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Single-site measurements over shallow aquifer

Posted: Sun Mar 06, 2016 10:54 pm
by Morphiner
Hello everyone,

It has been probably a decade since I last visited and posted in this forum, I am glad Mark keeps it alive and I see the contribution from all over the World.

I am battling with an issue regarding using ambient noise to define the presence of shallow aquifer. It can also be phrased in reverse, e.g. what do I expect taking measurements over a shallow aquifer? Literature doesn't offer much, I wonder if anyone ever had it in their mind and if there is more published out there.

Currently I am running some simulations where I split the top layer in two, keeping Vp the same and changing the rest of the parameters. The behavior of the HV curve is as expected of a two-layer-over-bedrock model and kind of fits the experimental data I have. I would like to hear from others if they ever came to a situation like this before.

Re: Single-site measurements over shallow aquifer

Posted: Sun Jul 10, 2016 12:40 am
by Leonard
Hi Morphiner

You don't give us a lot of details about your problem but i hope my next lines would help you. It's possible to detect changes in saturation using seismic methods. Two things to consider before to attempt your survey; 1) what is the geological context of your problem? ,the materials are soils, rocks,soils over rocks, unconfined or confined aquifers,? 2) Depth of investigation.

If your problem its about an unconfined aquifer (the most part of the stratigraphic column is unconsolidated materials) use the seismic refraction method to find the water table level tentatively, due the unconsolidated clays, sands and some gravels tend to have a p wave velocity lower than the p wave velocity of water * when those are in unsaturated state and when this materials are under the water table the p wave velocity increase asymptotically towards the water's velocity. Now use a surface wave method to obtain the s-wave profile considering the p- wave velocity limits given by the Refraction survey. Place your water table level contrasting the p-wave profile of the surface wave method and the refraction method.

If your problem is related with a confined aquifer you must consider the effect of the clay layer above your sand, the water table could be above its and the mentioned strategy must be modified; you would have two retractors now, one related with your water table level and the second with your saturated sand. Using surface waves methods, the sand-clay surface should be identifiable.

It's possible extend this methodology to the rocks domain considering faulting, joint's system and other geological stuff but having in mind that the bulk modulus increase with saturation, and the matric suction due low saturation's degree has a null effect in this parameter (This assumption is used in reflection seismology for oil and gas survey)

* You must know clays and silts (and some fine-graded sands) have a property called "suction", that is related with its moisture, the suction is a consequence of the surface tension phenomena inside the pore system of the fine soils. This suction has an effect in the Shear modulus; with the increase in suction, the shear modulus increase in a non-linear way, i think is possible find materials with p-wave velocity higher than the water's velocity when the moisture is very low near the residual water content value.

Best regards.

(Sorry for my English)

Re: Single-site measurements over shallow aquifer

Posted: Tue Jul 12, 2016 1:47 pm
by Morphiner
Hello Leonard,

Thank you for the detailed answer. I wanted to see if someone would initiate a reply, so I can be more detailed.

What I am trying to do is to investigate if I can define the presence of shallow aquifer in sediments. The best approach is to use either electric or electromagnetic methods to define the presence of water, and has been exhaustively used for decades. For those who want to look for aquifer siting on a non-porous material using seismic method the solution is also easy, you look for the bedrock, which can be defined with any seismic (active or passive) method.

Our case is a basin with alluvium sediments (~450m/sec), coastal area (Port of Spain, Trinidad & Tobago). The basin has 150-200m depth to bedrock, which is Triassic limestone/schist (>2500 m/sec). Fresh water aquifers are located in places at 30-40m and close to the coast we have saline water intrusion at even smaller depth.

We try to investigate if the presence of water has any effect on the wave signal, the HVSR curve or spectrum and if it does, how we can model the basin to inverse the synthetic waveform. My intuition tells me to treat it as another layer, but I would like to hear from other people if they ever considered with a problem like this.