Jiuping Chen

Three methods for mitigating airwaves in shallow water marine controlled-source electromagnetic data

In the past several years, marine controlled-source electromagnetic (MCSEM) techniques have been applied successfully in deep water (depth > 1 km) for oil and gas exploration. The application of this technology in shallow water is challenged, however, because of “airwaves” that mask the signal from the target reservoir at depth. Based upon the understanding that an airwave is a lateral wave, which can be analytically expressed in a dual-half-space resistivity model, we propose three airwave-mitigation approaches to reduce the effects of these airwaves on MCSEM data. In the EM “x-bucking” approach, the effect of the airwaves can be “bucked” out from two measurements by using the analytic expression of the airwave. The frequency derivative (dE/dFreq) approach takes advantages of the unique characteristics of the airwaves in frequency domain, enhancing the reservoir signals while suppressing the airwave. The magnetotelluric (MT) stripping method uses the plane-wave feature of the airwaves and subtraction of ...

Comparison of Sensitivity And Resolution With Two Marine CSEM Exploration Methods

This paper compares the sensitivity to, and resolution of the properties of a resistive target using marine controlled source electromagnetic measurements, with the frequency domain horizontal source-receiver method and the recently introduced vertical source-receiver time domain configuration. The problem is addressed from an analytical stand point, i.e. by analyzing closed form solutions of the 1D spatial and spectral distribution of the fields, and numerically, from 1D inversion of synthetic datasets as well as from 2D simulations of the response of finite lateral extent reservoirs. The 1D analysis demonstrates that the far offset measurement of the standard CSEM has more sensitivity to the presence of the resistive layer than the vertical source-receiver time domain measurement done at close offsets from the source. Closed form solutions derived for the guided mode of the fields yields increasing sensitivity of the standard CSEM configuration for thin resistors and increasing offsets from the source. The image term solution for the fields observed in the near offset vertical source-receiver configuration yields increasing sensitivity with decreasing frequency, i.e. towards the late times of the measurement. For the simplified single layer model a threshold offset is establish beyond which the standard CSEM method is more sensitive. However for a more realistic setting of a finite extent 2D reservoir this report shows that the guided mode driving the far offset sensitivity is only dominant for wide enough targets. The vertical source receiver is more sensitive to smaller targets, where the guided mode does not develop, and it has better resolution to the lateral extent of the reservoirs.

Three Methods for Mitigating Airwaves in Shallow Water Marine CSEM Data

Over the past several years, Marine Controlled-Source EM (MCSEM) techniques have been successfully applied in deep water (water depth >1 km) for oil/gas exploration. In contrast, application of this technology in shallow water, although available, is challenged due to ‘airwaves’ that mask the signal from the target reservoir at depth. Based upon ‘lateral wave’ theory, we propose three airwave-mitigation approaches to reduce the effects of these arrivals on MCSEM data. By comparing the detectability of a target in deep water versus shallow water for models including bathymetry, we show that the airwave effects in a shallow water environment can be reduced leading to a greater reservoir detectability.

Resolution And Uncertainty Analysis For Marine CSEM And Cross-well EM Imaging

Methods are presented for appraising resolution and uncertainty in images generated with large-scale nonlinear EM inversion schemes where singular value decomposition or direct matrix inversion is not possible. The methods explore the computation of the model resolution matrix (MRM) and model covariance matrix (MCM) using a conjugate gradient (CG) method. The proposed methods enable the computation of the MRM and MCM for all the inversion iterations without considerable sacrifices in the total computation time. Examples of the resolution and uncertainty analysis are provided for the inversion of Marine CSEM and cross-well EM synthetic and field data.