Validity of acoustic early‐arrival waveform tomography for near‐surface imaging

2011 ◽  
Author(s):  
Wenhuan Kuang ◽  
Lina Zhang ◽  
Jie Zhang
Keyword(s):  
Surface Imaging ◽  
Near Surface ◽  
Early Arrival ◽  
Waveform Tomography

Early arrival waveform tomography on near-surface refraction data

Geophysics ◽  
2006 ◽  
Vol 71 (4) ◽  
pp. U47-U57 ◽  
Author(s):  
Jianming Sheng ◽  
Alan Leeds ◽  
Maike Buddensiek ◽  
Gerard T. Schuster
Keyword(s):  
Ground Truth ◽  
Velocity Model ◽  
Frequency Method ◽  
Near Surface ◽  
Good Convergence ◽  
Traveltime Tomography ◽  
Early Arrival ◽  
Synthetic Test ◽  
Waveform Tomography ◽  
Tomography Method

We develop a waveform-tomography method for estimating the velocity distribution that minimizes the waveform misfit between the predicted and observed early arrivals in space-time seismograms. By fitting the waveforms of early arrivals, early arrival waveform tomography (EWT) naturally takes into account more general wave-propagation effects compared to the high-frequency method of traveltime tomography, meaning that EWT can estimate a wider range of slowness wavenumbers. Another benefit of EWT is more reliable convergence compared to full-waveform tomography, because an early-arrival misfit function contains fewer local minima. Synthetic test results verify that the waveform tomogram is much more accurate than the traveltime tomogram and that this algorithm has good convergence properties. For marine data from the Gulf of Mexico, the statics problem caused by shallow, gassy muds was attacked by using EWT to obtain a more accurate velocity model. Using the waveform tomogram to correct for statics, the stacked section was significantly improved compared to using the normal move-out (NMO) velocity, and moderately improved compared to using the traveltime tomogram. Inverting high-resolution land data from Mapleton, Utah, showed an EWT velocity tomogram that was more consistent with the ground truth (trench log) than the traveltime tomogram. Our results suggest that EWT can provide supplemental, shorter-wavelength information compared to the traveltime tomogram for both shallow and moderately deep seismic data.

Using Active and Passive Near-Field Hydrophones to Image the Near-Surface in Ultra-Shallow Waters Offshore Abu Dhabi

2021 ◽  
Author(s):  
Oleg Khakimov ◽  
Yaser Gholami ◽  
Bertrand Tertrais ◽  
Guillaume Cambois ◽  
Mohamed Mahgoub
Keyword(s):  
Near Field ◽  
Abu Dhabi ◽  
Shallow Waters ◽  
Surface Imaging ◽  
Single Source ◽  
Near Surface ◽  
Seismic Surveys ◽  
Site Survey ◽  
Dual Source ◽  
Water Bottom

Abstract Seismic surveys are generally designed to image deep reservoirs, which leaves the near-surface woefully under-sampled. This is particularly a challenge offshore Abu Dhabi, where a complex near-surface – with karstic collapses and meandering channels – contaminates the seismic image with strong footprints. To mitigate these effects, we use near-field hydrophone data, primarily designed to QC the airgun source, for near-surface imaging. Near-field hydrophones (NFH) are positioned about a meter above each airgun and are designed to record the source near-field pressure. They immediately capture dysfunctional or out-of-spec guns, which alerts the recording crew. Yet, in a shallow water environment, they unintentionally record seismic reflections from the near-surface, which we will use for seismic imaging. Streamer vessels usually use two source arrays, 50 meters apart, which shoot in a flip-flop mode. The active NFH refer to the recordings directly above the shooting guns, while the passive NFH refer to the recordings from the array that is not shooting. Because the passive NFH are less contaminated by the source near-field, they are typically the preferred choice for near-surface imaging. Waters are too shallow in offshore Abu Dhabi to use streamer vessels. Instead, seismic surveys involve ocean-bottom cables (OBC) or nodes (OBN) and smaller airgun arrays. The shooting vessels can be single-source or dual-source. While a single source vessel has only active NFH, a dual source vessel has both active and passive NFH. However, even if a dual-source vessel is used, the 50 m distance between the shooting source array and the passive NFH is too large to capture the water-bottom reflection for water-depths shallower than 25 m. For these reasons, we propose to combine both measurements, using active NFH for the very shallow section and passive NFH for the deeper section. We have applied this technique to a recent node survey acquired offshore Abu Dhabi. By combining the active and passive NFH, a very high-resolution shallow image was obtained, which allows the interpretation of geological layers just below the water bottom. Comparisons with high resolution 2D site survey images show good agreement. Given the NFH do not require any additional acquisition and are delivered as a byproduct of standard seismic surveys, we have demonstrated that proper use of NFH can provide high quality images for pre-site survey interpretation, which reduces the need for additional – and expensive – geotechnical surveys. This is the first published use of combined active and passive NFH in Abu Dhabi shallow waters for the purpose of imaging. The resolution of the shallow formation images allows detailed interpretation not achievable using conventional seismic data. In the long term, this technique may reduce the need for additional site survey acquisitions.

Detecting near-surface objects with seismic waveform tomography

Geophysics ◽  
2009 ◽  
Vol 74 (6) ◽  
pp. WCC119-WCC127 ◽  
Author(s):  
Brendan Smithyman ◽  
R. Gerhard Pratt ◽  
John Hayles ◽  
Ralph Wittebolle
Keyword(s):  
Seismic Attenuation ◽  
Blind Test ◽  
Near Surface ◽  
Traveltime Tomography ◽  
Seismic Waveform ◽  
Long Offset ◽  
Velocity Images ◽  
Weight Drop ◽  
Waveform Tomography ◽  
Geophysical Imaging

Three shallow, high-velocity, rubble-filled targets are imaged using waveform tomography in an engineering-scale clay embankment at Seven Sisters Falls, Manitoba, Canada, to locate targets buried at approximately [Formula: see text] as a blind test of geophysical imaging methods. Previous studies use near-offset reflection methods to image the targets; however, this test uses waveform tomography of the long-offset, refracted arrivals to image P-velocity and seismic attenuation. The targets are invisible to standard traveltime tomography. Using weight-drop data, with frequencies of 20–150 Hz, the subwavelength targets are resolved in the velocity images and complementary images of seismic [Formula: see text] are produced. The interpreted target locations are consistent with limited survey information from the embankment construction. Multiple quality-control efforts, paired with a very good fit between model and observed data, indicate the reliability of the results.

Testing of time‐lapse seismic monitoring with Early Arrival Waveform Tomography

2009 ◽  
Author(s):  
Chad Hogan ◽  
Ken Hedlin ◽  
Gary Margrave ◽  
Michael Lamoureux
Keyword(s):  
Time Lapse ◽  
Seismic Monitoring ◽  
Early Arrival ◽  
Waveform Tomography
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