Simulate sediment accumulation

pb210_simulate_accumulation() simulates the accumulation of lead-210 over time, returning a data frame with one row per time_step, whereas pb210_simulate_core() returns a data frame with one row per depth_step (like extruding a core). These functions were created to test the pb210_crs() and pb210_cic() functions, and to create the lead-210 theory vignette. Ages are in years, depths are in cm, masses are in kg, densities are in kg / m^3^, and specific activities are in Bq / kg. pb210_simulate_counting() simulates putting a subsample of each slice on a counter for a specified amount of time. This allows assigning an error to the known lead-210 activity, as well as provide more realistic data for simulations. See pb210_error_from_activity() for details.

pb210_simulate_accumulation(
  mass_accumulation = pb210_mass_accumulation_constant(),
  max_age = 500,
  time_step = 1,
  supply = pb210_supply_constant(),
  compressibility = pb210_compressibility_constant(),
  initial_density = pb210_density_constant(),
  decay_constant = pb210_decay_constant()
)

pb210_simulate_core(
  accumulation = pb210_simulate_accumulation(),
  depth_step = rep(0.5, 60),
  core_area = pb210_core_area()
)

pb210_simulate_counting(
  accumulation = pb210_simulate_core(),
  count_mass = 0.5/1000,
  count_time = lubridate::ddays(1)
)

Arguments

mass_accumulation	A function such as that generated by pb210_mass_accumulation_constant(). Values returned by the function should be in kg / m^2^ / year.
max_age	The maximum age of the simulation (years)
time_step	The time to consider in each step (years)
supply	A function such as that generated by pb210_supply_constant(). The flux of lead-210 to the surface, in Bq/ m^2^ / year
compressibility	A function such as that generated by pb210_compressibility_constant(). Values returned by the function should be in 1 / Pa.
initial_density	A function such as that generated by pb210_density_constant(). Values returned by the function should be in kg / m^3^.
decay_constant	The decay constant to use (see pb210_decay_constant())
accumulation	An age simulation, created by pb210_simulate_accumulation().
depth_step	A vector of depth steps to consider, from the top to the bottom of the core. There should be one element in the vector for each slice considered.
core_area	The internal area of the core in m^2^.
count_mass	The amount of mass each sample that was counted (kg). Can be a vector to specify different masses for each sample.
count_time	The time each sample was counted, as resolved by lubridate::as.duration(). Raw numbers are treated as a number of seconds, however we reccomend using lubridate::ddays() and/or lubridate::dhours() to make the units explicit.

Value

A tibble with columns:

• age (years): the mean age of the slice (weighted by mass)

• depth (cm): the midpoint depth of the slice

• activity (Bq/kg)

• age_top, age_bottom (years)

• depth_top, depth_bottom (cm)

• slice_mass (kg)

• slice_density (kg / m^3^)

The output of pb210_simulate_counting() also includes the columns:

• activity_estimate (Bq/kg): A plausible activity that might be obtained by a lead-210 counting method, sampled using stats::rpois().

• activity_se (Bq/kg): The standard error (estimate of the standard deviation) of activity_estimate. For the expected error, use pb210_error_from_activity() on

Examples

# 1 row per year
accumulation <- pb210_simulate_accumulation()
# 1 row per 0.5 cm
core <- pb210_simulate_core(accumulation)
counted_core <- pb210_simulate_counting(core)