Richard A. Krahenbuhl

Enhancement of Magnetic Data by Stable Downward Continuation for UXO Application

The magnetic method has been proven to be a successful geophysical tool for the detection of unexploded ordnance (UXO). Aeromagnetic surveys are advantageous since they can acquire data over large areas. The downside is that magnetic anomalies due to multiple metallic targets can overlap significantly due to flight height restrictions. Such overlap combined with the acquisition noise may significantly decrease the signal-to-noise ratio of data. These adverse effects can mask the true level of contamination at a site during the initial assessment based on the magnetic method as well as decrease the overall effectiveness of discrimination during the active clearance stage. We propose a method to ameliorate these difficulties using stable downward continuation, which reconstructs the field at a lower observation height from the observed data. The stable algorithm formulates the downward continuation as an inverse problem and incorporates the expected power spectrum of UXO anomalies. The power spectrum preserves the spectral properties and subdues the amplification of high-frequency noise. Synthetic and field examples show that the algorithm can reliably reconstruct the magnetic anomaly at the ground surface within the limitation imposed by the noise. The reconstructed field exhibits significant enhancement compared to the original data.

Hybrid optimization for lithologic inversion and time-lapse monitoring using a binary formulation

We have developed a hybrid optimization algorithm for binary inversion of geophysical data associated with lithologic inversion and time-lapse monitoring. The binary condition is designed to invert geophysical data for well-defined physical properties within discrete lithologic units, such as a salt body within a sedimentary host, or temporal changes in physical property associated with dynamic processes, such as the density and conductivity change in an oil and gas reservoir or groundwater aquifer. The solution of such inverse problems with discrete model values requires specialized optimization algorithms. To meet this need, we develop a hybrid optimization algorithm by combining a genetic algorithm with quenched simulated annealing. The former allows for easy incorporation of prior geologic information and rapid buildup of large-scale model features, whereas the latter guides genetic algorithm to faster evolution by rapidly adjusting the finer model features. In examining the performance of the hybrid ...

Understanding the applications and limitations of time-lapse gravity for reservoir monitoring

There is a strong need in our industry for advancing the understanding and applications of multiple geophysical techniques for time-lapse problems, such as for CO2–H2S sequestration and enhanced oil recovery (EOR) projects. Currently, monitoring efforts for such activities are dominated largely by 4D seismic, and there are numerous developments and successful applications throughout the geophysical literature to justify this state of affair (e.g., Lumley, 2010). The primary advantage of 4D seismic for these endeavors is that it can provide a high-resolution imaging tool that allows better reservoir management decisions and improved production efficiency.

3D Inversion of Airborne Gravity Gradiomentry For Iron Ore Exploration In Brazil

We present the 3D inversion results of gravity gradiometry data over iron ore deposits in Minas Gerais, Brazil. The host formation for the iron ore is largely the Caue Itabirite, along with the overlying Gandalera Formation and, to a lesser degree, the underlying Batatal. Consequently, the iron ore bodies have a distinctly high density contrast and produce well defined anomalies in airborne gravity gradiometry data. We have carried out a preliminary study to apply a 3D inversion to a 20-km2 subarea of data from a larger survey to demonstrate the utility of such an algorithm both in delineatng the ore deposit and in estimating the reserves. We focus on three components of the data (Tzz, Txz, and Tyz). The commonly discussed Tzz component is sufficient to produce geologically reasonable and interpretable results, while including the two horizontal components increases the resolution of the recovered density model and improves the ore delineation.

Genetic Algorithm to Optimize Layer Parameters in Light Weight Deflectometer Backcalculation

AbstractThe light weight deflectometer (LWD) is a portable, nondestructive testing device that can estimate pavement layer parameters, namely moduli. Conventional backcalculation of layer parameters from LWD deflections is formulated as an inverse problem where predicted vertical deflections are matched to observed vertical deflections using a gradient search algorithm. In this paper, we present an LWD backcalculation scheme to recover layer parameters, including top-layer thickness, of a two-layer earthwork system. Our approach resolves the problem using a dynamic finite-element (FE) model for the forward calculation of LWD deflection data and implements a genetic algorithm (GA) as the inverse solver. The objective function we minimize is formulated as a measure of the data misfit between predicted and observed data, normalized by the peak deflections, and it includes 180 data points from the dynamic deflection time history. The objective function contains multiple local minima that can potentially trap ...

Joint Inversion of Surface And Borehole 4D Gravity Data For Continuous Characterization of Fluid Contact Movement

In this paper, we demonstrate the feasibility of jointly inverting 4D gravity data collected on the surface and in a borehole setting to characterize fluid contact movement over time. As technology rapidly advances towards a practical borehole gravity meter capable of collecting data in horizontal monitoring wells, the foundations for properly inverting and interpreting these valuable data must simultaneously be developed. We demonstrate that 4D gravity method may contribute to improved production efficiency and reservoir management in-between the more traditional, and expensive, 4D seismic surveys. In our presentation, simulations are performed using a representation of the published Jotun Field in the Norwegian North Sea, a well-studied site demonstrating successful application of time-lapse (4D) seismic method after early rise in water cut, and therefore decline in production. Results demonstrate that 4D gravity monitoring in-between seismic surveys may have predicted this early water-cut at this site, thus providing additional valuable information to better optimize reservoir management.

Influence of Self-demagnetization Effect On Data Interpretation In Strongly Magnetic Environments

In this paper, we discuss the influence of the selfdemagnetization effect on magnetic data and present an alternative means of quantitatively interpreting such data in highly magnetic environments. In particular, we present two important results based on simulation which one might consider in their interpretation of magnetic data when selfdemagnetization is present. First, current methods for estimating total magnetization, which are typically applied to the problem of remanent magnetization, do not reliably recover this parameter when the anomalous source bodies have high magnetic susceptibilities. And second, a single value estimation of total magnetization does not provide adequate information to properly resolve subsurface geology through inversion. Numerical experiments demonstrate that directly inverting amplitude data, calculated from magnetic data yet weakly-dependent on magnetization direction, produces superior results when interpreting data generated in terrain with high magnetic susceptibilities.

Hybrid optimization for a binary inverse problem

We have developed a hybrid optimization algorithm for inversion of gravity data using a binary formulation. The new algorithm utilizes the Genetic Algorithm (GA) as a global search tool, while implementing Quenched Simulated Annealing (QSA) intermittently for local search. The hybrid has significantly decreased computational cost over GA or Simulated Annealing (SA) alone and has allowed for successful inversion of more realistic gravity problems. We illustrate the algorithm using a large 21⁄2D model derived from the SEG/EAGE 3D salt model, which has a complex background density profile and a pronounced nil zone.

4D Gravity Monitoring of Fluid Movement At Delhi Field, LA: A Feasibility Study With Seismic And Well Data

†Summary In this paper, we present a multi-faceted feasibility study for monitoring fluid movement in a reservoir at various injection times using 4D micro-gravity method. Simulations are performed using a representation of the Delhi Field, LA, constructed by directly integrating seismic and well data. We then analyze anomaly amplitudes and inversion performance with respect to data noise. Results demonstrate a strong likelihood of imaging bulk fluid movement over the life of the project, as well as at early stages of CO2 injection into the thicker down-dip sequences within the reservoir. In contrast, there is a limited ability to recover fluid contact movement in the thinner up-dip regions of the reservoir.

Gravity inversion using a binary formulation

When a nil zone is present in the subsurface salt structure, it effectively creates an annihilator of density contrast that gives rise to zero gravity response on the surface. As a result, part of the salt structure is invisible to the surface data and inversion algorithms often have difficulties in recovering the salt structure correctly. We develop a binary inversion technique in which the density contrast is restricted to being one of the two possibilities: either zero or the value expected at a given depth. The binary condition places a strong restriction on the admissible models so that the non-uniqueness caused by nil zones can be resolved. In this presentation, we will outline the formulation, discuss the solution strategy, and illustrate it with numerical examples.