dolomite (ρ = 2.8 to 3.1 g/cm3), calcite (ρ = 2.71 g/cm3), and clays like illite (ρ = 2.6 to 2.9 g/cm3) and kaolinite (ρ = 2.6 g/cm3). The grain density of shale (ρ = 2.65 to 2.8 g/cm3) is then a combination of these mineral densities at different proportions.

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Density

Density (ρ) is the mass (m) of a material divided by its volume (V), given in units of mass per unit
volume (kg/m3): ρ = m/V
The density of a material is a function of the density of the individual grains or crystals, the porosity of the material1, and the fluid filling the pore space. Even so, the variation in density of natural geologic materials is quite small, with only a 16% density increase from limestone to basalt.

Measurements of dry and wet samples are taken to calculate the dry density, wet density, and the grain density of a material. The dry density is the density of the material when the pore space is full of air, and is closely linked to porosity. The wet density is the density of the material when the pore space is filled with fluid (water or oil), and is related to the porosity and the fluid density. The grain density is the density of the solid material excluding porosity, and is closely linked to the mineralogy.

The density of a particular material will remain relatively stable under different circumstances, with little variation. Variation depends on saturation, the degree of compaction, cementation, or fracturing, and degree of metamorphism. Water has a higher density than air, so as water infills pore space, saturated materials are more dense than unsaturated materials. Materials that are more compact will be more dense as pore space is reduced, and intact materials will be more dense than those riddled by fractures or voids. Cementation can also infill void space, as can recrystallization, so more cemented or metamorphosed rocks are more dense than even compacted grains.

The critical density of a soil is the point at which a soil begins to demonstrate dilative behaviour when under high strain. This density will change with respect to the confining stress, the fabric of the material, the stress history, and the type and duration of loading, so is not constant for a particular geologic material under different conditions.

Specific gravity is the density of a soil normalized to the density of water, calculated as the material mass divided by the mass of an equal volume of water. It is a unitless measurement:
s.g. = ρsample/ ρwater

Because water density fluctuates with temperature, density and specific gravity measurements are made with respect to a reference temperature (typically 20◦C).

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Pancake Rocks

In a decidedly inedible celebration of Pancake Day, I’ve written about the Pancake Rock formation over on io9.

(If the link isn’t live yet, I’ve mucked up my cross-timezone scheduled posts. It’ll be out at 1pm EST.)

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Polar Bear Day!

It’s polar bear day! This year, have some adorable photos of a curious exploring bear, or learn about Google mapping polar-bear turf.

Polar bears are threatened by habitat loss, climate change, and trophy hunting. They are way too vicious and charismatic to die out that way. Plus, the males embrace alternative culture by getting their lips tattooed for tracking.

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Detecting Porosity & Permeability

Effective porosity can be measured with electromagnetic induction probes. Hydraulic conductivity can be measured in-situ by observing water-level fluctuations in drill-holes. For a system at equilibrium, the system needs to be disturbed to produce fluid flow by either pumping water out and observing the recharge rate, or injecting a slug of water and observing how quickly it dissipates out of the hole.

The purpose and environmental conditions of the test will impact what method is most appropriate. If permeability is being measured as part of contamination monitoring and modelling, the pumping or injection tests may further spread the contaminant. In some cases, it is best to substitute gas as the fluid, using equipment like the Core Laboratories Portable Probe Permeameter. The permeameter forces gas into an exposed rock face at a fixed initial pressure, then measures how quickly the gas dissipates into the outcrop as the pressure decays.

Borehole Logging

Flow zones may also be delineated through borehole surveys, usually involving temperature differentiation. A Temperature/Fluid Resistivity probe can be used to assess the temperature gradient, identify zones of variable water quality or salinity, and delineate features. An Impeller Flow Meter delineates water flow zones, while at a Heat Pulse Flow Meter can do the same thing but for lower flow rates.

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Characterization of Porosity & Permeability

Porosity and permeability are closely related, so it is unsurprising that measuring the properties share common techniques that involve determining interconnected pore volume.

Porosity

Porosity (n) is measured as the ratio of the volume of voids within a material to the total volume of the material. In a laboratory setting, this requires careful measurement of sample volume, and of pore volume. A right cylindrical core sample is extracted using a core drill press, rock saw, and surface grinder. X-ray CT scanners may be used to identify undamaged full-diameter sections for sampling. The dimensions of the sample – length (l) and diameter (d) – are measured using calipers. The volume (V ) of a right cylinder is then a simple calculation: Vtotal = πdl

Next, the sample is dried by baking it in an oven for 24 hours to ensure that no water remains in the pore space.

The pore space is filled with helium gas, which is both nonreactive (thus not altering the sample) and has a small nucleus to ensure the gas can quickly penetrate even small pore spaces provided the pores are interconnected and not isolated. The cylinder is placed in a helium pycnometer: a sample chamber and a reference chamber, both at a known volume and at a fixed temperature. The reference and sample chambers are pressurized with helium gas. Once the sample is inserted, the two chambers are connected, allowing the gas to flow out of the reference chamber into the sample chamber. The ratio of the initial and final pressures is used in conjunction with Boyle’s Law (P1V1 = P2V2) to calculate the solid volume of the sample: Vsolid = V2 = P1V1/P2

Finally, the pore volume is calculated as the difference of the total volume (determined by dimensional measurement) and the solid volume (determined by the helium pycnometer): Vpore = Vtotal − Vsolid

This technique is limited to materials with interconnected pore spaces. Isolated pores are not penetrated by the helium gas, and thus are not measured by this technique.

Alternately, liquid mercury can also be used following a similar process, where the size of infiltrated pores is proportional to the exerted pressure. The measured pore volume has the the same limitations, with the added risk of working with a neurotoxin. In the future, when helium resources have been bled away in party balloons, mercury will be the primary option, and careless graduate students will be the new mad hatters.

Permeability

Permeability is measured as the hydraulic conductivity (k), which is the the ratio of flow velocity (v) to the hydraulic gradient (i): k = v (3.5) i

Hydraulic conductivity is measured in a laboratory setting by placing a sample under standard temperature conditions, then measuring the rate of discharge of water through a cross-sectional area of the medium. The water must be under laminar flow conditions so that turbulence does not complicate the flow rate. The hydraulic gradient and the cross-sectional area are coordinated to produce unit measurements. A typical example of the laboratory equipment are the Matest Hoek cells to measure the flow of water through a rock specimen of the specified diameter. A similar process can be followed using flowing air instead of water for dissolvable materials

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Porosity and Permeability

Porosity is the number of pores per unit of volume in soil. Porosity may decrease through packing or compaction, increase through dilation during shear, or change when fines are transported from one area to another. Materials with higher porosity have a higher storage capacity to hold fluids within the void space. Porosity, in conjunction with the pore fluid properties, directly impacts resistivity.
Permeability is the capacity of a material to conduct a fluid. High permeability materials are less resistant to fluid flow and require less pressure to force the fluid through than lower permeability materials. Permeability is dependent upon the geometry of fissures, pores, and cracks.

Relationship with Geological Materials

Porosity and permeability are highest in coarse-grained poorly-consolidated sedimentary rocks, with decreasing porosity with finer grained and more compacted materials. Igneous rocks typically have low porosity, excepting extrusive tuffs and pumice. Metamorphic rocks usually have intermediate porosity, where compaction and recrystallization infill pore spaces. Weathering increases porosity and permeability, where increasing cracks, fractures, and void spaces all allow for greater pore fluid mobility.

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Geotechnical Properties

Some physical properties of geologic materials are important in geotechnical engineering, but are not useful for geophysical interpretation. These properties are the dry strength, dilatancy plasticity, and toughness of the material. All four properties may be easily quantitatively categorized in the field by handling the materials.

Dry Strength

Dry strength is how strong the material is when it is dry. The dry strength is categorized in the field by the engineer modelling a small ball, adding water if necessary until the material has the consistency of putty, then applying finger pressure. If the ball crumbles with the pressure of handling, it has no dry strength. Categories progress through low, medium, and high, with very high dry strength indicating a material that cannot be broken with the pressure applied by squeezing the sample between a thumb and a hard surface.

Dilatancy

Dilatancy is how the volume of a cohesion less soil will expand under loading or shear deformation. The dilatancy of a sample can be qualitatively categorized in the field using basic tools. The field technician will need to mould the specimen into a small ball, adding water until it is soft, then smoothing the surface with a blade or spatula. Then, he or she will shake the ball horizontally, striking the side of one hand against the other, and observing how quickly water appears on the surface of the ball. Then the engineer squeezes the balled sample by closing his or her hand, or by pinching the material, and observing if and how quickly the water disappears back into the ball. The dilatancy is then categorized as none, slow, or rapid depending on how quickly the water appears and disappears when the ball is manipulated.

Plasticity

Plasticity is how far a material may be deformed under constant stress, without cracking or dilatancy. The plastic limit can be determined by rolling the material into a thread, then folding and re-rolling the material until the thread crumbles. After reaching the plastic limit (when the thread crumbles), the field technician kneads the material into a lump, continuing to knead until the material crumbles. The plasticity is qualitatively categorized by how the material behaves with more or less water than the plastic limit, particularly how long it must be rolled to form a thread, then how a lump of the material behaves as it dries out.

Toughness

Toughness is the ratio of the plasticity index to the flow index. It is tested concurrently to the plasticity test. Qualitatively, it is categorized into low, medium, and high by how much pressure is required to rolling the material into a thread and kneading it into a lump during the plasticity test, and the stiffness of the thread and lump.

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Detecting Physical Properties (Fieldwork)

Geophysical field surveys are used to investigate the subsurface. Geophysics may utilize passive techniques, measuring and mapping changes in the ambient field, or as active techniques that measures how the geological environment responds to an input signal. In either instance, the geophysical signal is processed and interpreted to indicate particular physical properties of geological materials. Those physical properties are used to determine a geological interpretation for subsurface materials and structures that would plausibly result in the observed geophysical signals.

The advantage of geophysics surveying is that passive techniques are non-destructive, that measurements are taken close (or at) the area of interest, and that an appropriate survey can provide a wealth of information about the subsurface in a cost-effective manner. The disadvantage of field surveying is that active techniques can be destructive (depending on the type of source providing an active signal), that environmental conditions (weather, access, injury) can increase survey time beyond estimates, and that in-situ properties do not always match properties determined in controlled laboratory settings.

Passive techniques include any survey where sensors measure the ambient field. Examples include gravity and magnetic methods, and some resistivity techniques. Active techniques include any survey where a source provides an active signal, and sensors measure how that signal is altered by the environment. Examples include induced polarization and artificial-source seismic techniques.

Borehole Logging

Boreholes are a circular cross-section made in soil or rock, that is either cased with piping or left bare. A borehole log is the record of the depth, geologic units, sample recovery (if any), water level, and any other significant facts related to the drilling including geophysical data collected through borehole sondes. A sonde is a long, tubular object containing instruments that is attached to an armoured cable, lowered into the drill hole with a pulley and winch system. The sonde is lowered to depth, then data is collected during a slow pull up the hole.

The advantages of a borehole logging with geophysical probes is that we can obtain in-situ measurements close to the target area, can use core or drill chipping to guide geological interpretation of the geophysical data, and, for some types of surveys, obtain measurements for the undisturbed surface away from the borehole. The disadvantages of borehole logging is that drilling is expensive (particularly depending on the location, subsurface characteristics, depth, and if core is preserved), destructive, and may induce fracturing or breach impermeable layers spreading contamination plumes.

A sonde may contain one or more geophysical systems, most commonly sensors to measure resistivity, acoustic velocity, ground penetrating radar, spontaneous potential, neutron-neutron, natural gamma, gamma-gamma, temperature, fluid flow, gravity, or magnetic properties. A survey may consist of in individual hole in isolation, or a cross hole survey with the source in one hole and receivers in another.

Data extracted from geophysical borehole probes is interpreted in conduction with other information from the site. Two additional borehole probes are commonly used in conjunction with the geophysical sondes are calipers (or callipers for those across the ocean), providing a record of borehole diameter, and an optical televiewer, creating an oriented digital image of the borehole wall. Additional information on site conditions, elevation, diameter and angle of hole, material used during drilling or casing, observations on the extracted core or chip pings, and the people involved in the project are all part of standard field logs that can aid geophysical interpretation.

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Characterizing Physical Properties (Labwork)

Measurements are made to determine the physical characteristics of geological materials. This is done by placing a homogeneous sample in specialized equipment designed to measure that particular property under either controlled or in-situ conditions.

Extracting an intact sample is essential for testing of geological materials in controlled conditions.
The material – rock or soil – is obtained using a set standard of care intended to preserve the in- situ properties (such as structure, water content, or density) that are relevant to the particular tests. This is frequently accomplished through some variety of core drilling machine such as that manufactured by The Testwel Instruments Company.

The advantages of laboratory testing is the ability to measure the particular characteristics of the sample uncontaminated by ambient noise. The disadvantage of laboratory testing is that some physical properties change when removed from their in-situ environment, and extracting a sample is destructive. Particular care must be taken when determining the correct sample dimensions for size-dependent mechanical properties, like strength, where the specimen size strongly influences the characteristic.

Examples of physical characteristics that are measured in a laboratory setting, and the machines used to make those measurements Matest Hoek cells used to measure permeability, the Barrington Instruments MS3 for measuring magnetic susceptibility, and triaxial cells for measuring volumetric elasticity to determine the bulk modulus of a material.

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