diff --git a/mhkit/wave/resource.py b/mhkit/wave/resource.py
index 7a8fc141..6c7049d5 100644
--- a/mhkit/wave/resource.py
+++ b/mhkit/wave/resource.py
@@ -358,39 +358,45 @@ def surface_elevation(
         data=2 * np.pi * f, dims=frequency_dimension, coords={frequency_dimension: f}
     )
 
-    if phases is None:
-        np.random.seed(seed)
-        phase = xr.DataArray(
-            data=2 * np.pi * np.random.rand(S[var].size),
-            dims=frequency_dimension,
-            coords={frequency_dimension: f},
-        )
-    else:
-        phase = phases[var]
+    eta = xr.Dataset()
+    for var in S.data_vars:
+        if phases is None:
+            np.random.seed(seed)
+            phase = xr.DataArray(
+                data=2 * np.pi * np.random.rand(S[var].size),
+                dims=frequency_dimension,
+                coords={frequency_dimension: f},
+            )
+        else:
+            phase = phases[var]
 
-    # Wave amplitude times delta f
-    A = 2 * S[var]
-    A = A * delta_f
-    A = np.sqrt(A)
+        # Wave amplitude times delta f
+        A = 2 * S[var]
+        A = A * delta_f
+        A = np.sqrt(A)
 
-    if method == "ifft":
-        A_cmplx = A * (np.cos(phase) + 1j * np.sin(phase))
-        eta_tmp = np.fft.irfft(0.5 * A_cmplx.values * time_index.size, time_index.size)
-        eta = xr.DataArray(data=eta_tmp, dims="Time", coords={"Time": time_index})
-
-    elif method == "sum_of_sines":
-        # Product of omega and time
-        B = np.outer(time_index, omega)
-        B = B.reshape((len(time_index), len(omega)))
-        B = xr.DataArray(
-            data=B,
-            dims=["Time", frequency_dimension],
-            coords={"Time": time_index, frequency_dimension: f},
-        )
+        if method == "ifft":
+            A_cmplx = A * (np.cos(phase) + 1j * np.sin(phase))
+            eta_tmp = np.fft.irfft(
+                0.5 * A_cmplx.values * time_index.size, time_index.size
+            )
+            eta[var] = xr.DataArray(
+                data=eta_tmp, dims="Time", coords={"Time": time_index}
+            )
+
+        elif method == "sum_of_sines":
+            # Product of omega and time
+            B = np.outer(time_index, omega)
+            B = B.reshape((len(time_index), len(omega)))
+            B = xr.DataArray(
+                data=B,
+                dims=["Time", frequency_dimension],
+                coords={"Time": time_index, frequency_dimension: f},
+            )
 
-        # wave elevation
-        C = np.cos(B + phase)
-        eta = (C * A).sum(dim=frequency_dimension)
+            # wave elevation
+            C = np.cos(B + phase)
+            eta[var] = (C * A).sum(dim=frequency_dimension)
 
     if to_pandas:
         eta = eta.to_pandas()