Olduvai Gorge Project
Geophysical modeling of one of the world's most important fossil sites reveals the history of the site where early humankind evolved.
The field studies of this project were carried out by Gerard Schuster, Sherif Hanafy and doctoral student Kai Lu.
Olduvai George is located in north Tanzania, almost 6 hours drive distance from Arusha and 2 hours from Karato. The project was carried out over four trips.
Robust Imaging Methodology for Challenging Environments: Wave Equation Dispersion Inversion of Surface Waves(International Conference on Engineering Geophysics, Al Ain, United Arab Emirates, 9-12 October 2017, Society of Exploration Geophysicists, 2017-10-12) [Conference Paper]A robust imaging technology is reviewed that provide subsurface information in challenging environments: wave-equation dispersion inversion (WD) of surface waves for the shear velocity model. We demonstrate the benefits and liabilities of the method with synthetic seismograms and field data. The benefits of WD are that 1) there is no layered medium assumption, as there is in conventional inversion of dispersion curves, so that the 2D or 3D S-velocity model can be reliably obtained with seismic surveys over rugged topography, and 2) WD mostly avoids getting stuck in local minima. The synthetic and field data examples demonstrate that WD can accurately reconstruct the S-wave velocity distributions in laterally heterogeneous media if the dispersion curves can be identified and picked. The WD method is easily extended to anisotropic media and the inversion of dispersion curves associated with Love wave. The liability is that is almost as expensive as FWI and only recovers the Vs distribution to a depth no deeper than about 1/2~1/3 wavelength.
Wave-equation dispersion inversion(Geophysical Journal International, Oxford University Press (OUP), 2016-12-10) [Article]We present the theory for wave-equation inversion of dispersion curves, where the misfit function is the sum of the squared differences between the wavenumbers along the predicted and observed dispersion curves. The dispersion curves are obtained from Rayleigh waves recorded by vertical-component geophones. Similar to wave-equation traveltime tomography, the complicated surface wave arrivals in traces are skeletonized as simpler data, namely the picked dispersion curves in the phase-velocity and frequency domains. Solutions to the elastic wave equation and an iterative optimization method are then used to invert these curves for 2-D or 3-D S-wave velocity models. This procedure, denoted as wave-equation dispersion inversion (WD), does not require the assumption of a layered model and is significantly less prone to the cycle-skipping problems of full waveform inversion. The synthetic and field data examples demonstrate that WD can approximately reconstruct the S-wave velocity distributions in laterally heterogeneous media if the dispersion curves can be identified and picked. The WD method is easily extended to anisotropic data and the inversion of dispersion curves associated with Love waves.
Data: Olduvai Gorge Project Trip # 3(2016) [Dataset]The third trip to collect seismic data started on May 14th and ended on June 4th. Team members were Kai Lu (May 14 - 27) and Sherif Hanafy (May 25 to June 4). We recorded four lines, line 1 to 3 has 240 receivers each, and line 4 is 120 receivers: Line 1 starts at the NW corner towards SE, then line 2, line 3, and line 4 ends at the SE corner of the area.
Data: Olduvai Gorge Project Trip # 1(2016) [Dataset]The first trip to collect seismic data started on July 27 and ended on August 16. Team members are Kai (July 27 to August 16), Sherif (July 27 – August 9), and Jerry (August 7 to August 16). A total of 360 CSGs are recorded with shot interval of 10 m and receiver interval of 5 m. The total numbers of receivers are 720. Since we have only 240 receivers at the site, we recorded the data using three phases, phase 1 with receivers 1-240, phase 2 with receivers 241 to 480, and phase 3 with receivers 481 to 720. During shooting time only part of the receivers were active, we repeated some shots to have overlaps between the 3 phases, and the following table lists which receivers were active during shooting time. P.S. to have 480 receivers for some shots we had to repeat some of the shots twice, for example, CSG # 121 was recorded when the active receivers were 1 to 240, then repeated when the active receivers were 241 to 480, both files merged together to have CSG # 121 with 480 active receivers.
Data: Olduvai Gorge Project Trip # 2(2016) [Dataset]The second trip to collect seismic data started on January 5th and ended on January 17th. Team members were Kai Lu (January 5 - 12), Sherif Hanafy (January 10 - 17), and Gerard Schuster (January 5 - 12). We recorded data at two locations: 1. Profiles 6 and 7. They are recorded at the northeast of the camp location Contains ?? CSGs with 470 receivers. Receiver intervals are 5 m and shot intervals are 15 OR 30 m 2. Profiles 4 and 5. They are recorded next to profiles 1, 2, and 3 Contains 102 CSGs and 408 receivers with shot intervals of 20 m and receiver intervals of 5 m At the second location we also recorded passive (almost 6 pm to almost 2 am) and CNS (2 hours) only along profile 5.
Data: Olduvai Gorge Project Trip # 4(2016) [Dataset]The fourth trip to collect seismic data started on Sept. 27th and ended on Oct. 5th. Team members are Kai Lu (Sept. 27 – Oct. 4) and Sherif Hanafy (Oct. 3 to Oct. 5). We recorded four lines: Line 1: 233 receivers and 89 CSGs Line 2: 233 receivers and 118 CSGs Line 3: 233 receivers and 118 CSGs Line 4: 240 receivers and 90 CSGs