Lewis C. Bartel

Insight into the output of reverse-time migration: what do the amplitudes mean?

Summary With the purpose of attaching meaning to the waveforms imaged by reverse-time migration, we obtain an expression for the output of such an algorithm over a simple subsurface model of a dipping interface. We invoke the cross-correlation imaging condition and make extensive use of the stationary phase approximation to analyze the migrated image. Our result quantifies the meaning of the amplitudes output from reverse-time shot-profile migration and should have relevance for direct migration of passive seismic data and frequency-swept source signals. A numerical example of reverse-time migration supports our theoretical results.

Acquisition of Controlled-Source Audiofrequency Magnetotelluric(CSAMT) Data At an Active Steam-Drive Site

Mapping and monitoring of enhanced oil recovery processes are important to predict process behavior and to be able to control the process for maximizing oil recovery. If there are electrical resistivity contrasts between various phases of a process, then electrical and/or electromagnetic geophysical techniques may be useful to map and monitor the process. A limited controlled-source audiofrequency magnetotelluric (CSAMT) survey was performed at an active steam-drive oil recovery process site to address questions concerning data acquisition and data quality in a high electrical noise and pipe line environment. The results of the survey are: the data acquisition problems are not severe, reasonably good data quality can be achieved, and there are station-to-station variations in the measured apparent resistivity which may be indicative of the process. However, the precise interpretation of the station-to-station and time variations is yet to be made. 5 refs., 5 figs.

Comparison of Poroelastic And Elastic Full-Waveform AVO Responses

Full-waveform seismic reflection responses of an isolated porous sandstone layer are simulated with three-dimensional (3D) isotropic poroelastic and isotropic elastic finite-difference (FD) numerical algorithms. When the pore-filling fluid is brine water with realistic viscosity, there is about a ~10% difference in synthetic seismograms observed in an AVO recording geometry. These preliminary results suggest that equivalent elastic medium modeling is adequate for general interpretive purposes, but more refined investigations (such as AVO waveform analysis) should account for poroelastic wave propagation effects.

Grid Search Algorithm For 3D Seismic Source Location

The spatial and temporal origin of a seismic energy source are estimated by minimizing (in the weighted least squares sense) the misfit between observed and predicted arrival times at a set of receiver stations. A search is conducted for the best source position within a 3D gridded volume of trial locations. Rapid calculation of predicted traveltimes is achieved by evaluating closed-form formulae appropriate for a homogeneous or 1D layered velocity model. The method is applicable to microseismic event location for mapping hydraulic fracturing in a petroleum reservoir.

Application of Wave Migration to Borehole Vertical Electric Source EM Data

The wave extrapolation and migration method commonly used in the analysis of seismic data is extended to migrating electromagnetic (EM) waves within the audio frequency range. The method is then applied to migrating surface, secondary magnetic fields produced by a borehole, vertical electric source (VES). The approach used here is to Fourier transform the surface magnetic field data along a line of data stations to the horizontal wave vector domain. These fields are then propagated downward in fixed steps until the image condition is satisfied. The image condition is at time t=O the magnetic wave field is in phase and the imaginary part is zero. Once the image is determined in the horizontal wave vector domain, the results are Fourier transformed back into realspace to form the image. The surface magnetic fields produced by a VES are created by the portion of the geoelectric section which is not axially symmetric about the VES. Here two approaches for the z-component of the wave vector were examined: (1) the exact complex EM wave vector was used, and (2) a seismic analogue was used where the EM phase velocity was substituted for the seismic velocity. The use of the seismic analogue appears to image the model target better than the more exact EM wave vector. This points out that it is primarily the phase which produces the image. The wave extrapolation and migration method is applied to experimental surface magnetic field data acquired to map a shallow salt water injection.

West Pearl Queen CO2 sequestration pilot test and modeling project 2006-2008.

The West Pearl Queen is a depleted oil reservoir that has produced approximately 250,000 bbl of oil since 1984. Production had slowed prior to CO{sub 2} injection, but no previous secondary or tertiary recovery methods had been applied. The initial project involved reservoir characterization and field response to injection of CO{sub 2}; the field experiment consisted of injection, soak, and venting. For fifty days (December 20, 2002, to February 11, 2003) 2090 tons of CO{sub 2} were injected into the Shattuck Sandstone Member of the Queen Formation at the West Pearl Queen site. This technical report highlights the test results of the numerous research participants and technical areas from 2006-2008. This work included determination of lateral extents of the permeability units using outcrop observations, core results, and well logs. Pre- and post-injection 3D seismic data were acquired. To aid in interpreting seismic data, we performed numerical simulations of the effects of CO{sub 2} replacement of brine where the reservoir model was based upon correlation lengths established by the permeability studies. These numerical simulations are not intended to replicate field data, but to provide insight of the effects of CO{sub 2}.

Applications of EM holographic methods to borehole vertical electric source data to map a fuel oil spill

The multifrequency, multisource holographic method used in the analysis of seismic data is to extended electromagnetic (EM) data within the audio frequency range. The method is applied to the secondary magnetic fields produced by a borehole, vertical electric source (VES). The holographic method is a numerical reconstruction procedure based on the double focusing principle for both the source array and the receiver array. The approach used here is to Fourier transform the constructed image from frequency space to time space and set time equal to zero. The image is formed when the in-phase part (real part) is a maximum or the out-of-phase (imaginary part) is a minimum; i.e., the EM wave is phase coherent at its origination. In the application here the secondary magnetic fields are treated as scattered fields. In the numerical reconstruction, the seismic analog of the wave vector is used; i.e., the imaginary part of the actual wave vector is ignored. The multifrequency, multisource holographic method is applied to calculated model data and to actual field data acquired to map a diesel fuel oil spill.

Diagram Approach to the Plane-wave Electromagnetic Wave Impedance

The impedance function determined for electromagnetic plane waves impinging upon a horizontally-stratified earth can be expressed in terms of an infinite series. Each term in the series represents primary and/or primary plus multiple reflections from interfaces of conductivity contrasts in the earth. A methodology is outlined for determining terms in the summation where the terms are obtained from diagrams representing the reflection terms. The terms in the sum are transformed into the time domain using a cosine transform. The transform of the real part of the impedance function, normalized by the transform of the uniform earth response for the upper most layer, is found to be quite useful in analysis of magnetotelluric type data. The character of the resultant function can be interpreted in terms of electromagnetic wave reflections, primary and multiples, from horizontal boundaries where there ’ conductivity contrast. Peaks in the norA:lizeda impedance function are identified as due to twoway travel of the electromagnetic wave in the earth. However due to dispersion, the peaks broaden for increasing time

Reply to L. Szarka by the authors

We welcome the opportunity to respond to comments by Szarka on our recent paper. The main points he raised on our near‐field correction scheme for controlled‐source audio‐frequency magnetotelluric (CSAMT) data are the application of the correction scheme and the near‐field/far‐field demarcation in the presence of layers and the application in the presence of electrical structure beneath the transmitter location. In our paper, we addressed the application for three‐dimensional electrical structure beneath the receiver location with the transmitter over a homogeneous half‐space. In this reply we wish to clarify these points and point out possible limitations of our correction scheme.

Reply to H‐M Maurer by the authors

In our reply to the discussion by Szarka (this issue), we examined the near‐field correction scheme for two‐layer cases with a basal layer more conducting and less conducting than the surface layer where we only considered the broadside configuration. Maurer points out that for the collinear configuration the correction scheme does not produce an accurate correction particularly for the less conducting basal layer case. We wish to comment on the correction scheme.