Adrià Meléndez

Data-driven Layer-stripping Strategy for 3-D Joint Refraction and Reflection Travel-time Tomography

We present and critically evaluate a data-driven strategy to successively invert different layers with joint refraction and reflection travel-time tomography. We use TOMO3D, a new 3-D travel-time tomography code that we have developed, which retrieves the velocity distribution and the geometry of reflectors based on refraction and reflection arrival times. Travel times are computed using a ray-tracing technique combining the graph and bending methods, and the LSQR algorithm performs the iterative inversion. The forward problem solution, which takes by far most of the run time, is parallelised. For practicality reasons, the code is designed to handle a single reflector per inversion, and it is combined with a layer-stripping strategy in cases involving several reflectors. Layers are defined by consecutive reflectors, and the final velocity model is built layer by layer, sequentially extending it with each new inversion and sorting data by offset. This strategy allows us to introduce strong velocity contrasts associated to reflectors, reduces the negative effect of velocity–depth ambiguity, and poses simpler inverse problems. Layer stripping is applied to a complex synthetic case including two curved reflectors. We show that this strategy allows to recover the velocity of both layers and the geometry of intermediate and bottom reflectors.

TOMO3D - A New 3-D Joint Refraction and Reflection Travel-time Tomography Code for Active-source Seismic Data

TOMO3D is a code for three-dimensional refraction and reflection travel-time tomography of wide-angle seismic data that inverts for the velocity field and the geometry of a reflector. Ray tracing is performed by a hybrid method combining the graph and bending methods, and the inversion is iteratively solved using an LSQR algorithm. We present a series of benchmark tests with synthetic data for the forward and inverse problems, as well as other more complex tests comparing inversions with refraction travel times only (i.e. first arrivals) to others with refraction and reflection travel times. The combination of refraction and reflection data increases ray coverage and thus velocity resolution, while allowing the determination of major geological interfaces, and reducing the velocity-depth trade-off.

Appraisal of Joint Refraction and Reflection Travel-time Tomography in the Context of Weathering Correction

We propose an application of joint refraction and reflection travel-time tomography to determine both velocity and thickness of the weathering layer to be used in statics correction. This low velocity layer affects the quality of seismic images. The innovative aspect of our approach is the use of refracted and reflected phases. This remarkably reduces the trade-off between depth and velocity. First arrivals can be automatically picked and the reflection at the layer bottom can be easily identified because of the high impedance contrast with the basement. Synthetic data are generated for a reference model with an acquisition geometry simulating production experiments. The reference model is modified by adding a velocity perturbation and displacing the reflector to create the initial model for the inversion. Travel-time residuals decrease from ~1 s to ~0.1 s and the reflector is recovered to within ±2 m. A Monte Carlo analysis is performed to calculate the mean deviation and its reduction as a measure of the model parameters uncertainty. The improvement (10-50%) is more significant in areas covered by reflections and refractions, while the reflector is again limited to a ~2 m range. We are testing this application with real data.