Guy Flanagan

3D modeling of a deepwater EM exploration survey

Analysis of a current offshore prospect employed 3D numerical modeling of a controlled-source electromagnetic (CSEM) exploration system. The analysis considers the sensitivity of data presentations to assumptions about the background model. The numerical simulations show that false anomalies and significant distortion to anomaly magnitude can be caused by normalization of the observed electric fields by reference fields calculated from an incorrect or oversimplified background model. Bathymetry effects on the measured electric fields, if not accounted for, can produce anomalies as large as those of target sands. The maximum sensitivity to oil-water contacts or other strong lateral variations within the modeled channel sands is achieved by profiling along the length of the channel. Profiles run offset from a simulated oil-water contact by as little as 2 km show a response below the expected noise levels. Good background models can be constructed by taking advantage of the magnetotelluric data recorded by m...

Comparison of grid Euler deconvolution with and without 2D constraints using a realistic 3D magnetic basement model

We describe the application of a 2D-constrained grid Euler deconvolution method which is able to determine for each solution window whether the source structure is two dimensional, three dimensional, or poorly defined and to estimate the source location and depth. In each solution window, eigenvalues and eigenvectors are derived from the Euler equations and compared to threshold levels. A single eigenvalue below the given threshold and lying in the x–y-plane is shown to indicate a 2D source, while the absence of such an eigenvalue indicates a 3D source geometry. Two small eigenvalues indicate the field in the window has no distinct source. Applying these criteria to each solution window allows us to generate a map of source-geometry distribution. We evaluate the effectiveness of 2D-constrained grid Euler deconvolution using synthetic magnetic data generated from a 3D basement model based on real topography from an area with surface-exposed faulting. This modeling strategy provides a complex, nonidealized data set that compares Euler depth estimates directly to the known basement surface depth. Our results indicate that noninteger structural indices can be the most appropriate choice for some data sets, and the 2Dconstrained grid Euler method images magnetic basement structure more clearly and unambiguously than the conventional grid Euler method.

Testing Magnetic Local Wavenumber Depth Estimation Methods using a Complex 3D Test Model

Williams et al (2002, 2003, 2004) have proposed the use of a realistic 3D test model in order to properly evaluate methods of estimating depth-to-magnetic-basement and other magnetic interpretation methods. The strategy has involved the taking of a real topographic dataset from an area near Bishop, California, contained numerous exposed fault scarps of varying sizes and orientations and then scaling them to provide an analogue for part of a basement surface beneath a large scale sedimentary basin. Taking this basement surface (Fig. 1a) as the top of magnetic basement overlain by non-magnetic sediments, synthetic magnetic anomalies can be computed. The aim of this approach is to provide a magnetic test dataset with a realistic level of complexity to quantitatively evaluate the accuracy of inversion methods for structures with varying strike within a depth range of ~1km to 9 km.

Realistic models of basement topography for depth to magnetic basement testing

Summary The analysis of potential field data is an important tool for studying basement structure in hydrocarbon exploration settings. Numerous techniques have been developed to aid the interpretation of such data. Traditionally the effectiveness of each method has been demonstrated on highly idealized model data and/or real data where the correct interpretation is uncertain. In this paper we propose a new approach for evaluating the effectiveness of ‘depth to magnetic basement’ estimators. We present inversion results for a model magnetic dataset generated from a modified topography dataset. This strikes a balance between geological realism and ground-truthing.

Hybrid Euler Magnetic Basement Depth Estimation: Bishop 3D Tests

We propose an improved magnetic basement depth estimator using a hybrid of two Extended Euler methods. We test the method on the realistic Bishop model. In a significant advance on previous practice, we estimate basement depths independent of any structural index assumptions. We derive structural indices separately. The derived depths follow the general depth trends of the model with some discrepancies which are beyond the capabilities of magnetic depth estimation methods.

Grid Based Euler Deconvolution: Completing the Circle With ¿2D Constrained Euler¿

Mushayandebvu et al. (2001) introduced a second equation, described as a rotational constraint. The profile based method is known as profile ‘extended Euler’ where is assumed zero in the above equation. For each operator window along a profile the solution location derived from the second equation is compared to that derived from conventional Euler and only solutions having similar locations are accepted, The accepted solutions are well resolved 2D source structures and give spatially more consistent results than conventional 2D Euler. A further advantage of this method is that dip and susceptibility contrast can be determined for contacts and dyke models. Extended Euler was then applied to grid data by Mushayandebvu et al. (2000). y T/∂ ∂

3D numerical simulation of a deepwater EM exploration survey

An analysis of a current offshore prospect has been carried out using three-dimensional numerical modeling of a controlled source electromagnetic (CSEM) exploration system. The analysis considers the sensitivity of data presentations to assumptions about the background model. The numerical simulations show that false anomalies and significant distortion to anomaly magnitude can be caused by normalization of the observed electric fields by fields calculated from an incorrect or oversimplified background model. Bathymetry effects on the measured electric fields can, if not accounted for, produce anomalies as large as those of target sands. Good background models can be constructed by taking advantage of the magnetotelluric data recorded by marine receivers during times when the CSEM transmitter is not in operation.