Broad-band waveforms and robust processing for marine CSEM surveys

Abstract

S U M M A R YIn the marine controlled-source electromagnetic method, the Earth response varies in frequency; therefore, using a wide range of frequencies may better constrain geological structure than using a single frequency or only a few closely spaced frequencies. Binary waveforms, such as the square wave, provide a number of frequencies, though many are limited in usefulness because of the rapid decline of amplitude with frequency. Binary waveform design can be improved by recognizing that the class of doubly symmetric waveforms has special properties: they are compact, have controlled phase, are never polarizing and can be described by a simple closed-form mathematical solution. Using this solution, we discovered a compact waveform in which the amplitudes of the third and seventh harmonics are maximized and which has a signal-to-noise advantage at higher frequencies over several other common waveforms.Compact waveforms make possible improved methods for time-series processing. Using short time windows and a first-difference pre-whitener lessens spectral contamination from magnetotelluric signal and oceanographic noise; robust stacking reduces bias from time-series noise transients; and accurate variance estimates may be derived from averages of waveformlength Fourier transform windows of the time-series.

Bayesian inversion of marine CSEM data from the Scarborough gas field using a transdimensional 2-D parametrization

“…8). Details of the CSEM acquisition and MT mapping of a deep subhorizontal conductive layer can be found in Myer et al (2011Myer et al ( , 2012 and Myer et al (2013), respectively. In this work, we have concentrated our efforts on inverting the inline electric field CSEM data at frequencies of 0.25, 0.75, 1.75 and 3.25 Hz acquired over the ∼50-km-long Line 2 to the south.…”

Non Stationary, Broad-band Waveforms for CSEM - An Analysis with Synthetic Data

“…Using a chirp as source waveform, we can chose any bandwidth and we can account amplitude attenuation over source-receiver distance, since both the amplitude and frequency can be tailored to specific needs. Both are typical problems for CSEM discussed by Myer et al (2010) among others. As an example, by reducing low frequency amplitude in favour for high frequency amplitude, source signal range could be increased for high frequencies assuming a given power generator output.…”

“…The receiver transfer function between electric and magnetic fields at receiver location depends on conductivity beneath the receiver, source-reciever distance and other (mainly geometric) factors. This renders CSEM sensitive to the subsurface conductivity structure and thus had led to its use in resource exploration (Myer et al, 2010, and references therein). Since the frequency for peak sensitivity is normally unknown prior exploration, it is desirable to acquire the transfer functions for a broad range of frequencies and specialised waveforms have been researched in order to optimise some given criteria that maximises the success in respective situations.…”

Controlled-source electromagnetic and seismic delineation of subseafloor fluid flow structures in a gas hydrate province, offshore Norway

“…We applied Key & Lockwood (2010) orthogonal procrustes rotation analysis (OPRA) inversion technique to calculate the orientation of the OBE dipoles (Table 1). Myer et al (2011) processing method was customized here to account for limitations in the data that were imposed during the acquisition stage.…”

“…To assess and interpret the data in the frequency domain the time series data were processed into the frequency domain using the procedure detailed in Myer et al (2011) and summarized here. The CSEM data were fast Fourier transformed using time windows of 1 s to give amplitude and phase data in the frequency domain.…”