diff --git a/swash/processing/post.py b/swash/processing/post.py
index 0de324e..502d0f4 100644
--- a/swash/processing/post.py
+++ b/swash/processing/post.py
@@ -37,7 +37,7 @@ dt = config.getfloat("post", "dt")
 f = 1 / dt
 log.info(f"Computing reflection coefficient at x={x0}")
 
-eta = (data["dep"] - data["botl"])[t > t0, x0]
+eta = data["dep"][t > t0, x0] - data["botl"][x0]
 u = data["vel"][t > t0, 0, x0]
 
 phi_eta = np.abs(sgl.csd(eta, eta, f))