_Applied_  
_sciences_  
_Article_  
**Integrated MASW and ERT Imaging for Geological**  
**Definition of an Unconfined Alluvial Aquifer**  
**Sustaining a Coastal Groundwater-Dependent**  
**Ecosystem in Southwest Portugal**  
**[PERSON] 1,2,*, [PERSON] 3,4, [PERSON] 5, [PERSON] 6(), [PERSON] 7() and [PERSON] 5,4**  
1 CIQuBio, Barreiro School of Technology, Polytechnic Institute of Settibal, 2839-001 Lavradio, Portugal  
2 Instituto Dom Luiz, Faculdade de Ciencias, Universidade de Lisboa, 1749-016 Lisbon, Portugal  
3 Instituto Geologico y Minero de España (IGME), 28003 Madrid, Spain; [EMAIL_ADDRESS]  
4 Instituto de Ciencias Quimicas Aplicadas, Facultad de Ingenieria, Universidad Autonoma de Chile, Santiago 7500138, Chile  
5 Civil Engineering Research and Innovation for Sustainability (CERIS), Instituto Superior Tecnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal; [EMAIL_ADDRESS] (A.M.)  
6 Department of Mining and Civil Engineering, Universidad Politecnica de Cartagena, 30203 Cartagena, Spain; p.martinez@upctes  
7 Engineering Sciences, Pontificia Universidad Catolica Madre y Maestra,  
Santo Domingo 2748, Dominican Republic; [EMAIL_ADDRESS]  
* Correspondence: [EMAIL_ADDRESS]  
Received: 6 July 2020; Accepted: 22 August 2020; Published: 26 August 2020  
**Abstract:** This paper integrates multichannel analysis of surface waves (MASW) and time-lapse electrical resistivity tomography (ERT) to define aquifer geometry and identify transient groundwater features of the Cascalheira Stream Basin Holocene alluvial aquifer (aquifer H), which contributes to the Santo Andre Lagoon, part of a coastal groundwater-dependent ecosystem (GDE), located in southwest Portugal. MASW measures shear-wave velocity (VS), allowing one to obtain steady geological models of the subsurface, and ERT measures subsurface electrical resistivity (ER), being subjected to ambient changes. MASW enables disambiguation of geological structures in low ER environments, such as coastal areas. This research covered one natural year and involved one MASW campaign, four ERT campaigns, and additional geological field surveys and groundwater monitoring to assist interpretation of results. In the area, the conjugate NW-SE and NE-SW strike-slip fault systems determine compartmentalization of geological structures and subsequent accommodation space for Holocene sedimentation. MASW and ERT surveys show how the NW-SE system deepens these structures toward the coast, whereas the NE-SW system generates small horsts and grabens, being one of these occupied by aquifer H. From upstream to downstream, aquifer H thickness and width increase from 10 m to 12 m and from 140 m to 240 m, respectively. Performance of VS and ER models was satisfactory, with a normalized error of the VR and ER models in the 0.01-0.09 range, meaning that a quantitative quota of uncertainty can be segregated from the overall uncertainty of groundwater models without substantially affecting its simulations accuracy. This methodology seeks to improve the design of shallow groundwater research in GDE preservation policies.  
**Keywords:** multichannel analysis of surface waves; electrical resistivity tomography; time-lapse inversion; aquifer geometry; groundwater-dependent ecosystem; Santo Andre Lagoon; Portugal