Christopher D. Walker

Spectrogram analysis of low to mid frequency marine mammal clicks

Previous investigators have proposed explanations for some sperm whale click structure and pointed out that the separation of individual pulses within the click might be used to determine approximately the size of the sperm whales. Recently, Mohl et al. [J. Acoust. Soc. Am. 114, 1124–1154 (2003)] have shown that echo‐location click structure is highly dependent on the received angle. In data measured by the Littoral Acoustic Demonstration Center using bottom‐moored hydrophones in the northern Gulf of Mexico in the summers of 2001 and 2002, rich click structures were observed in the spectrograms of many click trains, some of which exhibit strikingly consistent spectral nulls across the train. Although this structure in the spectra could be due to propagation effects, investigations to date suggest this possibility is highly unlikely, as discussed in the next abstract. Therefore it is at least plausible that the structure could be used to identify individual animals. This is known to be a difficult problem ...

Automated detection of sperm whale sounds as a function of abrupt changes in sound intensity

An algorithm designed to detect abrupt changes in sound intensity was developed and used to identify and count sperm whale vocalizations and to measure boat noise. The algorithm is a Matlab routine that counts the number of occurrences for which the change in intensity level exceeds a threshold. The algorithm also permits the setting of a ‘‘dead time’’ interval to prevent the counting of multiple pulses within a single sperm whale click. This algorithm was used to analyze digitally sampled recordings of ambient noise obtained from the Gulf of Mexico using near bottom mounted EARS buoys deployed as part of the Littoral Acoustic Demonstration Center experiment. Because the background in these data varied slowly, the result of the application of the algorithm was automated detection of sperm whale clicks and creaks with results that agreed well with those obtained by trained human listeners. [Research supported by ONR.]

Time and frequency analysis of seismic airgun calibration data from an environmental acoustic recording system (EARS) buoy

In the summer of 2003 two Environmental Acoustic Recording System (EARS) buoys were deployed in the northern Gulf of Mexico by the Littoral Acoustic Demonstration Center. The buoys were collocated and recorded ambient noise and seismic airgun array shots up to approximately 25 kHz. The gains and hydrophone sensitivities were set such that one EARS buoy could record the seismic shots without clipping and the other could record ambient noise. The M/V Kondor towed an airgun array on parallel linear tracks with horizontal closest points of approach to the buoy of 0, 500, 1000, 2000, and 5000 m, giving experimental data for a wide range of horizontal distances (up to 7 km) and arrival angles. The raw data were calibrated using the EARS system parameters to produce calibrated pressure time series for each shot. These data are analyzed in both the time and frequency domains. Maximum pressures for each shot as well as sound exposure levels (pressure squared integrated over time for 200 ms in this case) are presen...

Understanding the effects of modern marine 3‐D geometries

Advances in marine acquisition technology have resulted in a significant increase in the number of towed streamers per sail line, often achieved using multiple vessels. The subsequent increase in the width of the acquisition spread means that the design of multistreamer 3D surveys has become more complex due to the need to consider aspects of the acquisition system response relating to increased lateral separation between the sources and receivers.

Schiehallion 4D: From time‐lapse repeatability study to reservoir monitoring

Seismic 3D time-lapse (4D) technology has been successfully applied to the Schiehallion oil field in the west of Shetlands in North Sea. Advanced time-lapse processing methodology has been developed and applied to the conventional towed streamer data, acquired in 1993 and 1996 for the repeatability study, and 1999 for the reservoir monitoring, respectively. No oil production occurred between the 1993 and 1996 surveys.

Schiehallion: A 3‐D time ‐ lapse processing case history

We present the results of comparing two vintage streamer surveys over the Schiehallion field, west of the Shetland Isles. The objective of the study was to determine the detectability threshold for towed streamer time-lapse 3-D data since there had been no production from the field between the two surveys. The primary area of concern was whether we could reprocess both surveys and then produce a meaningful difference volume. Meaningful in this case means that the difference amplitudes would be diagnostic of reservoir changes and not differences arising from changes in the acquisition scheme or processing methodology. This is a particularly challenging task when dealing with streamer data. Our results indicate that we can process streamer data and generate difference volumes 20 db below the original survey amplitudes.