diff --git a/simpegEM/Base.py b/simpegEM/Base.py
index 7f938bf6..9e213825 100644
--- a/simpegEM/Base.py
+++ b/simpegEM/Base.py
@@ -1,6 +1,16 @@
-from SimPEG import Survey, Problem, Utils, Models, np, sp, Solver as SimpegSolver
+from SimPEG import Survey, Problem, Utils, Models, Maps, PropMaps, np, sp, Solver as SimpegSolver
 from scipy.constants import mu_0
 
+class EMPropMap(Maps.PropMap):
+    sigma = Maps.Property("Electrical Conductivity", defaultInvProp = True)
+    mui = Maps.Property("Inverse Magnetic Permeability", defaultVal = 1./mu_0)
+
+    # rho = Maps.Property("Electrical Resistivity") 
+    # mu = Maps.Property("Inverse Magnetic Permeability", defaultVal = 1./mu_0)
+
+    # Do some error checking: only 1 of sigma, rho can be InvProp similar story with mu and mui 
+    # Also ensure that sigma and rho are reciprocals of one another "" 
+
 class BaseEMProblem(Problem.BaseProblem):
 
     def __init__(self, mesh, **kwargs):
@@ -9,6 +19,8 @@ class BaseEMProblem(Problem.BaseProblem):
 
     surveyPair = Survey.BaseSurvey
     dataPair = Survey.Data
+    
+    PropMap = EMPropMap
 
     Solver = SimpegSolver
     solverOpts = {}
@@ -29,89 +41,98 @@ class BaseEMProblem(Problem.BaseProblem):
     ####################################################
 
     # Mu
-    @property
-    def mu(self):
-        if getattr(self, '_mu', None) is None:
-            # if getattr(self, '_mui', None) is not None:
-            #     self._mu = sel
-            self._mu = mu_0
-        return self._mu
-    @mu.setter
-    def mu(self, value):
-        if getattr(self, '_MfMui', None) is not None:
-            del self._MfMui
-        if getattr(self, '_MfMuiI', None) is not None:
-            del self._MfMuiI
-        if getattr(self, '_MeMu', None) is not None:
-            del delf._MeMu 
-        if getattr(self, '_MeMuI', None) is not None:
-            del self._MeMuI
-        self._mu = value
+    # @property
+    # def mu(self):
+    #     if getattr(self, '_mu', None) is None:
+    #         # if getattr(self, '_mui', None) is not None:
+    #         #     self._mu = sel
+    #         self._mu = mu_0
+    #     return self._mu
+    # @mu.setter
+    # def mu(self, value):
+    #     if getattr(self, '_MfMui', None) is not None:
+    #         del self._MfMui
+    #     if getattr(self, '_MfMuiI', None) is not None:
+    #         del self._MfMuiI
+    #     if getattr(self, '_MeMu', None) is not None:
+    #         del delf._MeMu 
+    #     if getattr(self, '_MeMuI', None) is not None:
+    #         del self._MeMuI
+    #     self._mu = value
     
     # TODO: hardcoded to assume diagonal mu
-    @property
-    def mui(self):
-        if getattr(self, '_mui', None) is None:
-            self._mui = 1./mu_0
-        return self._mui
-    @mui.setter
-    def mui(self, value):
-        if getattr(self, '_MfMui', None) is not None:
-            del self._MfMui
-        if getattr(self, '_MfMuiI', None) is not None:
-            del self._MfMuiI
-        if getattr(self, '_MeMu', None) is not None:
-            del delf._MeMu 
-        if getattr(self, '_MeMuI', None) is not None:
-            del self._MeMuI
-        self._mui = value
+    # @property
+    # def mui(self):
+    #     if getattr(self, '_mui', None) is None:
+    #         self._mui = 1./mu_0
+    #     return self._mui
+    # @mui.setter
+    # def mui(self, value):
+    #     if getattr(self, '_MfMui', None) is not None:
+    #         del self._MfMui
+    #     if getattr(self, '_MfMuiI', None) is not None:
+    #         del self._MfMuiI
+    #     if getattr(self, '_MeMu', None) is not None:
+    #         del delf._MeMu 
+    #     if getattr(self, '_MeMuI', None) is not None:
+    #         del self._MeMuI
+    #     self._mui = value
 
     # Sigma
-    # deleteTheseOnModelUpdate = ['_MeSigma', '_MeSigmaI','_MfSigmai','_MfSigmaiI']
-    @property
-    def sigma(self):
-        if getattr(self, '_sigma', None) is None:
-            self._sigma = self.curModel.transform
-        return self._sigma
-    @sigma.setter
-    def sigma(self, value):
-        if getattr(self, '_MeSigma', None) is not None:
-            del self._MeSigma
-        if getattr(self, '_MeSigmaI', None) is not None:
-            del self._MeSigmaI
-        if getattr(self, '_MfSigmai', None) is not None:
-            del delf._MfSigmai 
-        if getattr(self, '_MfSigmaiI', None) is not None:
-            del self._MfSigmaiI 
-        self._sigma = value
+    # @property
+    # def sigma(self):
+    #     if getattr(self, '_sigma', None) is None:
+    #         self._sigma = self.curModel.transform
+    #     return self._sigma
+    # @sigma.setter
+    # def sigma(self, value):
+    #     if getattr(self, '_MeSigma', None) is not None:
+    #         del self._MeSigma
+    #     if getattr(self, '_MeSigmaI', None) is not None:
+    #         del self._MeSigmaI
+    #     if getattr(self, '_MfSigmai', None) is not None:
+    #         del delf._MfSigmai 
+    #     if getattr(self, '_MfSigmaiI', None) is not None:
+    #         del self._MfSigmaiI 
+    #     self._sigma = value
 
     # def dsigma_dm(self):
     #     return self.curModel.transformDeriv
 
 
     # TODO: hardcoded to assume diagonal sigma
-    @property
-    def sigmai(self):
-        if getattr(self, '_sigmai', None) is None:
-            self._sigmai = 1./self.curModel.transform
-        return self._sigmai
-    @sigmai.setter
-    def sigmai(self, value):
-        if getattr(self, '_MeSigma', None) is not None:
-            del self._MeSigma
-        if getattr(self, '_MeSigmaI', None) is not None:
-            del self._MeSigmaI
-        if getattr(self, '_MfSigmai', None) is not None:
-            del delf._MfSigmai 
-        if getattr(self, '_MfSigmaiI', None) is not None:
-            del self._MfSigmaiI 
-        self._sigma = value
+    # @property
+    # def sigmai(self):
+    #     if getattr(self, '_sigmai', None) is None:
+    #         self._sigmai = 1./self.curModel.transform
+    #     return self._sigmai
+    # @sigmai.setter
+    # def sigmai(self, value):
+    #     if getattr(self, '_MeSigma', None) is not None:
+    #         del self._MeSigma
+    #     if getattr(self, '_MeSigmaI', None) is not None:
+    #         del self._MeSigmaI
+    #     if getattr(self, '_MfSigmai', None) is not None:
+    #         del delf._MfSigmai 
+    #     if getattr(self, '_MfSigmaiI', None) is not None:
+    #         del self._MfSigmaiI 
+    #     self._sigma = value
 
 
     ####################################################
     # Mass Matrices
     ####################################################
 
+    # TODO: Link to EMPropMap 
+    # if Prop
+    # deleteTheseOnModelUpdate = ['_MeSigma', '_MeSigmaI','_MfSigmai','_MfSigmaiI']
+    @property
+    def deleteTheseOnModelUpdate(self):
+        toDelete = []
+        if self.mapping.sigmaMap is not None:
+            toDelete += ['_MeSigma', '_MeSigmaI','_MfSigmai','_MfSigmaiI']
+        return toDelete
+    
     @property
     def Me(self):
         if getattr(self, '_Me', None) is None:
@@ -129,13 +150,13 @@ class BaseEMProblem(Problem.BaseProblem):
     @property
     def MfMui(self):
         if getattr(self, '_MfMui', None) is None:
-            self._MfMui = self.mesh.getFaceInnerProduct(self.mui)
+            self._MfMui = self.mesh.getFaceInnerProduct(self.curModel.mui)
         return self._MfMui
 
     @property
     def MfMuiI(self):
         if getattr(self, '_MfMuiI', None) is None:
-            self._MfMuiI = self.mesh.getFaceInnerProduct(self.mui, invMat=True)
+            self._MfMuiI = self.mesh.getFaceInnerProduct(self.curModel.mui, invMat=True)
         return self._MfMuiI
 
     @property
@@ -155,7 +176,7 @@ class BaseEMProblem(Problem.BaseProblem):
     @property
     def MeSigma(self):
         if getattr(self, '_MeSigma', None) is None:
-            self._MeSigma = self.mesh.getEdgeInnerProduct(self.sigma)
+            self._MeSigma = self.mesh.getEdgeInnerProduct(self.curModel.sigma)
         return self._MeSigma
 
     # def dMeSigma_dsigma(self, u):
@@ -164,7 +185,7 @@ class BaseEMProblem(Problem.BaseProblem):
     @property
     def MeSigmaI(self):
         if getattr(self, '_MeSigmaI', None) is None:
-            self._MeSigmaI = self.mesh.getEdgeInnerProduct(self.sigma, invMat=True)
+            self._MeSigmaI = self.mesh.getEdgeInnerProduct(self.curModel.sigma, invMat=True)
         return self._MeSigmaI
 
     # def dMeSigmaI_dsigma(self,u)