Mehran Gharibi

Two‐ and three‐dimensional electrical resistivity imaging at a heterogeneous remediation site

Geometrically complex heterogeneities at a decommissioned sour gas plant could not be adequately characterized with drilling and 2D electrical resistivity surveys alone. In addition, 2D electrical resistivity imaging profiles produced misleading images as a result of out‐of‐plane resistivity anomalies and violation of the 2D assumption. Accurate amplitude and positioning of electrical conductivity anomalies associated with the subsurface geochemical distribution were required to effectively analyze remediation alternatives. Forward and inverse modeling and field examples demonstrated that 3D resistivity images were needed to properly reconstruct the amplitude and geometry of the complex resistivity anomalies. Problematic 3D artifacts in 2D images led to poor inversion fits and spurious conductivity values in the images at depths close to the horizontal offset of the off‐line anomaly. Three‐dimensional surveys were conducted with orthogonal sets of Wenner and dipole–dipole 2D resistivity survey lines. The ...

Resolution of 3-D Electrical Resistivity Images from Inversions of 2-D Orthogonal Lines

Three-D electrical resistivity imaging (ERI) using sets of orthogonal of 2-D survey lines provides an efficient and cost effective tool for site characterization in environmental and engineering investigations. A 3-D survey design using sparse sets of lines reduces the survey time at the expense of the resolution. The effects of line spacing on the resolution of 3-D electrical resistivity images were investigated using numerical modeling with synthetic and field data for two standard configurations, dipole-dipole and Wenner arrays. Synthetic data studies indicate that dipole-dipole configuration produces a more accurate map of the subsurface than the Wenner configuration. A severely under-sampled 3-D survey could result in introducing small-scale shallow spurious artifacts at the surface of the resistivity model caused by the projection of the anomalies located in the deeper parts of the model. Results from inversion of the real and synthetic data showed that lines should be separated by no more than four...

Time-lapse electrical resistivity monitoring of salt-affected soil and groundwater

Summary Time-lapse electrical geophysics is used to monitor the remediation of oilfield brines in soil and groundwater. Challenges with comparing geophysical surveys done during different seasons at different temperatures were overcome through temperature monitoring and geophysical processing techniques. The background electrical conductivity structure captured by the geophysics compared well with push tool conductivity logs collected and had strong correlation with salt samples taken. However the changes in electrical conductivity observed in the timelapse geophysics provided information about the remediation effectiveness that was not available from the logs and point measurements. From the observed changes in the geophysics we interpret that depression focused recharge has a large impact on remediation effectiveness. This research indicates that time-lapse electrical geophysics is an effective and low cost remediation monitoring method at appropriate sites.

Automatic 1-D inversion of magnetotelluric data: Finding the simplest possible model that fits the data

We demonstrate that automatic layered inversion of plane‐wave electromagnetic data can be carried out by modifying standard least‐squares inversion schemes. The modifications include a logarithmic reparameterization of the unknown model parameters, whereby all layer parameters are forced to remain within given bounds. However, the most important modification to help the optimization process find the best model and to avoid local minima is to split the data into several subbands, starting from the highest frequencies. By this stripping procedure, the shallower part of the model becomes well estimated first. As more data are introduced, more layers may be required to improve the data fit. The new inversion procedure has been applied to many sets of theoretical data representing increasingly complicated layering that is often found in near‐surface studies. The main result of these simulations is that there is a very strong coupling between the resolution power of plane‐wave data and their random errors. Part...