GSA 2014 Vancouver TweetUp

Are you coming to GSA 2014 in Vancouver? We’re holding a meet-up at Steamworks Brew Pub on Sunday night (October 19th) at 8pm. The venue is less than a 5-minute walk from the conference and across the parking lot from Waterfront Station (the terminus station for all Skytrains and the seabus).

GSA meetup Sunday October 19, 2014 at 8pm.

GSA meetup Sunday October 19, 2014 at 8pm.

Steamworks is an on-site microbrewery and well-lit all-ages restaurant. Please let me know if you feel uncomfortable with this venue for a professional-social meetup, and we can try to figure out how to adjust things to address your concerns.

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Geoscientists Are Descending on Vancouver

Vancouver is drawing geoscientists into its grasp this October with the arrival of the Geological Society of America meeting. If you’re coming to the science-party, here’s a few ideas on how to spend your time outside the conference:

That’s it for now; as always you can contact me in the comments here, on Twitter, or by writing to mika @ this domain.

If you’re wondering why this blog has been so quiet in recent months, that’s because I’m writing daily about Earth, space, and planetary sciences for io9!

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Things I Wrote This Week

For the past two months, I’ve been writing for the Space subsite on io9. If you haven’t checked it out before, here’s some stories stories from this week that you might find interesting:

I also have a massive In Case You Missed It post covering the Space subsite for April, just in case you want to spiral down a recursive network of link-clicking. But really, I wrote some cool things last month that you might find awesome if this is the first you’ve heard of me writing for io9.

This is the last month of my trial period as an io9 Recruit. If I don’t make 300k US People in traffic for the month of May, I will no longer be writing on the io9 website. If that fills you with a gasp of denial, spread the love by sharing a story you liked on Facebook, Twitter, or whatever social media platform you desire to help me reach new readers.

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Tips for Students: fieldwork

The summer field season is rappidly approaching. If you missed it last year, check out the EGU’s Geology for Global Development mapping projects guidelines.

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Detecting Radioactive Anomalies

The low penetrating power of alpha and beta particles make them unlikely to be detected during field surveys. Instead, most radiometric surveys concentrate on detecting gamma rays. The low attenuation rate of gamma rays passing through air make both ground and airborne surveys effective.

Because radioactive decay is an ongoing process, readings must take place over long enough times to be statistically meaningful. Statistical error (σ) is proportional to the number of detected events (n): σ = 100/ √n
such that 10,000 counts produces a 1% error. This is not practical in areas with low count rates, so it is common practice to slowly traverse the area, pausing for time-consuming total-count readings when encountering radiometric anomalies.

The substantial variation in attenuation in different materials and moisture levels makes detailed field notes of surficial features where measurements are taken. The presences of bare rock, overburden, standing water (including puddles) and recent and ongoing weather conditions may all influence readings. Gamma-ray spectography is particularly effective at dry, exposed bare rock outcrops.

Some quartz gravity meters contain a small amount of radioactive material to ionize residual gas and prevent build-ups of static electricity which can produce a detectable anomaly. Thus it is unwise to simultaneously conduct a radiometric and a gravity survey with those instruments to prevent introducing noise.

Radiometric surveys are undertaken to prospect for uranium or other industrial mineral deposits, for regional geologic mapping and correlation, and to determine phosphate concentrations in-situ. Surveys can also be used to monitor buried radioactive waste for leaks, to track groundwater by adding radioactive tracers, or to establish a public health risk from radon gas exposure.

Alpha Particle Monitors

Radon produces radon gas, an odourless, colourless, upwardly buoyant gas that can easily penetrate soil and rock, and causes public health problems (including increased risk of lung cancer) when in- haled. It may be present without significant gamma radiation, and is more reliably detected through measuring alpha particles.

Alpha particles are detected by leaving a small detector on site for at least 12 hours. In field surveys, the detector is sometimes suspended within a small chamber and buried.

Borehole Logging

Nuclear logs may be taken in open or cased boreholes, above or below the water table. However, the measured radiation will change from above to below the water table even if accompanied by no change in lithology due to changes in the oxidation/reduction processes.

Natural Gamma Log

A natural gamma log is a passive measurement of naturally emitted gamma radiation using a scintillometer, usually measured in American Petroleum Institute (API) units. Lithographies with more uranium, potassium, or thorium will produce higher emitters of gamma radiation. A spectral log of the natural gamma can be made to differentiate between uranium, thorium, and potassium sources.
Natural gamma is used to define lithologies, make stratigraphic correlations between boreholes, and assess the relative sand, silt, and clay content of a unit. It is also used in well completion studies, and during uranium, coal, and shale exploration.

Gamma-Gamma Log

A gamma-gamma log is an active measurement produced by irradiating the surrounding rock with a small gamma radiation source (usually caesium, 137Cs), and recording the back-scattered gamma radiation. Because gamma rays are inversely proportional to the density of irradiated rock, a gamma-gamma log can be used to create a density log of the borehole. It is useful for defining lithology, lithological correlation between boreholes, and estimating bulk density and moisture content of the material.

Neutron-Neutron Log

A neutron-neutron log is an active measurement produced by irradiating the surrounding rock with higher-energy neutrons (from a plutonium-beryllium source), then recording the how many neutrons are recorded in a detector a short distance (30+ centimetres) away from the source. The neutrons lose energy to collisions within the rock, especially to hydrogen nucleus which have a similar mass to the neutrons, so a lower count is typically indicative of high hydrogen concentrations. Neutron-neutron logs can help define lithology, and aid in defining zones of saturated porosity where water or oil fill the pore spaces.

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Characterizing Radioactivity

Detection of radioactivity depends on the type of radiation being targeted. Geiger Counters Gieger counters are short tubes filled with a low-pressure mixture of gas with electrodes maintained at a large potential difference. When radiation entered through the window, the … Continue reading

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Radioactivity of Geological Materials

Primeval elements with long half-lives such as potassium (40K), thorium (232Th), and uranium (235U, 238U produce elements with shorter half-lives as part of the decay process. Other primeval elements such as calcium (48Ca), vanadium (50V), and nickel (58Ni) are either rare or very weakly radioactive, so are not significant when determining radioactivity of specific minerals.

Potassium typically decays via beta emission into calcium, or by electron capture and gamma ray emission to form argon. All other geologically important radioactive elements produce daughter elements which are themselves radioactive, creating a radioactive series. Although uranium and thorium have more than one possible decay mode, with potentially highly complex decay chains, both eventually decay into stable lead (Pb). The stages of the decay chains, and the half-lives of the intervening steps, significantly impact the number and energy of the gamma rays produced.

Acid igneous rocks, evaporates, and rocks formed in reducing environments all have a higher proportion of primeval elements than other rocks. The same equipment is used to measure the radioactivity of a sample to determine its physical properties, or while in a field situation detecting anomalies.

Due to strong attenuation, radioactivity is only detected in a thin surface layer, so geometry plays an important role in anomaly detection. Anomalies with a large lateral extent will be easily detected, while those with a small lateral extent may be missed if the distance to the detector is too great.

Typically, shales and sylvite contain more potassium so have high natural gamma, while limestone, sandstone, and gypsum have very little potassium so have low natural gamma. Igneous rocks with feldspar and mica weather into clays with the ability to accommodate large radioactive ions, so will also have higher natural gamma.

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Radioactivity

Some elements are naturally radioactive, releasing alpha particles, beta particles, or gamma radiation as they decay to more stable elements. The decay in a fixed period of time is proportional to the number of unstable atoms, with exponential decay dictating a half-life for radioactive elements.

Alpha Particles

Alpha particles (α) consist of two neutrons and two protons, resulting in a positively-charged relatively large mass equivalent to a stable helium nucleus. Emission of an alpha particle causes the radioactive element to decrease in atomic mass by four, and in atomic number by two. The alpha particle is relatively large with large kinetic energy, but loses that energy quickly to collisions.

At thermal energies, the alpha particle will quickly acquire two electrons, evolving into a regular helium atom over an average distance of less fractions of a millimetre in solid rocks. This means that alpha particles have extremely weak penetration, and can be blocked by a piece of paper. They are easily blocked by any overburden, even a thin layer of sand or gravel, so may only be detected from bare rock exposed at the surface.

Beta Particles

Beta particles (β) consist of electrons ejected from the nuclei, resulting in a negatively-charged small mass equivalent with higher kinetic energy than a normal electron. Emission of a beta particle causes the radioactive element to become an isotope of the original element. The beta particle loses kinetic energy quickly to collisions, particularly by colliding with other electrons.

The beta particle will quickly lose its excess kinetic energy, becoming indistinguishable from surrounding electrons within centimes of travel within solids or liquids. This means that beta particles have weak penetration, and can be blocked by a thin sheet of aluminium. Like alpha particles, beta particles are also easily blocked by any overburden.

Gamma Radiation

Gamma rays (γ) are electrically neutral photos with energy (≥ 0.1 MeV) proportional to frequency (≥ 0.25 x 1020 Hz ) and a very short wavelength (≈ 10 m). Emission of a gamma ray results in the original element reducing from an excited state to an unexcited state. Not all decay stages produce gamma rays, and not all gamma rays have equal energy.

Because gamma rays are electrically neutral, they have greater penetrating power than electrically charged alpha and beta particles and are more likely to be detected through overburden, giving them greater utility during geological surveys. Even so, approximately 90% of detected particles originate in the top 20 to 30 centimetres of rock, or 50 centimetres of soil. Water increases the attenuation of gamma rays, so measuring dry rock is preferred to avoid absorption by moisture. Attenuation is frequency dependent, with higher frequency rays penetrating better, although all frequencies can be blocked by centimetres of lead.

Natural gamma rays can be proceed by terrestrial or cosmic sources. The terrestrial sources are related to stages in the decay chains, while those from cosmic sources are typically much higher energy. The background noise of gamma radiation consists of very high energy gamma rays produced by cosmic sources, gamma rays that have lost energy to electron-ejecting collisions (Compton scatter- ing), and gamma rays that are absorbed in electron-ejecting collision (photoelectric effect). This background radiation curve must be accounted for when processing gamma ray spectra.

Additionally, the particular gamma ray frequency produced by one decay series might overlap with the frequency of gamma rays produced by a different decay chain. This must be corrected by stripping the measured gamma ray windows, an automated instrument-dependent process. Geologically important peaks are at 2.62 MeV (for thorium decay into 208Th), 1.76 MeV (for uranium decay into 214Bi), and 1.46 MeV (for potassium decay into 40K). If the primeval elements are at equilibrium with the daughter products, the corrected gamma rays measured as a window around can be used in a spectrum analysis to calculate concentrations of the three parent elements.

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LA Earthquake: Don’t panic.

I wrote an anti-hysterical guide to the earthquakes in Los Angeles for io9. Head over for the geology, seismology of recent earthquakes, interpreting risk forecasting, and preparing for the Big One without freaking out.

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Detecting Density Anomalies

Gravity field surveying is a passive technique to detect gravity anomalies. Gravity is a global field that always points vertically down, with anomalies caused by very small variations in density of the underlying materials.

A gravity base station measures the rate of a mass falling over a known distance, but it is difficult to measure absolute gravity to the accuracy necessary for interpretation. Instead, gravimeters are used to measure relative gravity with respect to deviations from a fixed base station. A gravimeter consists of a weight offset with a spring stiffened to compensate for local regional gravity conditions. The weight will then move up or down in the presence of a positive or negative anomaly.

Processing Corrections

Elevation and topography impact gravimeter measurements, and require corrections in order to process the collected data when interpreting density of the underlying materials. For higher elevations, it is necessary to subtract the Bouger correction and add the Free-Air correction, while the inverse applies to lower elevations. Topography corrections are always positive. For airborne surveys, the Eo ̈tvo ̈s correction compensates for the aircraft’s speed and direction at the time of the measurement.

The Earth’s gravity field also has regional variations independent of material properties. Because the Earth is not a perfect sphere, and is instead a flattened spheroid, an observer at the poles is closer to the centre of mass while one at the equator has more mass between them and the center of mass. Add in that the Earth is rotating such that centripetal acceleration is stronger at the equators than the poles, and the net impact is that gravity is approximately +0.53% stronger at the poles than at the equator. When interpreting gravity survey data, latitude corrections may be necessary. Latitude corrections are based on the current geodetic model of the appropriate scale, adding a correction to survey measurements south of the base station, and subtracting a correction for survey measurements north of the base station.

Even base stations vary with time, with cyclic variation from tides that may automatically calculated and corrected, and with instrumental drift caused by spring creep or variations in temperature or pressure that needs to be manually corrected with a drift curve. For surveys that go beyond a local region and latitude corrections become necessary, the survey must overlap base stations, and the base stations tied to the nearest network station where absolute gravity has been accurately measured.

All of these corrections make it relatively difficult to process and interpret gravity data compared to other forms of geophysical surveys. However, surveys can be conducted from almost any platform: ground, air, marine, satellite, and from boreholes.

Time Lapse Microgravity Surveys

Time-lapse microgravity studies are an alternate approach to processing. The same area is surveyed at different times, with only cyclic corrections applied, leaving anomalies from nearby structures and other artifacts intact. The readings are then compared to each other such that any stable anomaly cancels out, leaving only unexplained temporal changes. This can be used to monitor response to fluid injection, reservoir withdrawal, or subsidence.

Borehole Logging

The response of irradiated rock can be used to interpret the density of that rock. This will be discussed next month.

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