Martin Landrø

Calendar time interpolation of amplitude maps from 4D seismic data

A calendar time interpolation method for 2D seismic amplitude maps, done in two steps, is presented. The contour interpolation part is formulated as a quadratic programming problem, whereas the amplitude value interpolation is based on a conditional probability formulation. The method is applied on field data from the Sleipner CO2 storage project. The output is a continuous image (movie) of the CO2 plume. Besides visualization, the output can be used to better couple 4D seismic to other types of data acquired. The interpolation uncertainty increases with the time gap between consecutive seismic surveys and is estimated by leaving a survey out (blind test). Errors from such tests can be used to identify problems in understanding the flow and possibly improve the interpolation scheme for a given case. Field-life cost of various acquisition systems and repeat frequencies are linked to the time-lapse interpolation errors. The error in interpolated amplitudes increased by 3%-4% per year of interpolation gap for the Sleipner case. Interpolation can never fully replace measurements.

Acoustic generation of underwater cavities—Comparing modeled and measured acoustic signals generated by seismic air gun arrays

Underwater vapor cavities can be generated by acoustic stimulation. When the acoustic signals from several air guns are reflected from the sea surface, the pressure drop at some locations is sufficient for cavity growth and subsequent collapse. In this paper the generation of multiple water vapor cavities and their collapses are numerically modeled and the results are validated by comparing with field data from a seismic air gun array test. In a first modeling attempt where cavity interaction is neglected, a correspondence between measured and modeled data is found. Then, this correspondence is improved by assuming that the acoustic signal generated by the other cavities changes the hydrostatic pressure surrounding each cavity. This modeling can be used to estimate the amount and strength of high frequency signals generated by typical marine air gun arrays, given that a calibration step is performed prior to the modeling.

Sensitivity analysis and application of time-lapse full-waveform inversion: synthetic testing and field data example from the North Sea, Norway

Time-lapse refraction can provide complementary seismic solutions for monitoring subtle subsurface changes that are challenging for conventional P-wave reflection methods. The utilization of refraction time lapse has lagged behind in the past partly due to the lack of robust techniques that allow extracting easy-to-interpret reservoir information. However,with the recent emergence of the full-waveform inversion technique as a more standard tool, we find it to be a promising platform for incorporating head waves and diving waves into the time-lapse framework. Here we investigate the sensitivity of 2D acoustic, time-domain, full-waveform inversion for monitoring a shallow, weak velocity change (−30 m/s, or −1.6%). The sensitivity tests are designed to address questions related to the feasibility and accuracy of full-waveform inversion results for monitoring the field case of an underground gas blowout that occurred in the North Sea. The blowout caused the gas to migrate both vertically and horizontally into several shallow sand layers. Some of the shallow gas anomalies were not clearly detected by conventional 4D reflection methods (i.e., time shifts and amplitude difference) due to low 4D signal-to-noise ratio and weak velocity change. On the other hand, full-waveform inversion sensitivity analysis showed that it is possible to detect the weak velocity change with the non-optimal seismic input. Detectability was qualitative with variable degrees of accuracy depending on different inversion parameters. We inverted, the real 2D seismic data from the North Sea with a greater emphasis on refracted and diving waves’ energy (i.e., most of the reflected energy was removed for the shallow zone of interest after removing traces with offset less than 300 m). The full-waveform inversion results provided more superior detectability compared with the conventional 4D stacked reflection difference method for a weak shallow gas anomaly (320 m deep).

Acoustically induced cavity cloud generated by air-gun arrays—Comparing video recordings and acoustic data to modeling

For seismic air-gun arrays, ghost cavitation is assumed to be one of the main mechanisms for high-frequency signal generation. Ghost cavitation signals are weak for seismic frequencies (<300 Hz) and do not contribute to seismic reflection profiling. In the current experiment, the ghost cavity cloud is monitored by a high-speed video camera using 120 frames per second. This is, as far as the authors know, the first convincing photographic evidence of ghost-induced cavitation. In addition to video recording, acoustic signals were recorded with a sampling rate of 312.5 kHz using broadband hydrophones suspended 17 m below the array. The pressure drop around the source array is estimated using air-gun modeling followed by a phenomenological modeling of the growth and collapse of each vapor cavity. The cumulative effect of cavity collapses is modeled based on linear superposition of the acoustic signals generated by individual cavities. The simulated acoustic ghost cavitation signal and the corresponding cavity cloud show good agreement with the field data.

Experiments on the sound transmission at the water-air interface for different source-interface distances

The water-air interface is a nearly perfect reflecting boundary for acoustic waves due to the high impedance contrast between the two media. In many marine applications, ranging from geophysical measurements to bioacoustics, the sea surface has an important impact. A rough sea surface topography leads to complex scattering and interference for acoustic frequencies with similar wavelength or higher than the wavelength of the interface. In geophysical applications and infrasound the frequencies of interest often have larger wavelengths. At the same time, the source within these applications is mostly located close to the interface with respect to its wavelength. In this case, an increased transmission could be expected as experiments with acoustic transducers demonstrate [D. C. Calvo et al., J. Acoust. Soc. Am. 134, 3403-3408 (2013)]. We design two experiments with sources close to the interface while changing the distance between the source and the water-air surface. For the first experiment, a water gun, ...

Calendar Time Interpolation of Amplitude Maps from 4D Seismic Data

A calendar time interpolation method for two dimensional amplitude maps is used on real data from the Sleipner CO2 storage project. Interpolation uncertainty increases with the time gap between consecutive seismic surveys and is estimated by leaving a survey out (blind test). Field-life cost of various acquisition systems and repeat frequencies are linked to the time-lapse interpolation errors.