PASSIVE SURFACE-WAVE INVESTIGATION
Microtremor Array Measurements (MAM)
Estimate subsurface shear-wave velocity from ambient vibrations using array geometry, dispersion analysis and site-specific quality checks.
ENGINEERING OVERVIEW
Listening to ambient ground vibration
Microtremor Array Measurements (MAM) use ambient ground vibration recorded at several sensors to analyse surface-wave dispersion and estimate an interpreted shear-wave velocity (Vs) structure. The method is passive: it does not require a controlled impact source during recording. Its output depends on array geometry, ambient-wavefield properties, selected dispersion branches and inversion constraints.
Projects may need seismic stiffness information where active-source energy is restricted or deeper velocity structure is of interest. MAM can complement active multichannel surface-wave measurements when sufficient and sufficiently varied ambient vibration exists. But the absence of a controlled source means wavefield directionality, frequency content and array processing deserve close scrutiny.
MEASUREMENT & INTERPRETATION
What the method actually tells you
Ambient wavefield
Traffic, machinery, wind-driven or other environmental sources create surface-wave energy; these are not always spatially uniform or stable over the recording period.
Array-based dispersion
Multiple receivers provide spatial relationships, enabling methods such as spatial autocorrelation (SPAC) to estimate Rayleigh-wave phase velocity versus frequency.
Vs profile is inverted
Dispersion curves are not direct depth logs. Different velocity models can fit similar dispersion data unless constrained by geology or other measurements.
MAM is not a synonym for HVSR
Single-station horizontal-to-vertical spectral ratio (HVSR) explores resonance characteristics; MAM uses an array to constrain a dispersion relation. The techniques are complementary, not interchangeable.
PRACTICAL SURVEY FLOW
From site question to defensible interpretation
Set the depth objective
Identify which depth interval and engineering parameter matter; decide whether MAM alone can address the question.
Design multiple apertures
Select receiver array geometry and practical layouts with attention to wavelength range, access and coupling.
Record ambient signals
Collect sufficient duration and repeat data across the array; check synchronisation, noise bursts and spatial consistency.
Derive and invert dispersion
Assess SPAC or equivalent processing, dispersion reliability, alternative Vs models and agreement with active surveys or boreholes.
WHERE IT ADDS VALUE
Engineering applications and use cases
Seismic site characterisation
Develop Vs structure relevant to site-response assessment when supported by suitable wavefield and model sensitivity.
Broad subsurface stiffness screening
Compare candidate geological layering along accessible ground or multiple survey locations.
Mixed-method investigation
Combine passive MAM with active MASW, seismic refraction and borehole data to reduce ambiguity.
MEASUREMENT BOUNDARIES
Limitations and conditions of use
Interpretation quality depends as much on survey suitability and independent evidence as on processing software.
- Coherent directional vibration, weak low-frequency energy and array aliasing may distort dispersion estimates.
- Available wavelengths constrain usable depth; larger arrays do not automatically guarantee an unambiguous deep model.
- Inversion depends on assumed layer structure and dispersion mode selection.
- MAM-derived Vs should not be converted mechanically into strength or a site class without the relevant context and uncertainty.
METHOD SELECTION
How to choose the right approach
MAM versus active MASW
MAM uses ambient vibration and array statistics; active MASW uses a controlled source and an acquisition geometry that can improve repeatability.
MAM versus HVSR
MAM estimates dispersion for Vs inversion. HVSR estimates spectral resonance behaviour and does not by itself provide a unique full Vs profile.
MAM versus refraction
MAM derives a model from surface-wave dispersion; refraction derives constraints from body-wave first arrivals and layer-velocity contrasts.
Choose MAM when ambient vibrations and survey access can support suitable array observations and when a passive Vs constraint adds value. Consider an active-passive combination rather than treating MAM as a universal replacement for MASW.
ENGINEERING ASSURANCE
Field quality, interpretation and review
A traceable project record should retain the following information where applicable:
- Receiver timing, orientation, coupling and station coordinates
- Array aperture, geometry, usable frequency and spatial aliasing checks
- Record windows, noise-source directionality and repeatability
- Dispersion reliability, mode assumptions and alternative inversion fits
PROJECT DELIVERABLES
What a useful technical report should contain
01 · Project output
Array layout, sensor metadata and ambient-vibration record summary
02 · Project output
Dispersion observations and processing-quality assessment
03 · Project output
Inverted Vs profile or section with confidence and modelling assumptions
04 · Project output
Comparative interpretation alongside boreholes or complementary geophysics
COMMON QUESTIONS
Frequently asked questions
Does MAM need a hammer or vibrator?
Is MAM equivalent to a single-sensor HVSR survey?
Can MAM provide Vs30?
TECHNICAL REFERENCES
Further technical reading
- USGS — MAM with SPAC analysis
- USGS — Multimethod seismic site characterisation
- USGS — Microtremor HVSR data and context
References describe the underlying methods and recommended data-quality principles; interpretation for any particular site must address its own evidence and constraints.
PROJECT ENQUIRIES
Discuss your ground investigation challenge.
GEOOE · GEOORIGIN ENGINEERING LIMITED
Share the site location, investigation objective, expected depth range, available boreholes and decisions the results need to support. We can discuss appropriate technical methods, limitations and project-specific collaboration.