The Electric Double Layer (EDL) near a mineral surface can influence bulk conductivity in several ways.
In this paper I exploit the similarity between the structure of the EDL near a mineral
surface and near an electrode to examine surface conductivity as pore water freezes.
High contact resistance in frozen ground is a serious problem for
resistivity and induced polarization surveys in cold climates. Recent work on the electrical properties of
ice has shown that contact resistance can in some cases be drastically lowered by applying a small potential
to the electrodes while they are being frozen into the formation. This paper describes my recent work on this
Injection electrode polarization occurs when a current is driven through the interface between the
metallic electrode and the pore electrolyte. The
polarization potential, which is dependent on the current density and the polarity of the electrode can reach several Volts and may take
many minutes to decay once the current has been shut off. I demonstrate the conditions
under which this can occur and make some recommendations for resistivity and induced polarization surveys.
Electrode impedances contribute to the overall impedance between an electrode and the ground.
They are caused by processes that occur at the electrode electrolyte contact, such as,
activation overvoltage, concentration overvoltage, ohmic overvoltage and electro-osmosis.
They can vary by several orders of magnitude in seconds and are a concern in resistivity and induced polarization.
The rotational dynamics of a planet are sensitive to the size and state (fluid or solid) of its core. In this work we examine the possibility of determining the state of the Martian core by VLBI measurements of its rotation.
Fractures in till can greatly increase the electrical and hydraulic conductivity. In this paper, Mark and I
describe an experiment to characterize the fracture pattern in a well-studied field site near Birsay, Saskatchewan.
Inhomogeneities, a gravel stringer and variations in the thickness of the overburden, greatly increase the
difficulty of extracting the fracture pattern from azimuthal resistivity measurements.
The potash mining machine is a broadband source of both P wave and S waves. In this papaer we describe experiments
to use the mining machine as a continuous seismic source.
The response method is a technique originally developed for the analysis of sea level data. It permits the separation of a linear response to the tide raising potential from a non-linear response, or a non-tidal response. In this paper I use synthetic data to show how the response method can be used to extract a small non-tidal signal arbitrarily close in frequency to a large amplitude tidal signal and real data to extract degree three diurnal and semi-diurnal tides and atmospheric pressure tides at coincident earth tide frequencies.
This paper examines the response of a shallow conducting target in a moderatly
conducting host. Three separate modes of response are recovered: a galvanic mode, an
induction or vortex mode, and a viscous magnetic remanence. These are identified on
the basis of spatioal characteristics and temporal decay. Many other geophysical
techniques have been used on this target. Most of these show either no response or a
very weak response from one mode. Time domain em is the only technique that
shows a response from all three modes.
The reference level stability is a critical issue for superconducting gravimeters.
In this paper my co-authors and I describe a series of nine experiments in which an
absolute gravimeter and a superconducting gravimeter were operated in close proximity.
The experiments lasted for about a week at a time and were conducted over a two and a
half year period between Feb. 1998 and June 2000. These reveal that the superconductor
has drifted by no more than 3 microgals during this time. Short term offsets between the
absolute gravimeter and the superconductor of several microgal amplitude have been
observed, suggesting that one or both instruments may show spurious changes in gravity.
We are continuing these experiments.
Co-authored with D.E. Smylie and O. Francis. This paper looks at a controversial inner core mode called the center of mass mode. Implications for inner core density and viscosity of the outer core near the inner core boundary are discussed.
This paper examines a set of superconducting gravimeter observations from a global distribution of sites in a search for the inner core translation mode. My contribution to this paper was the processing of the Cantley data and the tidal analysis for most of the other stations.
This paper outlines the goal of the Global Geodynamics Project and describes the network of superconducting gravimeters that has been set up.
Zonal tides are lunar and solar tides that alternately increase and decrease the axial moment of inertia of the Earth and consequently change the rotational speed of the Earth. In this investigation, we used Very Long Baseline Interferometry to measure these changes in the length of day. We were able to demonstrate that all of the observed dispersion was due to ocean tides. Precise knowledge of the Earth's orientation in space is crucial for space navigation, astronomy, and navigation on Earth with systems like the Global Positioning System.
The sensitivity, low drift and stability of the superconducting gravimeter allow it to detect small signals that could not be detected by any other gravimeter. In this paper I showed that the Cantley gravimeter was responding to the loading effect of non-linear ocean tides in the Bay of Fundy and North Atlantic. This is a remarkable result. It confirms the gravimeter as a tool to investigate global ocean dynamics, because I was able to show that our residuals were sensitive to changes in sea level of a tenth of a mm! The technique I used to separate the small non-linear tides from the much larger linear has broader applicability and is being used to look for vibrations in the Earth's core. After the loading effects of non-linear tides are accounted for, there is no remaining artifact of non-linear response in the solid Earth, or in the instrument. This is still the most stringent limit on anelastic response of the Earth at tidal periods.
The Nearly Diurnal Free Wobble, is a small, globally coherent, periodic change in latitude that exists only because the Earth's core is fluid. It has a counterpart in nutation called the Free Core Nutation. The period of the Nearly Diurnal Free Wobble is about one day, and of the Free Core nutation 430 days. The exact period of both is a function of how the fluid core interacts with its container - the mantle, and mostly this is determined by the shape of the core mantle boundary. My measurement requires the interaction between the core and the mantle to be slightly tighter than anticipated. Other measurements of this phenomenon, as a wobble or as a mutation, also require a tighter coupling. The gravity results and astronomical results disagree on the damping time - most gravity measurements indicate a shorter damping time than the astronomical measurements. More recent gravity measurements have been in close agreement with my result.
Prior to the deployment of the superconducting gravimeter, tide tables, or catalogues, contained enough information to adequately analyse the tides the instruments were observing. The superconducting gravimeter is much more sensitive, and once these became available I, several others, attempted to assess the various tide catalogues to determine if they were adequate, and if not, what was needed. The software described in this paper was intended to test tide predictions based on tide tables. I found that two of the recently produced catalogues were adequate for work with superconducting gravimeter data. This demonstration has leant confidence to conclusions, such as those in the above papers, that are really dependent on the tide tables.
Atmospheric pressure affects gravity at a level 90dB above the sensitivity of the superconducting gravimeter. Local atmospheric pressure causes most of the effect, but in this paper I showed that the instrument is responding to the global atmospheric pressure field. I showed what was required to do an adequate atmospheric pressure correction and in a later paper I showed how a less accurate correction could be done very quickly.