Seismic · Method
MASW or ReMi: where each one fails
Both produce a shear-wave velocity profile. They break down under opposite site conditions, which is why the firm you ask tends to decide the answer.
A contractor who owns one instrument will recommend that instrument for every site. It is not dishonesty so much as the shape of the business: the alternative is turning work away. But MASW and ReMi fail under opposite conditions, and a site that defeats one is often the site where the other works best.
What separates them
MASW uses a source you control — a sledgehammer on a plate, or a heavier seismic source, striking at known offsets along a geophone spread. Because you control the energy you control the frequency content, and you can repeat the shot until the record is clean.
ReMi uses the noise already there. Traffic, plant, machinery, the general hum of a developed site. No source, no striking, longer recordings, and the energy arrives from wherever it happens to originate.
That single difference drives everything else.
Where MASW struggles
On a noisy site. An operating plant, a live carriageway, a construction site with equipment running — ambient energy competes with your source, and a clean record means either stopping the noise or striking harder than the setting allows. Neither is usually available.
MASW also needs room. A clear run for the spread, and physical access to strike along it. A congested site with no uninterrupted line makes the geometry awkward and the deeper part of the profile suffers first.
Where ReMi struggles
On a quiet site. A greenfield parcel some distance from any road has little to record. ReMi depends on noise existing, and the quieter the ground the longer the recording and the weaker the low-frequency content that carries the deeper part of the profile.
It also depends on that noise arriving from varied directions. A site with one dominant source — a single highway along one boundary — produces a biased wavefield, and the interpretation has to account for it.
Why we run both
The two methods sample the wavefield differently and extend the dispersion curve at opposite ends: MASW resolves the shallow section, ReMi reaches deeper. Run together they constrain a profile neither would constrain alone.
The more useful point is what happens when they disagree. Two independent methods agreeing turns a V̄s30 from a judgement into a measurement with corroboration. Two methods disagreeing has told you something about the ground that one method alone would have concealed — and it is considerably better to learn it before the number reaches a permit set than after a reviewer asks how it was derived.
On a noisy industrial site, ReMi carries the result and MASW confirms the shallow section. On a quiet greenfield, MASW carries it and ReMi adds less. Most sites sit somewhere between, which is the argument for having both on the truck.
What to ask before you commission
Ask which methods the surveyor operates. If the answer is one, ask what they do on a site where that method is the wrong one. The answer tells you whether you are buying a measurement or an instrument.