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USGS2018

Inversion of data from electrical resistivity imaging surveys in water-covered areas

Electrical resistivity imaging surveys widely used in many environmental and engineering studies have also been conducted in water-covered areas. Surveys in water-covered areas include conventional surveys using multi-electrode resistivity systems where part of the survey line crosses a river or stream, and surveys conducted entirely within a water-covered environment. Surveys that are located entirely within a water-covered environment utilise electrodes mounted on a streamer, towed behind a boat. The streamer can be dragged along the water bottom, or float on the water surface. In this paper, the smoothness-constrained least-squares inversion method commonly used to interpret electrical resistivity imaging data from land surveys is adapted for underwater surveys. To accommodate the water bottom topography, a distorted finite-element grid is used to calculate the apparent resistivity values for the inversion model. The first few rows of elements are used to model the water layer, while the lower part of the grid is used for the sub-bottom resistivity distribution. For robust inversion, the water column resistivity and geometry must be known accurately as a large proportion of the current flows through the water layer. The section of the Earth below the bottom surface is subdivided into a large number of rectangular cells. The water column resistivity and geometry in the earth model is fixed, and the inversion program attempts to determine the resistivity of the cells that would most accurately reproduce the observed resistivity measurements. Implementation of water column resistivity and geometric constraints is demonstrated using numerical simulations and field data. Examples of electrical resistivity imaging surveys conducted on and across water bodies including rivers and near-shore marine environments are shown.

Exploration Geophysics↗

USGS2013

Uranium redox transition pathways in acetate-amended sediments

Redox transitions of uranium [from U(VI) to U(IV)] in low-temperature sediments govern the mobility of uranium in the environment and the accumulation of uranium in ore bodies, and inform our understanding of Earth’s geochemical history. The molecular-scale mechanistic pathways of these transitions determine the U(IV) products formed, thus influencing uranium isotope fractionation, reoxidation, and transport in sediments. Studies that improve our understanding of these pathways have the potential to substantially advance process understanding across a number of earth sciences disciplines. Detailed mechanistic information regarding uranium redox transitions in field sediments is largely nonexistent, owing to the difficulty of directly observing molecular-scale processes in the subsurface and the compositional/physical complexity of subsurface systems. Here, we present results from an in situ study of uranium redox transitions occurring in aquifer sediments under sulfate-reducing conditions. Based on molecular-scale spectroscopic, pore-scale geochemical, and macroscale aqueous evidence, we propose a biotic–abiotic transition pathway in which biomass-hosted mackinawite (FeS) is an electron source to reduce U(VI) to U(IV), which subsequently reacts with biomass to produce monomeric U(IV) species. A species resembling nanoscale uraninite is also present, implying the operation of at least two redox transition pathways. The presence of multiple pathways in low-temperature sediments unifies apparently contrasting prior observations and helps to explain sustained uranium reduction under disparate biogeochemical conditions. These findings have direct implications for our understanding of uranium bioremediation, ore formation, and global geochemical processes.

PNAS↗

USGS2014

Spatially robust estimates of biological nitrogen (N) fixation imply substantial human alteration of the tropical N cycle

Biological nitrogen fixation (BNF) is the largest natural source of exogenous nitrogen (N) to unmanaged ecosystems and also the primary baseline against which anthropogenic changes to the N cycle are measured. Rates of BNF in tropical rainforest are thought to be among the highest on Earth, but they are notoriously difficult to quantify and are based on little empirical data. We adapted a sampling strategy from community ecology to generate spatial estimates of symbiotic and free-living BNF in secondary and primary forest sites that span a typical range of tropical forest legume abundance. Although total BNF was higher in secondary than primary forest, overall rates were roughly five times lower than previous estimates for the tropical forest biome. We found strong correlations between symbiotic BNF and legume abundance, but we also show that spatially free-living BNF often exceeds symbiotic inputs. Our results suggest that BNF in tropical forest has been overestimated, and our data are consistent with a recent top-down estimate of global BNF that implied but did not measure low tropical BNF rates. Finally, comparing tropical BNF within the historical area of tropical rainforest with current anthropogenic N inputs indicates that humans have already at least doubled reactive N inputs to the tropical forest biome, a far greater change than previously thought. Because N inputs are increasing faster in the tropics than anywhere on Earth, both the proportion and the effects of human N enrichment are likely to grow in the future.

Proceedings of the National Academy of Sciences of↗

USGS2002

Diffuse fluid flux through orogenic belts: Implications for the world ocean

Fifty years ago a classic paper by W. W. Rubey [(1951) Geol. Soc. Am. Bull. 62, 1111-1148] examined various hypotheses regarding the origin of sea water and concluded that the most likely hypothesis was volcanic outgassing, a view that was generally accepted by earth scientists for the next several decades. More recent work suggests that the rate of subduction of water is much larger than the volcanic outgassing rate, lending support to hypotheses that either ocean volume has decreased with time, or that the imbalance is offset by continuous replenishment of water by cometary impacts. These alternatives are required in the absence of additional mechanisms for the return of water from subducting lithosphere to the Earth's surface. Our recent work on crustal permebility suggests a large capacity for water upflow through tectonically active continental crust, resulting in a heretofore unrecognized degassing pathway that can accommodate the waer subduction rate. Escape of recycled water via delivery from the mantle through zones of active metamorphism eliminates the mass-balance argument for the loss of ocean volume or extraterestrial sources.

Proceedings of the National Academy of Sciences of↗

USGS2025

State shifts in the deep Critical Zone drive landscape evolution in volcanic terrains

Understanding the near-surface environment where atmospheric and solid earth processes interact, often termed the “Critical Zone,” is important for assessing resources and building resilient societies. Here, we examine a volcanic landscape in the Oregon Cascade Range, an understudied Critical Zone setting that is host to major regional water resources, pervasive silicate weathering, and significant geohazards. We leverage a bedrock age chronosequence to show that the volcanic Critical Zone undergoes a structural shift, from depth extents of >1 km to meters, over timescales of ~1 My. We map an active groundwater volume comparable to major continental lakes, stored at the Cascade Range crest. This state shift makes volcanic landscape evolution a unique probe of deep coupling between Earth systems.

PNAS↗

USGS2010

Pb-isotopic systematics of lunar highland rocks (>3.9 Ga): Constraints on early lunar evolution

The present lead (Pb)-isotopic database of over 200 analyses from nearly 90 samples of non-mare basalt, lunar highland rocks (>3.9 Ga) delineate at least three isotopically distinct signatures that in some combination can be interpreted to characterize the systematics of the entire database. Two are fairly new sets of lunar data and are typical of Pb data from other solar-system objects, describing nearly linear arrays slightly above the “geochron” values, with 207 Pb/ 206 Pb values <0.9. In aggregate, those data allow a relatively new interpretation of the Pb-isotopic evolution of the Moon, helping to identify and characterize a planetary evolutionary stage not preserved by Earth rocks. From the Pb-isotopic compositions, a source 238 U/ 204 Pb (μ) value can be calculated; differences in μ values from one Pb signature to another can have important implications on the age and evolution of Moon-forming events. In particular, the U/Pb ratio is one method of measuring the fractionation between refractory and volatile elements, an important indicator when considering large-scale planetary differentiation stages. The oldest Pb signature is represented solely by ferroan anorthosite 60025, presumably a piece of the early plagioclase-rich lunar crust produced from a magma ocean, and is characterized by source μ values between 35 and 100 at ~4.43 Ga. Another prominent and uniquely lunar Pb signature, identified more than 20 years ago, is representative of many ferroan anorthosites and most high-Mg suite rocks (particularly 15415, 62237, 76535, and 78235), and is characterized by extremely high 207 Pb/ 206 Pb values (∼1.45) that require extremely high source μ values (>500). Although the age and origin of this exotic Pb is not well constrained, it is interpreted to be related to the entrapment of incompatible-element-rich (U, Th) melts within the lunar upper mantle and crust between 4.36 and 4.46 Ga (urKREEP residuum?). The latest discovered Pb signature is found only in lunar meteorites and is characterized by relatively low source μ values between 10 and 50 at 3.9 Ga. The fact that most lunar crustal rocks (>3.9 Ga) exhibit high 207 Pb/ 206 Pb values requires that they were derived from, mixed with, or contaminated by Pb produced from early-formed, high-μ sources. The ubiquity of these U-Pb characteristics in the sample collection is probably an artifact of Apollo and Luna sampling sites, all located on the near side of the Moon, which was deeply excavated during the basin-forming event(s). However, the newest Pb-isotopic data support the idea that the Moon originally had a μ value of ~8 to 35, slightly elevated from Earth values, and that progressive U-Pb fractionations occurred within the Moon during later stages of differentiation between 4.36 and 4.46 Ga.

International Geology Review↗

USGS2014

Auroral omens of the American Civil War

Aurorae are a splendid night-time sight: coruscations of green, purple, and red fluorescent light in the form of gently wafting ribbons, billowing curtains, and flashing rays. Mostly seen at high latitudes, in the north aurorae are often called the northern lights or aurora borealis, and, in the south, the southern lights or aurora australis. The mystery of their cause has historically been the subject of wonder. The folklore and mythology of some far-northern civilizations attributed auroral light to celestial deities. And, in ironic contrast with their heavenly beauty, unusual auroral displays, such as those seen on rare occasions at lower southern latitudes, have sometimes been interpreted as portending unfavorable future events. Today we understand aurorae to be a visual manifestation of the dynamic conditions in the space environment surrounding the earth. Important direct evidence in support of this theory came on September 1, 1859. On that day, an English astronomer named Richard Carrington was situated at his telescope, which was pointed at the sun. While observing and sketching a large group of sunspots, he saw a solar flare—intense patches of white light that were superimposed upon the darker sunspot group and which were illuminated for about a minute. One day later, a magnetic storm was recorded at specially designed observatories in Europe, across Russia, and in India. By many measures, the amplitude of magnetic disturbance was the greatest ever recorded. In the United States, the effects of the Carrington storm could be seen as irregular backand-forth deflections of a few degrees in the magnetized needle of a compass. Rapid magnetic variation also induced electric fields in the earth’s conducting lithosphere, and interfered with the operation of telegraph systems. The Carrington magnetic storm, and an earlier storm that had occurred on August 28, 1859, caused spectacular displays of aurora borealis in the night-time sky over the entire United States and the western hemisphere, possibly all the way down to the equator. This was extremely unusual, so much so that an auroral event o

Weatherwise↗

USGS2009

Remote sensing of snow and ice

Monitoring of snow and ice on the Earth's surface will require increasing use of satellite remote sensing techniques. These techniques are evolving rapidly. Active and passive sensors operating in the visible, near infrared, thermal infrared, and microwave wavelengths are described in regard to general applications and in regard to specific USA or USSR satellites. Meteorological satellites (frequent images of relatively crude resolution) and Earth resources satellites such as Landsat (less frequent images of higher resolution) have been used to monitor the areal extent of seasonal snow, but problems exist with cloud cover or dense forest canopies. Snow mass (water equivalent) can be measured from a low-flying aircraft using natural radioactivity, but cannot yet be measured from satellite altitudes. A combination of active and passive microwave sensors may permit this kind of measurement, but not until more is known about radiation scattering in snow. Satellite observations are very useful in glacier inventories, correcting maps of glacier extent, estimating certain mass balance parameters, and monitoring calving or surging glaciers. Ground ice is virtually impossible to monitor from satellites; ice on rivers and lakes can be monitored only with very high-resolution sensors. Microwave sensors, due to their all-weather capability (the ability to see through clouds) provide exciting data on sea ice distribution. Analysis of digital tapes of satellite data requires the archiving and scanning of huge amounts of data. Simple methods for extracting quantitative data from satellite images are described.

Hydrological Sciences Bulletin↗

USGS1999

International river basins of the world

It is becoming acknowledged that water is likely to be the most pressing environmental concern of the next century. Difficulties in river basin management are only exacerbated when the resource crosses international boundaries. One critical aid in the assessment of international waters has been the Register of International Rivers a compendium which listed 214 international waterways that cover 47% of the earth's continental land surface. The Register, though, was last updated in 1978 by the now defunct United Nations Department of Economic and Social Affairs. The purpose of this paper is to update the Register in order to reflect the quantum changes that have taken place over the last 22 years, both in global geopolitics and in map coverage and technology. By accessing digital elevation models at spatial resolutions of 30 arc seconds, corroborating at a unified global map coverage of at least 1:1 000 000, and superimposing the results over complete coverage of current political boundaries, we are able to provide a new register which lists 261 international rivers, covering 45.3% of the land surface of the earth (excluding Antarctica). This paper lists all international rivers with their watershed areas, the nations which share each watershed,their respective territorial percentages, and notes on changes in or disputes over international boundaries since 1978.

International Journal of Water Resources Developme↗

USGS1998

North American landscape characterization project: The production of a continental scale three-decade Landsat data set

The North American Landscape Characterization (NALC) project is a component of the National Aeronautics and Space Administration (NASA) Landsat Pathfinder program. Pathfinder projects are focused on the investigation of global change utilizing current remote sensing technologies. The NALC project is a cooperative effort between the U. S. Environmental Protection Agency (EPA), the U.S. Geological Survey (USGS), and NASA to make Landsat data available to the widest possible user community for scientific research and general public interest. The NALC project is principally funded by the EPA Office of Research and Development and the USGS's Earth Resources Observation Systems (EROS) Data Center (EDC). The objectives of the NALC project are to produce standardized remote sensing data sets, develop standardized analysis methods, and derive standardized land cover change products for a large portion of the North American continent (the conterminous United States and Mexico) (Lunetta and Sturdevant, 1993). The standard product is the NALC “triplicate”;, consisting of co‐registered Landsat multispectral scanner data for the years 1973, 1986, and 1991 (plus or minus one year), plus co‐registered 3 arcsecond digital terrain elevation data. Processing began with the 1986 scene, which was precision corrected (with full terrain correction) to a 60 meter Universal Transverse Mercator base. Automated cross‐correlation procedures were used to co‐register the 1970's and 1990's data to the 1980's base, and independent verifications of registration quality were performed on all triplicate components. The pertinent metadata were compiled in a relational database, which includes WRS2 path/rows, scene ID's, image dates, solar azimuth and elevation, verification RMSE's, and the number of verification control points. NALC triplicate data sets are being used for a number of applications, including the analysis of urbanization patterns, dynamics of climatic fluctuations, deforestation studies, and vegetation classification and mapping. These data are being distributed through the Earth Observing System Data and Information System (EOSDIS) Information Management System (IMS) at a cost of $15(U.S.) for each triplicate.

Geocarto International↗

USGS2019

An introduction to the “Oceans and Society: Blue Planet” Initiative

We live on a blue planet, and Earth’s waters benefit many sectors of society. The future of our blue planet is increasingly reliant on the services delivered by marine, coastal and inland waters and on the advancement of effective, evidence-based decisions on sustainable development. “Oceans and Society: Blue Planet” (hereafter denoted as “GEO Blue Planet”) is an initiative of the Group on Earth Observations (GEO) that aims to ensure the sustained development and use of ocean and coastal observations for the benefit of society. GEO Blue Planet works to advance and exploit synergies among the many observational programmes devoted to ocean and coastal waters; to improve engagement with a variety of stakeholders for enhancing the timeliness, quality and range of information delivered; and to raise awareness of the societal benefits of ocean observations at the public and policy levels. This paper summarizes the role of GEO Blue Planet, current activities and considerations for future directions.

Journal of Operational Oceanography↗

USGS2009

Review of FEWS NET biophysical monitoring requirements

The Famine Early Warning System Network (FEWS NET) provides monitoring and early warning support to decision makers responsible for responding to famine and food insecurity. FEWS NET transforms satellite remote sensing data into rainfall and vegetation information that can be used by these decision makers. The National Aeronautics and Space Administration has recently funded activities to enhance remote sensing inputs to FEWS NET. To elicit Earth observation requirements, a professional review questionnaire was disseminated to FEWS NET expert end-users; it focused upon operational requirements to determine additional useful remote sensing data and, subsequently, to assess whether such data would be beneficial as FEWS NET biophysical supplementary inputs. The review was completed by over 40 experts from around the world. Reviewers were asked to evaluate the relative importance of environmental variables and spatio-temporal requirements for Earth science data products, in particular for rainfall and vegetation products. The results showed that spatio-temporal resolution requirements are complex and need to vary according to place, time, and hazard; that high resolution remote sensing products continue to be in demand; and that rainfall and vegetation products are valued as data that provide actionable food security information.

Environmental Research Letters↗

USGS2019

The NASA Roadmap to Ocean Worlds

In this article, we summarize the work of the NASA Outer Planets Assessment Group (OPAG) Roadmaps to Ocean Worlds (ROW) group. The aim of this group is to assemble the scientific framework that will guide the exploration of ocean worlds, and to identify and prioritize science objectives for ocean worlds over the next several decades. The overarching goal of an Ocean Worlds exploration program as defined by ROW is to “identify ocean worlds, characterize their oceans, evaluate their habitability, search for life, and ultimately understand any life we find.” The ROW team supports the creation of an exploration program that studies the full spectrum of ocean worlds, that is, not just the exploration of known ocean worlds such as Europa but candidate ocean worlds such as Triton as well. The ROW team finds that the confirmed ocean worlds Enceladus, Titan, and Europa are the highest priority bodies to target in the near term to address ROW goals. Triton is the highest priority candidate ocean world to target in the near term. A major finding of this study is that, to map out a coherent Ocean Worlds Program, significant input is required from studies here on Earth; rigorous Research and Analysis studies are called for to enable some future ocean worlds missions to be thoughtfully planned and undertaken. A second finding is that progress needs to be made in the area of collaborations between Earth ocean scientists and extraterrestrial ocean scientists.

Astrobiology↗

USGS2020

Coseismic and post-seismic gravity disturbance induced by seismic sources using a 2.5-D spectral element method

I present a prescription for computing free-air coseismic and post-seismic gravity changes induced by seismic sources in a viscoelastic earth model. I assume a spherical earth geometry and a 2.5-D calculation, that is, 3-D motions that satisfy the equations of quasi-static equilibrium on a 2-D viscoelastic structure. The prescription permits application to regional gravity computations where a 2-D structure adequately represents the structural heterogeneity. I use a hybrid approach where deformation is computed on a discretized domain and the resulting density perturbations are expanded with spherical harmonics to produce the free-air gravity field. Starting with a solution to the equations of quasi-static displacements in the Laplace transform domain for a given dislocation source, I solve Poisson’s equation using Lagrangian interpolation on spectral element nodes to compute the required deformation quantities that contribute to free-air gravity. A numerical inverse Laplace transform then yields time domain results. This methodology is tested with analytic solutions on a spherically stratified viscoelastic structure, then applied to evaluate the effect of a descending slab of relatively high viscosity on post-seismic gravity in a megathrust faulting setting.

Geophysical Journal International↗

USGS2015

A Green's function approach for assessing the thermal disturbance caused by drilling deep boreholes in rock or ice

A knowledge of subsurface temperatures in sedimentary basins, fault zones, volcanic environments and polar ice sheets is of interest for a wide variety of geophysical applications. However, the process of drilling deep boreholes in these environments to provide access for temperature and other measurements invariably disturbs the temperature field around a newly created borehole. Although this disturbance dissipates over time, most temperature measurements are made while the temperature field is still disturbed. Thus, the measurements must be ‘corrected’ for the drilling-disturbance effect if the undisturbed temperature field is to be determined. This paper provides compact analytical solutions for the thermal drilling disturbance based on 1-D (radial) and 2-D (radial and depth) Green's functions (GFs) in cylindrical coordinates. Solutions are developed for three types of boundary conditions (BCs) at the borehole wall: (1) prescribed temperature, (2) prescribed heat flux and (3) a prescribed convective condition. The BC at the borehole wall is allowed to vary both with depth and time. Inclusion of the depth dimension in the 2-D solution allows vertical heat-transfer effects to be quantified in situations where they are potentially important, that is, near the earth's surface, at the bottom of a well and when considering finite-drilling rates. The 2-D solution also includes a radial- and time-dependent BC at the earth's surface to assess the impact of drilling-related infrastructure (drilling pads, mud pits, permanent shelters) on the subsurface temperature field. Latent-heat effects due to the melting and subsequent refreezing of interstitial ice while drilling a borehole through ice-rich permafrost can be included in the GF solution as a moving-plane heat source (or sink) located at the solid–liquid interface. Synthetic examples are provided illustrating the 1-D and 2-D GF solutions. The flexibility of the approach allows the investigation of thermal drilling effects in rock or ice for a wide variety of drilling technologies. Numerical values for the required radial GFs G R are available through the Advanced Cooperative Arctic Data and Information Service at doi:10.5065/D64F1NS6.

Geophysical Journal International↗

USGS2018

Rayleigh wave ellipticity measurement uncertainty across the IRIS/USGS and New China Digital Seismograph Networks

Long-period Rayleigh wave horizontal to vertical amplitude (H/V) ratios at a station provide information about local earth structure that is complementary to phase velocity. However, a number of studies have observed that significant scatter appears in these measurements making it difficult to use H/V ratio measurements to resolve earth structure. Some of the scatter in these measurements has been attributed to local geological structure while some has remained unaccounted for. Most Global Seismographic Network (GSN) stations contain two nearby high-quality broad-band seismometers (e.g. in the same vault, but on different piers or in different boreholes). For each broad-band sensor in the IRIS/USGS component of the GSN, we estimate H/V ratios of fundamental mode Rayleigh waves using M > 6.5 earthquakes from 2001 to 2018 (around 19 000 measurements). We compute these ratios at a number of discrete periods (25, 50, 75, 100 and 150 s) and find that for well-isolated Rayleigh waves (windows where the correlation coefficients between radial and the phase-shifted vertical components are greater than 0.9) significant scatter in H/V ratios occurs between colocated sensors (greater than 25 per cent at 100 s period). This suggests the scatter in H/V ratio measurements can be at least partially attributed to extremely local phenomena such as sensor emplacement in the vault. We also find that H/V ratios can vary as a function of event backazimuth, indicating that care must be taken when computing average ratios for a station, as a large number of events from a given region could bias H/V ratio measurements at a station.

Geophysical Journal International↗

USGS2019

Variability in synthetic earthquake ground motions caused by source variability and errors in wave propagation models

Numerical simulations of earthquake ground motions are used both to anticipate the effects of hypothetical earthquakes by forward simulation and to infer the behaviour of the real earthquake source ruptures by the inversion of recorded ground motions. In either application it is necessary to assume some Earth structure that is necessarily inaccurate and to use a computational method that is also inaccurate for simulating the wavefield Green's functions. We refer to these two sources of error as ‘propagation inaccuracies’, which might be considered to be epistemic. We show that the variance of the Fourier spectrum of the synthetic earthquake seismograms caused by propagation inaccuracies is related to the spatial covariance on the rupture surface of errors in the computed Green's functions, which we estimate for the case of the 2009 L'Aquila, Italy, earthquake by comparing erroneous computed Green's functions with observed L'Aquila aftershock seismograms (empirical Green's functions). We further show that the variance of the synthetic seismograms caused by the rupture variability (aleatory uncertainty) is related to the spatial covariance on the rupture surface of aleatory variations in the rupture model, and we investigate the effect of correlated variations in Green's function errors and variations in rupture models. Thus, we completely characterize the variability of synthetic earthquake seismograms induced by errors in propagation and variability in the rupture behaviour. We calculate the spectra of the variance of the ground motions of the L'Aquila main shock caused by propagation inaccuracies for two specific broad-band stations, the AQU and the FIAM stations. These variances are distressingly large, being comparable or in some cases exceeding the data amplitudes, suggesting that the best-fitting L'Aquila rupture model significantly overfits the data and might be seriously in error. If these computed variances are typical, the accuracy of many other rupture models for past earthquakes may need to be reconsidered. The results of this work might be useful in seismic hazard estimation because the variability of the computed ground motion, caused both by propagation inaccuracies and variations in the rupture model, can be computed directly, not requiring laborious consideration of multiple Earth structures.

Geophysical Journal International↗

USGS2019

Improvements in seismic resolution and current limitations in the Global Seismographic Network

Station noise levels play a fundamental limitation in our ability to detect seismic signals. These noise levels are frequency-dependent and arise from a number of physically different drivers. At periods greater than 100 s, station noise levels are often limited by the self-noise of the instrument as well as the sensitivity of the instrument to non-seismic noise sources. Recently, station operators in the Global Seismographic Network (GSN) have deployed several Streckeisen STS-6A very broadband borehole seismometers. These sensors provide a potential replacement for the no-longer-produced Streckeisen STS-1 seismometer and the GeoTech KS-54000 borehole seismometer. Along with showing some of the initial observational improvements from installing modern very broadband seismometers at depth, we look at current limitations in the seismic resolution from Earth tide periods 100,000 s (0.01 mHz) to Nyquist at most GSN sites (0.02 s or 50 Hz). Finally, we show the potential for improved observations of continuously excited horizontal Earth hum as well as the splitting of very long-period torsional modes as a result of installing instruments at depth. Both of these observations make use of the low horizontal noise levels which are obtained by installing very broadband borehole seismometers at depth with noise levels similar to the Streckeisen STS-1.

Geophysical Journal International↗