Yuanzhong Fan

Synthetic aperture controlled source electromagnetics

Controlled?source electromagnetics (CSEM) has been used as a de?risking tool in the hydrocarbon exploration industry. Although there have been successful applications of CSEM, this technique is still not widely used in the industry because the limited types of hydrocarbon reservoirs CSEM can detect. In this paper, we apply the concept of synthetic aperture to CSEM data. Synthetic aperture allows us to design sources with specific radiation patterns for different purposes. The ability to detect reservoirs is dramatically increased after forming an appropriate synthetic aperture antenna. Consequently, the types of hydrocarbon reservoirs that CSEM can detect are significantly extended. Because synthetic apertures are constructed as a data processing step, there is no additional cost for the CSEM acquisition. Synthetic aperture has potential for simplifying and reducing the cost of CSEM acquisition. We show a data example that illustrates the increased sensitivity obtained by applying synthetic aperture CSEM source.

3-D Controlled Source Electromagnetic (CSEM) interferometry by multi-dimensional deconvolution

SUMMARY Controlled Source Electromagnetic (CSEM) is an important technique in hydrocarbon exploration because it uses the large contrast in electrical resistivity to distinguish between water and hydrocarbons. In a shallow sea environment, the airwave that is refracted from the air-water interface dominates the recorded signal at large offsets. Therefore, the hydrocarbon detection ability of the CSEM is weakened because the airwave is independent of the properties of the subsurface. For a layered earth model, we apply multi-dimensionaldeconvolution interferometry to synthetic 3D CSEM data and estimate the reflection response of the subsurface. The difference in the models with and without a resistive layer is significantly increased by the employed interferometric analysis. However, the required receiver spacing is much denser than that of current CSEM surveys. In order to apply this technique to a field survey, we are currently working on how to relax the required receiver criterion for this technique.

Steering and focusing diffusive fields using synthetic aperture

Although beam steering and focusing have been used for waves in many important ways, the application of these concepts to diffusive fields has not been wide spread because of the common belief that diffusion lacks directionality and therefore can neither be steered nor focused. We use the similarities between diffusion and waves and prove that diffusive fields can be steered and focused both in the frequency domain and in the time domain. This finding has the potential of extending the use of diffusive fields as a diagnostic tool in science.

Solving Spatial Sampling Problems in 2D-CSEM Interferometry using Elongated Sources

With interferometry by multidimensional deconvolution (MDD), all effects from the air-water interface in marine Controlled Source Electromagnetics (CSEM) data can be removed. Unfortunately, to apply interferometry byMDD, a very dense receiver sampling is necessary. We show, that the critical sampling distance is equal to the larger of the two parameters: source height and source length. Consequently, by using an elongated source, the sampling criteria can be relaxed also for small vertical source-receiver distances.

Time-lapse controlled-source electromagnetics using interferometry

In time-lapse controlled-source electromagnetics, it is crucial that the source and the receivers are positioned at exactly the same location at all times of measurement. We use interferometry by multidimensional deconvolution (MDD) to overcome problems in repeatability of the source location. Interferometry by MDD redatums the source to a receiver location and replaces the medium above the receivers with a homogeneous half-space. In this way, changes in the source position and changes of the conductivity in the water-layer become irrelevant. The only remaining critical parameter to ensure a good repeatability of a controlled-source electro-magnetic measurement is the receiver position.

CSEM Interferometry Using a Synthetic Aperture Source

With interferometry by multidimensional deconvolution for Controlled Source Electromagnetics, the medium above the receivers is replaced with a homogeneous halfspace and the source is redatumed to the receiver level. The resulting retrieved reflection response is completely free of any airwave effect. Since the source is redatumed, the original source position becomes irrelevant, which is a useful property for time-lapse surveys. So far, interferometry by multidimensional deconvolution required a densely spaced receiver array. We show that this restrictive sampling requirement can be overcome by combining a lot of source positions to one very long source. By creating such a synthetic aperture source, we were able to apply interferometry for a receiver spacing as large as 1280 m.

Synthesized 2D CSEM-interferometry Using Automatic Source Line Determination

Interferometry by multidimensional deconvolution applied to Controlled-Source Electromagnetic data replaces the medium above the receivers by a homogeneous halfspace, suppresses the direct field and redatums the source positions to the receiver locations. In that sense, the airwave and any other interactions of the signal with the air-water interface and the water layer are suppressed and the source uncertainty is reduced. Interferometry requires grid data and cannot be applied to line data unless the source is infinitely long in the crossline direction. To create such a source, a set of source lines is required. We use an iterative algorithm to determine the optimal locations of these source lines and show that more source lines are required if the source is towed closer to the sea bottom and closer to the receivers.

Controlled Source Electromagnetic Interferometry: the Illumination Function

With interferometry for controlled-source data, a reflection response can be retrieved as if the sources are relocated at receiver positions, which are for example in a borehole in seismics or at the ocean bottom in Controlled-Source Electromagnetics. Interferometry can be done with a cross-correlation process or with a multidimensional deconvolution process. We show that interferometry by cross-correlation, which assumes a lossless medium, is related to interferometry by multidimensional deconvolution via the illumination function. This illumination function can be used to judge the improvements of multidimensional deconvolution over cross-correlation interferometry, without actually computing the expensive matrix inversion in the multidimensional deconvolution approach.

Increasing the sensitivity of controlled source electromagnetics by using synthetic aperture

Controlled-source electromagnetics (CSEM) has been used as a de-risking tool in the hydrocarbon exploration industry. Although there have been successful applications of CSEM, this technique is still not widely used in the industry because the limited types of hydrocarbon reservoirs CSEM can detect. In this paper, we apply the concept of synthetic aperture to CSEM data. Synthetic aperture allows us to design sources with specific radiation patterns for different purposes. The ability to detect reservoirs is dramatically increased after forming an appropriate synthetic aperture antenna. Consequently, the types of hydrocarbon reservoirs that CSEM can detect are significantly extended. In this paper, we mainly show one type of synthetic aperture antenna whose field can be steered into a designed angle. Consequently, the field concentrates on the target reservoir and the airwave is reduced. We show a synthetic example and a data example to illustrate the increased sensitivity obtained by applying synthetic aperture CSEM source. Because synthetic apertures are constructed as a data processing step, there is no additional cost for the CSEM acquisition. Aside from the applications to marine CSEM, synthetic aperture can be widely applied to other electromagnetic methods such as on land electromagnetics and bore hole electromagnetics.

Source distribution in interferometry for wave and diffusion

For the wave equation, the Green’s function that describes the wave propagating between two receivers can be reconstructed by cross-correlation if the receivers are enclosed by sources on a closed surface. This technique is normally called interferometry. The ordinary operator used in this technique is cross-correlation. The same technique for Green’s function extraction can be applied to the solution of the diffusion equation if there are sources throughout in the volume. In practice, we only have a finite number of active sources. We address the question what minimum source density is needed for the accurate extraction of the Green’s function, and how these sources should be located on the source in the wave problem and if it is possible to reconstruct the Green’s function of the diffusion equation by using a limited number of sources within a finite volume. We study these questions for homogeneous and isotropic media for both wave propagation and diffusion using numerical simulations. These simulations show that for the used model, the angular distribution of sources is critical in wave problems. For diffusion, the sensitivity of the sources decays away from the center of the two receivers. The required width of the source distribution decreases with frequency, and therefore the required source distribution for early time and late time reconstruction is different.