The Andean foothills area is characterized by a complex subsurface, a rough topography with rapid elevation changes and a complex near-surface from low velocity unconsolidated alluvial deposits to high velocity hard rock formations. Survey design is an optimization process to acquire a high quality subsurface image subject to critical environmental and economical constrains. Project workflow included the following processes:

•         Ray tracing analysis based on a geological model from existing 2D lines

•         Coherent and random noise modeling

•         Finite difference elastic modeling

•         Imaging of synthetic data with different noise levels

Predicted results from survey design are validated during the following stages:

- Data acquisition - requires detailed control of source parameters (charge and depth).

- Data processing - with special effort in statics for complex near surface conditions, noise attenuation and an iterative structurally driven imaging processing.

 Ray tracing analysis based on a geological model from existing 2D linesA geological model is created integrating existing 2D lines and well log data.Validation of the model is initially accomplished by ray tracing. Synthetic gathers are generated to examine the cinematic characteristics of events. Ray tracing helps to visualize that wide offset acquisition provides additional illumination of the deeper horizons in the shadow zone of the fault. These shadow zones are related to ray bending due to lateral velocity change through faults.  Ray tracing also provides information on migration aperture needed for proper imaging of these complex structures. Figure 1.contains an example showing the limitations of a narrow offset - short line length 2D seismic acquisition.

Coherent and random noise modeling
Coherent noise to represent real characteristics of relative amplitude (N/S) and spatial variation was extracted from the existing 2D seismic data. 2D synthetic seismic shot gathers were created using wave equation finite difference modeling. These gathers were the input for shot profile wave-equation pre-stack depth migration (PSDM) to obtain the images of subsurface structures using different levels of noise.

 In the finite-difference modeling, a Ricker wavelet with a central frequency of 25 Hz and maximum frequency of 75 Hz was used as the source. The synthetic traces were 5 seconds long and sampled at 4 ms.