I am monitoring injections (tophole pressures upwards of 300 psi and flows near 2000 gpm) in unconsolidated and noncohesive soils. I know I am washing out fine grained material and leaving the coarser fraction. I want to quantify the porosity change.
Hi Dale,
In water injections in the terrain we have never used high pressure, only the natural infiltration rate of the water into the terrain. So, I've always considered that water is moving naturaly, and there is no washing out of fine grained material. I've never used only resistivity to evaluate changes of porosity during the process of water injection, only to identify the water pathways.
I developed a joint use of resistivity and refraction to reach 2D sections of porosity (see one of my articles on this). So, did you considered the possibility of performing a joint survey of both these methods before (to have the reference) and after the injection process?
Have only used resistivity to estimate changes in pore fluid volume/resistivity. Obviously there are non-resistivity logging methods to get at porosity estimates.
If you do this in lab. As an excercies to get amodel with stability for other parameters then the resistivity may give you a good result but if you do this in the field (in situ) resistivity as single tool is not enoph to estimate porosity chang
If you do this in lab. As an excercies to get amodel with stability for other parameters then the resistivity may give you a good result but if you do this in the field (in situ) resistivity as single tool is not enoph to estimate porosity chang
Dale, I think that you are performing (or you carried out) a time lapse monitoring with ERT. And I think you are acquiring good data well sampled in space and time (not aliased as you well know). If you has good time-lapse data my first "rough idea" is to attempt the porosity change estimation based on the variation of formation factor. In fact in the case of "saturated" unconsolidated and non-cohesive soils, if the process is monitored in a short time (we should be able to follow the process without alias), the variation should be mainly dependent of the resistivity of the injected solution (that should be constant) and of the changed porosity. In other words, when the hydro-geohysical process is stabilized, ("steady state in a confined saturated volume condition"), it should be mainly governed by the washing out of fine material, then the perturbation in formation factor should be mainly related, or hypothetical only, to the porosity variations. Overpressures could be have an sort of influence in your case. Obviously we should also hypothize that the resistivity of fine material is quite equal to that of coarser. May be tested it in lab. What is wrong in my rough idea?
I am working in time lapse with injection, for remediation aims, and I see that when we employ high pressures and flows (not comparable with yours) the systems (unconsolitaded soils) reaches steady states in very short times, but the inverted perturbations, or resistivity , in these steady states, in my opinion are comparable with the repeatibility errors determined in background test and measurements. The inversion is the greatest problem in this kind of detailed and fine applications. But I have not washing out (almost observed) of materials, and I hope your experiment, very impressive, could induce greater variations.
Thanks, Roberto. I am monitoring with time lapse resistivity with one of our in-house manufactured systems that allow us to produce a snapshot every 20 minutes. You are right in that I think aliasing is not a problem. However, regularization is and I can induce small changes in the final distribution of resistivity by simply increasing or decreasing the temporal dampening factor. So, it would seem the best I could do in my estimation is to obtain the change in porosity and not the porosity itself. Unless I know a porosity value at either the beginning or end of the injection.
Hi Dale,
Our group has employed the Time-lapse ERT method to monitoring the progress of rainfall infiltration. However, it is hard to established the fracture zone by only the resistivity maps (with 10 days) obtained from inversion of ERT data sets. We transformed the ERT maps to the relative water saturation (RWS) maps (the changes of water saturation with contiguous two days). The RWS maps can describe the fracture zone well and show the preferential passway of the rainfall infiltration.
Waterless porosity shows near-infinite electrical resistance, and porosity with water shows very low electericsl resistivity.
Porosity - Resistivity Relation is reverse relation in ratios
based on numerous parameters and it mostly empirical relation change from area to another
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