Export 8 results:
Sort by: [ Author  (Asc)] Title Type Year
A B C D E F G H I J K L [M] N O P Q R S T U V W X Y Z   [Show ALL]
Malinverno, A, Parker RL.  2006.  Two ways to quantify uncertainty in geophysical inverse problems. Geophysics. 71:W15-W27.   10.1190/1.2194516   AbstractWebsite

We present two approaches to invert geophysical measurements and estimate subsurface properties and their uncertainties when little is known a priori about the size of the errors associated with the data. We illustrate these approaches by inverting first-arrival traveltimes of seismic waves measured in a vertical well to infer the variation of compressional slowness in depth. First, we describe a Baye-Sian formulation based on probability distributions that define prior knowledge about the slowness and the data errors. We use an empirical Bayes approach, where hyperparameters are not well known ahead of time (e.g., the variance of the data errors) and are estimated from their most probable value. given the data. The second approach is a non-Bayesian formulation that we call spectral, in the sense that it uses the power spectral density of the traveltime data to constrain the inversion (e.g., to estimate the variance of the data errors). In the spectral approach, we vary assumptions made about the characteristics of the slowness signal and evaluate the resulting slowness estimates and their uncertainties. This approach is computationally simple and starts from a few assumptions. These assumptions can be checked during the analysis. On the other hand, it requires evenly spaced traveltime measurements, and it cannot be extended easily (e.g., to data that have gaps). In contrast, the Bayesian framework is based on a general theory that can be generalized immediately, but it is more involved computationally. Despite the conceptual and practical differences, we find that the two approaches give the same results when they start from the same assumptions: The allegiance to a Bayesian or non-Bayesian formulation matters less than what one is willing to assume when solving the inverse problem.

McKenzie, DP, Parker RL.  1967.  The North Pacific: an example of tectonics on a sphere. Nature. 216:1276-1280.   10.1038/2161276a0   AbstractWebsite

Individual aseismic areas move as rigid plates on the surface of a sphere. Application of the Mercator projection to slip vectors shows that the paving stone theory of world tectonics is correct and applies to about a quarter of the Earth's surface.

McKenzie, D, Parker RL.  1974.  Plate tectonics in ω space. Earth and Planetary Science Letters. 22:285-293.   10.1016/0012-821x(74)90137-x   AbstractWebsite

The relative angular velocity vectors of the plates covering the earth form a three-dimensional closed polyhedron, for which we propose the name geohedron. All forms of plate evolution produce simple changes in the geohedron. Corresponding bodies exist for relative angular accelerations and an attempt is made to determine the value of the relative accelerations of the plates forming a single triple junction when they are governed by kinematic effects alone, but the resulting values do not agree with the available observations.

McMillan, DG, Constable CG, Parker RL, Glatzmaier GA.  2001.  A statistical analysis of magnetic fields from some geodynamo simulations. Geochemistry Geophysics Geosystems.   10.1029/2000GC000130   AbstractWebsite

We present a statistical analysis of magnetic fields simulated by the Glatzmaier-Roberts dynamically consistent dynamo model. For four simulations with distinct boundary conditions, means, standard deviations, and probability functions permit an evaluation based on existing statistical paleosecular variation (PSV) models. Although none closely fits the statistical PSV models in all respects, some simulations display characteristics of the statistical PSV models in individual tests. We also find that nonzonal field statistics do not necessarily reflect heat flow conditions at the core-mantle boundary. Multitaper estimates of power and coherence spectra allow analysis of time series of single, or groups of, spherical harmonic coefficients representing the magnetic fields of the dynamo simulations outside the core. Sliding window analyses of both power and coherence spectra from two of the simulations show that a 100 kyr averaging time is necessary to realize stationary statistics of their nondipole fields and that a length of 350 kyr is not long enough to full characterize their dipole fields. Spectral analysis provides new insight into the behavior and interaction of the dominant components of the simulated magnetic fields, the axial dipole and quadrupole. Although we find spectral similarities between several reversals, there is no evidence of signatures that can be conclusively associated with reversals or excursions. We test suggestions that during reversals there is increased coupling between groups of spherical harmonic components. Despite evidence of coupling between antisymmetric and symmetric spherical harmonics in one simulation, we conclude that it is rare and not directly linked to reversals. In contrast to the reversal model of R. T. Merrill and P. L. McFadden, we demonstrate that the geomagnetic power in the dipole part of the dynamo simulations is either relatively constant or fluctuates synchronously with that of the nondipole part and that coupling between antisymmetric and symmetric components occurs when the geomagnetic power is high.

McMillan, DG, Constable CG, Parker RL.  2002.  Limitations on stratigraphic analyses due to incomplete age control and their relevance to sedimentary paleomagnetism. Earth and Planetary Science Letters. 201:509-523.   10.1016/s0012-821x(02)00747-1   AbstractWebsite

A major limitation in the analysis of physical quantities measured from a stratigraphic core is incomplete knowledge of the depth to age relationship for the core. Records derived from diverse locations are often compared or combined to construct records that represent a global signal. Time series analysis of individual or combined records is commonly employed to seek quasi-periodic components or characterize the timescales of relevant physical processes. Assumptions that are frequently made in the approximation of depth to age relationships can have a dramatic and harmful effect on the spectral content of records from stratigraphic cores. A common procedure for estimating ages in a set of samples from a stratigraphic core is to assign, based on complementary data, the ages at a number of depths (tie points) and then assume a uniform accumulation rate between the tie points. Imprecisely dated or misidentified tie points and naturally varying accumulation rates give rise to discrepancies between the inferred and the actual ages of a sample. We develop a statistical model for age uncertainties in stratigraphic cores that treats the true, but in practice unknown, ages of core samples as random variables. For inaccuracies in the ages of tie points, we draw the error from a zero-mean normal distribution. For a variable accumulation rate, we require the actual ages of a sequence of samples to be monotonically increasing and the age errors to have the form of a Brownian bridge. That is, the errors are zero at the tie points. The actual ages are modeled by integrating a piecewise constant, randomly varying accumulation rate. In each case, our analysis yields closed form expressions for the expected value and variance of resulting errors in age at any depth in the core. By Monte Carlo simulation with plausible parameters, we find that age errors across a paleomagnetic record due to misdated tie points are likely of the same order as the tie point discrepancies. Those due to accumulation rate variations can be as large as 30 kyr, but are probably less than 10 kyr. We provide a method by which error estimates like these can be made for similar stratigraphic dating problems and apply our statistical model to an idealized marine sedimentary paleomagnetic record. Both types of errors severely degrade the spectral content of the inferred record. We quantify these effects using realistic tie point ages, their uncertainties and depositional parameters. (C) 2002 Elsevier Science B.V. All rights reserved.

McMillan, DG, Constable CG, Parker RL.  2004.  Assessing the dipolar signal in stacked paleointensity records using a statistical error model and geodynamo simulations. Physics of the Earth and Planetary Interiors. 145:37-54.   10.1016/j.pepi.2004.02.011   AbstractWebsite

Stacks of globally distributed relative paleointensity records from sediment cores are used to study temporal variations in the strength of the geomagnetic dipole. We assess the intrinsic accuracy and resolution of such stacks, which may be limited by errors in paleointensity, non-dipole field contributions, and the age scales assigned to each sediment core. Our approach employs two types of simulations. Numerical geodynamo models generate accurate predictions of time series of magnetic variations anywhere in the world. The predicted variations are then degraded using an appropriate statistical model to simulate expected age and paleointensity errors. A series of experiments identify the major contributors to error and loss of resolution in the resulting stacks. The statistical model simulates rock magnetic and measurement errors in paleointensity, and age errors due to finite sampling and approximations inherent in interpolation, incomplete or inaccurate tie point information, and sedimentation rate variations. Data sampling and interpolation to a designated age scale cause substantial decorrelation, and control the maximum level of agreement attainable between completely accurate records. The particular method of interpolation appears to have little effect on the coherence between accurate records, but denser tie point data improve the agreement. Age errors decorrelate geomagnetic signals, usually at shorter periods, although they can destroy coherence over a broad range of periods. The poor correlation between neighboring paleomagnetic records often observed in real data can be accounted for by age errors of moderate magnitude. In a global dataset of 20 records, modeled after the SINT800 compilation and spanning 300 kyr, our results show that dipole variations with periods longer than about 20 kyr can be recovered by the stacking process. Reasonable contributions to error in the paleointensity itself have a modest influence on the result, as do non-dipole field contributions whose effect is minor at periods longer than 10 kyr. Modest errors in the ages of tie points probably account for most of the degradation in geomagnetic signal. Stacked sedimentary paleomagnetic records can be improved by denser temporal sampling and careful selection of independent high-quality tie points. (C) 2004 Elsevier B.V. All rights reserved.

McNutt, MK, Parker RL.  1978.  Isostasy in Australia and evolution of the compensation mechanism. Science. 199:773-775.   10.1126/science.199.4330.773   AbstractWebsite

A linear transfer function analysis has been applied to gravity and topographic data from Australia to calculate the isostatic response function of Dorman and Lewis. The Australian response function is considerably different from that calculated for the United States. The differences can be explained on the basis of an apparent evolution of the isostatic compensation mechanism in which viscoelastic creep occurs in the lithosphere and relaxes the initial long-wavelength elastic stresses.

Medin, AE, Parker RL, Constable S.  2007.  Making sound inferences from geomagnetic sounding. Physics of the Earth and Planetary Interiors. 160:51-59.   10.1016/j.pepi.2006.09.001   AbstractWebsite

We examine the nonlinear inverse problem of electromagnetic induction to recover electrical conductivity. As this is an ill-posed problem based on inaccurate data, there is a critical need to find the reliable features of the models of electrical conductivity. We present a method for obtaining bounds on Earth's average conductivity that all conductivity profiles must obey. Our method is based completely on optimization theory for an all-at-once approach to inverting frequency-domain electromagnetic data. The forward modeling equations are constraints in an optimization problem solving for the electric fields and the conductivity simultaneously. There is no regularization required to solve the problem. The computational framework easily allows additional inequality constraints to be imposed, allowing us to further narrow the bounds. We draw conclusions from a global geomagnetic depth sounding data set and compare with laboratory results, inferring temperature and water content through published Boltzmann-Arrhenius conductivity models. If the upper mantle is assumed to be volatile free we find it has an average temperature of 1409-1539 degrees C. For the top 1000 km of the lower mantle, we find an average temperature of 1849-2008 degrees C. These are in agreement with generally accepted mantle temperatures. Our conclusions about water content of the transition zone disagree with previous research. With our bounds on conductivity, we calculate a transition zone consisting entirely of Wadsleyite has < 0.27 wt.% water and as we add in a fraction of Ringwoodite, the upper bound on water content decreases proportionally. This water content is less than the 0.4 wt.% water required for melt or pooling at the 410 km seismic discontinuity. Published by Elsevier B.V.